Beyond Streamlining: Drag reduction in the 21st century

Martt,

Thanks for the kind words. It's a truly exciting time in our movement. Just as Tom Poberezny said, the next big thing in aircraft innovation will come out of this organization.

Regarding biomimicry as a design strategy, the latest discoveries show that we have a very long way to go before we'll be in a position to apply most of the lessons taught by the animal kingdom. Our best aircraft aren't even in the same league as flying/swimming creatures, whose functionality appears to consider a myriad of highly advanced drag reduction and propulsive concepts we're not yet using.

When I was growing up, the educated elite among scientists seemed to treat birds, bats, and insects as primitive flight technologies compared to our enlightened conquests of the sky. I recall in particular that bats were held up as an example of evolutionary limitation; crude and inferior to birds.

A little humility seems in order. I recently read a study in the Journal of Aircraft revealing how bats achieve the incomprehensively stunning achievement of generating a maximum lift coefficient of SIX (at low Reynolds numbers!) while also creating extremely low induced drag. Their wake vortex is shed in a manner that causes self-interference, and with each upstroke, it is detached and a counter vortex is created that destroys the primary vortex to quickly yield a minimum syntropy displacement of air. (All this in a sonar guided, intensely aerobatic insect collector [2x We] having an eight hour-plus endurance.)

Fixed wing flight has no such mechanism for cutting the vortex loose. This adds all the more to our need to elevate minimum induced drag to a driving priority as we crudely apply fluid dynamic principles seen masterfully used throughout nature.

John, A place that takes a hit in effiency I think would be the placement of the propeller

A tractor type, it seems, would blow air back over the fuselage or wing and creat all kinds of drag. Would that reduce a super effient prop to a mediocre one.

Then on a pusher, it sits inline with the hot exhaust and engine compartment gases and turbulent airstream from the fuse. reduceing its effiency

How would a designer consider that, or does it really matter that much?

I think the better setup would be a pusher with an electric motor(less heat) and fuselage that would not produce too much turbulence.

                            Just some of the things I think about, Eric

1. 
   

 OOOH! Good question, Eric.

This one has driven me crazy for thirty years. Propeller placement is critical to propulsive efficiency, fuel efficiency, noise reduction, laminar flow, and economical use of open thermodynamic principles... yet we virtually ignore it. When we do take notice of the issue, our discussion tends to miss the point while we argue about specific case studies showing the results we prefer, or which place well-intentioned yet irrelevant constraints on aerodynamic design.

It is important that we limit a discussion of aerodynamic issues to the aerodynamics, before trying to apply them to aircraft. Many times, we shortcut that process by too quickly pointing out a structural limitation or a cooling issue or whatever; therefore we fail to listen long enough to learn what really drives the relationships or to discern fundamental aerodynamic truths. If we had all the facts, we could take aim at limiting aspects from a new angle more favorable to delivering the big picture results we desire.

The truth is that wake immersed propulsion (putting the propulsor in the back) is LESS EFFICIENT. For the propulsor.

It is far, far, far more efficient for overall propulsive efficiency of the properly-designed object we wish to move through a fluid medium. (Goldschmied verified numbers in wind tunnel testing that if bluntly restated here would most likely get me run off the forum as a lunatic.) The trick, therefore, is to improve propulsor efficiency (by thinking of it, not as a prop, but as an impeller) while maximizing the design integration of the airframe and propulsor.

Our examples in the world of aircraft usually fail to demonstrate this truth, but occasionally we let it sneak through in spite of ourselves. The O2A you pictured, for example, surprised investigators by revealing 5% greater propulsive efficiency when running on the rear engine than on the front. This is an exploitable phenomenon made quite a bit more evident in the world of hydrodynamics.

Have you ever seen a tractor-prop submarine? Torpedo? Boat? Fish? In these fields we have to be careful not to vary the disk inflow angle or induce a bunch of turbulence ahead of the prop, but if those obvious mistakes have been avoided, the results are clear: a submarine with a well designed wake immersed impeller will deliver more than 20% greater propulsive efficiency.

We can do the same to aircraft, but even our best examples so far (I like the Wright Stagger EZ as a beautiful example) would barely pass if they were scrutinized on the level of an underwater system. The good news is that we're not using several principles that make attainment much easier than at present.

The keys to avoiding a poor 'pusher' configuration are many, and they start with calling it a pusher. Pushing the air is ALWAYS a bad idea given an alternative. A well designed pusher is actually a sucker. It will make more thrust ahead of the prop disk than behind it. One of the ways it can do this actually steps beyond propulsion into the field of pressure thrust-based drag reduction, which is how Goldschmied's 176%-198% propulsive efficiency demonstration came to be possible.

Tractor configuration aircraft have taught us that it's really important to make the fuselage as small and unobtrusive as possible. But in the wake-sucking propulsive paradigm, the more work we can get the fuselage to do, the less work the propulsor has to do. A big-ole FAT laminar flow fuselage -less than four times as long as it is bulbous- can part the air like a maul can split cedar. Having done so, the mandantory collapse of the air onto the aft body returns energy to the system like a slowly stretched rubber band quickly returning to its original diameter. The engine can be used to help this process along by creating low pressure ahead of the prop disk but behind the maximum body diameter. Proper geometry can accelerate the inflow velocity of air into the impeller, improving its efficiency while allowing the geometry of the inrushing air to vector a thrust force against the cowling.

Folks, this is a good thing. If a body with twice the diameter can show half the drag, why are we still arguing about it? If
the number one GA noise reduction strategy is a decrease in propeller tip speed, why don't we look at how to achieve that goal without compromise?

A properly designed wake immersed impeller will trend to showing the following basic differences over a prop: 1) smaller diameter. 2) more blades. 3) more pitch. All of this is good for noise, and in the context of hyper-efficient flow design, results in variable-pitch performance with fixed-pitch weight and cost.

Impeding the cause is the standard air-cooled aviation engine. Not only is wake propulsion our best chance to use liquid cooling effectively, but torque at low RPM becomes the only thing we really care about. Electric motors and hybrid electric impeller drives will deliver far more torque than our present internal combustion-derived prop designs have optimized to use. When people talk about pushers having low efficiency, what they're actually talking about is the near total absence of appropriate prop designs. Seven bladed, low aspect ratio tail fans may one day prove to be more common than a two bladed tractor.

Long before we really got to play with designs of this sort experimentally, the idea that putting counter-rotating props on them  was championed in the hypothesis that they could improve flow vectoring of the propwash to eliminate unwanted swirl.

But since when is swirl unwanted? Certainly we pay a price for it in the tractor configuration, but one of the many fundamental mistakes of our first century of flight is to try to straighten out viscous fluid flow. Not only does the mechanical cost of such an effort cost more than is gained, but the inherent nature of fluid motion is to swirl.

Let it do so, choosing optimums in size and strength. At Oshkosh this year many of us saw the aircraft parts-based kinetic fountain sculpture on display in one of the vendor Hangars. It presented an unbroken, unbelievably smooth column of water, about 5/16" in diameter, dropping from a laminar flow nozzle above to a splashless entry into a small pool impossibly far below. How is this possible, we ask? Rotating laminar flow.

The bottom line is that a wake-immersed propulsion design, ingesting the aft boundary layer of a fat NLF fuselage by means of 'open thermodynamic pressure thrust'... sucking most of its thrust while spitting out the rest in a smoothly displaced laminar column... slowly spinning a high efficiency impeller... is the ultimate in aircraft propulsion integration in the GA speed range. One final bonus, as you've clearly considered, is that heat and exhaust can be put to work by keeping them out of the prop but exiting the aircraft with the prop stream. Exhausting through the prop hub or by accelerated nozzle flow ahead of the prop allows this energy to create thrust, not cooling drag.

John, Wicked Cool Ansewer!

Now I know what pressure thrust is, sort of.

Do fat and faster planes who's fuse. is bulbous like the Venture or Polliwagen benefit from it? Not really a serious question.

                                                                                                             Thanks, Eric

Ya puts in yer quarter, ya gets yer ride. Hurry, hurry, step right up....

I guess now we know why 2600 people have looked at this thread and half a dozen have written!

Actually, there's a lot more to recommend 'prop in back', but since others have covered those aspects of the topic pretty well I'll shut up now.

Your 'not serious' question is dead serious to me, though, and deserves a brief response. In short, yes! They do benefit from low 'fineness ratio', the length to width proportion whose optimum varies with Reynolds number. Using this insight, most GA aircraft could be redesigned for more room and less drag if there was sufficient incentive.

The recent crop of Euro-designed LSAs look that way for a reason. When Cessna, facing this competition, announced they would soon show their own LSA, I was thrilled to see how they'd sieze this perfect opportunity for progress and set a template for the aerodynamic design of their next generation aircraft. To say their UNinspired, UNimproved anachronism was UNderwhelming is an UNderstatement.

The Polliwagen was not well known to me before seeing your avatar, and the first thing that struck me was its sensible fatness. Is it as roomy as it looks?

John,

A question that comes to mind with impellers on aircraft: What about on takeoff? Before there is pressure thrust being created? It reminds me of the scram jets: they need to be at speed to create more speed, is the pressure thrust fuselage and impeller the same? Is it a matter of creating the impeller to be not perfectly efficient at speed to create more lift at slower speeds/takeoff?

Justin

 Justin,

No, a well designed wake impeller gets the best of both worlds, acting as an optimal airscrew at high speeds and as a high thrust volume pump at low speeds. It's helpful to forget the fuselage for a second to visualize things.

A spinning propeller draws in air from in front and accelerates it out the back, as we all know. The result of the acceleration is that the stream tube behind the prop is a contracted column of air. What's going on in front of the prop is a lot less consistent depending on speed. At high speeds, the air moved in response to the low pressure created in front of the prop comes from a column more directly forward of the prop disk, whereas at low speeds or when held stationary, the prop draws from a big mushroom-like volume ahead and to the sides. (If the prop diameter is the 'stem', the 'cap' can reach clear over it, drawing air even from behind the plane of the prop!)

The problem of takeoff is that this larger volume of air causes the inflow speed to be drastically lower than the prop would see at the same RPM in cruise, causing a high angle of attack at the blades. Generally we compensate most effectively by reducing pitch in response to this slower inflow velocity. But there are other ways.

In the ducted fan approach, the duct constrains the airflow ahead of the impeller by limiting its cross sectional area, thereby accelerating it to a much more ideal inflow velocity regardless of flight speed, and moving the source volume forward. This is the main reason why ducted fans can make monster takeoff thrust compared to other fixed pitch systems. The inflow is accelerated, putting the impeller in an angle of attack closer to cruise mode. At high speeds, the same physics apply, however, and unless there is compensation this inherent acceleration works against efficiency. At high speeds props are free to be airscrews, whereas ducted fans are still trying to be volume pumps. The drag of the duct gets the blame, but that's not really the problem. The problem is that as the aircraft accelerates, the source volume begins to exceed the pump volume.

Another way to accelerate the inflow into a prop is to block it. Placing a large body directly ahead of a prop drastically reduces the source volume it draws from. Regardless of flight speed, therefore, the prop disk inflow velocity is increased. This is why a wake prop must counterintuitively increase in pitch.

In the static thrust condition at takeoff, the same mushroom cap of source volume exists as in the first example, but we've cut its entire center volume out by blocking it with our fuselage. Whatever the proportion is between the unblocked volume and the blocked volume will reflect the value of the inflow acceleration we've created. This inflow acceleration is primarily what increases static thrust, potentially making a pressure thrust configuration superior to a fixed pitch tractor prop. If properly executed within the limits, an ideal relationship between low speed and high speed performance will exist given adequate torque.

In addition to static thrust increase through flow accceleration, pressure thrust reaction adds to the advantage of wake propulsion at takeoff. What we want in designing the 'inflow guiding aftbody' is for the guided airflow to meet the prop disk at an angle that makes sense, (although 'straight through' is not desired!) and for the inrushing air coming from the perimeter to 'bank shot' off the structure in a way that imparts some of its momentum to a forward thrust reaction vector. A concave 'pressure thrust aftbody' must be a structural item!

Last but certainly not least, we need this 'inflow guide' to get along with the much larger NLF fuselage we can join to the front of it. The relationships are daunting but pretty simple in principle, and in all cases depend upon a proper understanding and analysis of laminar flow transition. We desire the airflow (we're flying now, at cruise) to flow over the forward fuselage in as much natural laminar flow as prudent for the aircraft and flight speed. Just as it starts to turbulate, we trip the flow intentionally by volume reduction, then begin suction. Most follow the Ringloeb cusp 'circumferential slot' approach mentioned by Goldschmied, but I favor simple delta p: a pressure differential adequate to suck the flow tight to the aftbody. It doesn't take much. Suction in this manner augments what nature is trying to do with the flow anyway.

A problem I sense is that too many people are looking to the literature as if it is a recipe book. The experiments of the past should be used to grasp principle, not to constrain design using that principle. Taking Goldschmied's pressure thrust body and adding a prop, for example, won't work very well. The reason is that the geometry of pressure thrust recovery and the geometry of prop inflow are not optimized relative to their conflicting demands on the airflow. The prop will overpower the suction.

There are many ways to replicate the phenomenal results crudely achieved in testing 30, 40, and 50 years ago, but we have yet to do so publicly.

I say we can do even better than that. Our science has described a simplified mathematical model for 'inviscid' flight that is a world unto itself. But since what its says we can do and what we can actually do are two different things entirely, it will require that we reduce our dependence on limited math and use more of the creativity we find among our better examples.

John, you wrote "I guess now we know why 2600 people have looked at this thread and half a dozen have written!"

I have looked over this post several times, and am finally getting up the nerve to jump in with questions.  I am not sure if I am nerdy enough or smart enough to completely know what you are writting about,but will take a chance and ask for help and/or advice.

I would like to build a two place, tandem seat motor glider.  I have a few hours of glider time in the Schweitzer 2-33, and loved the visibility!  To my dismay, I seem to be striking out on finding plans or kits available in this configuration.  It appears I might need to design my own if I want this configuration.

Like 99.5% of all the other people on this forum, money is tight and my budget will be very low.  Also, I need to try to keep things as simple as feasible due to several perceived constraints.  My comfort level at this time is with aluminum or wood and fabric construction.  Also, even when the engine is running I want a very quiet aircraft (my ears aren't as good as they used to be).

My avatar photo is a Ryson ST-100 "Cloudster" from 1979, I think at Oshkosh or San Diego.  The ST-100 was a one off prototype by Ryson but the owner of the company died and the aircraft seems to be orphaned.  The other aircraft that seems promising is the Lucas L6A motorglider.  However, my attempts to find plans or locate Emile Lucas are not working.

I am currently working through the John Roncz 1990-91 series of Sport Aviation articles on aircraft design, and plan on also going through the Willford articles between 2000-2002.  I did get a copy of the 1980 Janes' "Gliders of the World" which also provided some interesting data.

At this time it appears the aircraft I want will have a wingspan of 40-45 feet, 130-150 SF wing area, weigh about 1200-1400 lbs and will likely have an engine of 75-100 hp.  With the aspect ratio around 10 or 12 to 1, I have been warned about torsional problems that the wing structure will need to counter.  I would like a stall speed of 40-45 knots, cruise at 100-110 knots.

First question, on post 1 you had a picture of a 4 blade propeller.  Who made this, I like it!  Could this prop have controlable pitch?  I suspect with the smaller diameter it can run to higher RPM, which means I might not need a speed reducer on the engine (keeping things simple?)

Any advice on picking an airfoil for use?  Most common glider airfoils appear to be NACA 23000 and 63 series, and Wortmann FX series.  What about Ribblett series?

To conclude this lengthy post, you stated the obvious "light is good!"  In your opinion, (and considering this old dog does not want to learn too many new tricks) aside from composites, would you advise all aluminum or wood and fabric as having the best potential for the lightest airframe?

After seeing the opinions I get back from these questions, I will post additional questions.  (Maybe)

 Reid,

well, you done did it now. First of all, as a Cloudster/Lucas fan, you're not only qualified in requisite nerdiness, you may have accumulated enough points just reading this thread to receive card carrying privileges. As to smart enough... well if that's a requirement it's due to ineffective communication on my part. This forum is about basic stuff we can relate to intuitively. Academicians will jargon it up later I'm sure.

People who truly appreciate motorgliders seem to be a rare breed. Thanks for carrying the torch. (I'd state for the record that glider pilots are real pilots and the rest of us haven't qualified. Flying around in 8:1 bricks, we may yet get the chance. Once.)

Since I was a kid I never could understand why putting an engine on an aircraft could destroy its L/D so completely. By the time of my student pilot training in high school I could recite the stock answer, but that doesn't mean I accepted it. There is no excuse for a powered aircraft being aerodynamically inferior to an unpowered aircraft, yet they typically are. So rather than cue off your desire to own a commercially extinct species, my brain goes to what the real options are.

Why not own something that does exactly what you want? Chances are it's on the drawing board already as a Synergy derivative; if not, it will be.

I can almost see the readers racing to fire off posts reminding all of us that airplanes are exercises in compromise and that we can't simply ignore the cherished limits of the past.

OK, why not? Referring us back to the black charts on post #12, why should we accept that there is no way to occupy the space above AEI =10 at any speed we desire to fly? Why not admit that there could be another path, a fork in the road we didn't take, long, long ago? One that leads, not to a dead end Aerodynamic Efficiency Index of 6, 7, or 8... but to double or triple those numbers. There is such a way. I'm racing to fly its full scale proof to OSH in 2011 via the CAFE Green Prize  .

Every future aircraft comprehensively applying the lessons we're discussing here is a superb motorglider. They motor quickly up, and they glide slowly down, whenever we want and at whatever speed we design them to. In between times they zip smoothly coast to coast at twice the speed conventional wisdom allows for their horsepower. Since the catalyst is low induced drag in a stable and structurally robust package, high altitude performance and tremendous useful loads provide utility you'll never find in an older design. Ideal volumetric displacement waveform and assisted hybrid NLF takes care of the rest.

I can show you one if you want. The technology it takes isn't public yet, but it will be soon. When it is, kit manufacturers can license a simple, safe, inexpensive way to triple their fuel efficiency and double their speed in a quieter, roomier, slower-landing aircraft.

Regarding your prop question from post #1, it is one of mine. It's just a quick workup for the wake propulsion aircraft I designed as a conventional baseline to compare Synergy to, seen in post #43. It's not a finished product, but the visual  design tool developed to model it parametrically now gives us the capability to quickly model and analyze real props for any application, using a novel system of priorities. This is what I spoke about at Oshkosh last year in the forum "Prop Design at the Bleeding Edge."

Awesome props are needed in order to deliver the kind of performance possible at this level. The prop you refer to is a quiet, constant Reynolds number, multi-blended, Mach-adapted, NLF airfoil wake prop of about 93% efficiency in the freestream. In the wake impeller role its efficiency exceeds that considerably. The reference pitch at each station is based upon inflow velocity variation due to fuselage effects.


Things that have advanced beyond this concept rendering include using a blade-and-hub design and new low-Reynolds NLF airfoils specifically designed for delivering super L/D over the unique AoA range of each prop station.

When you have a constant Reynolds number, each prop station is, as far as it is concerned, flying at the same speed. That takes a huge variable out of the equation and allows custom airfoils to be idealized into blendable families. In props that vary the Reynolds number, resulting non-optimum lift distribution is the source of excessive noise and poor efficiency. We need to be able to vary the lift of a blade along its length to optimize disk loading and lower induced drag, but the ideal means of doing this should consider not only pitch (angle of attack), but airfoil camber.

Near the hub, a highly cambered airfoil can be set at lower angles of attack, delivering the right lift (lots of it!) while minimizing rotational drag at runup. (The powerful inflow caused by the prop disk as a whole immediately reduces the actual AoA at the prop hub to a value that's not only reasonable, but is inside the stall range of the airfoil. That too reduces drag.) Outboard, camber is reduced, and at the tips it may be symmetrical or even reverse depending on the airfoil family. Compressibility effects with increasing Mach number toward the tips are considered in the airfoils for greatly reduced drag, instability, and noise.

Props need to be designed for engines and airframes together. Whether you can achieve the greatly desirable objective of direct drive depends on torque, RPM, and prop location. But as in post #43 above, a wake impeller is the way to go to get direct drive with high RPM engines.

My answer to your airfoil question is 180 degrees from the one I'd have given before starting to design airfoils. The bottom line is that for my own needs I can create airfoils at will that are far better than the ones you mention for a given application. I'd have gravitated toward Riblett and NASA NLFs previously depending on Reynolds range. A more complete graph comparing two Synergy airfoils to three common sections is attached as a .pdf below the one shown.

Regarding materials, for a quiet, efficient motorglider using your skills I'd go with wood. Bear in mind that wood is composite construction! Nothing stands in the way of designing and finishing it in a way that allows the final product to be comparable to a glass ship.

Reid, thanks for the post. 2593 more victims to go...

I'm not sure if you're trying to flatter me with your comment about my alledged nerdiness, but like they say "flattery will get you somewhere"

Your impeller.propeller bares a striking resemblance to what I vaguely recall seeing in a recent Sport Aviation article.  Were you behind that work, or are you aware of the parties involved?  (I am on a business trip, so I cannot recall which issue or name of article, sorry.)

Regarding exercises in compromises, life is an exercise in compromises.  Why should airplanes be different?  I am willing to look at and make some compromises based on what I want , what I can do, and what I can afford.  (My favorite Clint Eastwood/Dirty Harry quote "A man has to know his limitations.")

In addition to my design exercise, I have considering buying a set of plans to (heavily) modify.  Do I convert a Xenos from side by side to tandem seating?  Do I modify an RV-4 to 40 foot wings?  Do I buy a Schweizer 2-32, and mount an 80 hp engine on it?

So many questions, so little time to find answers.

Anyway, I do appreciate your advice.  Stay tuned for more questions qnd ramblings from a nominated Nerd.  When do I get my card?

BTW, I have never flown a motor glider.  I do have about 20 hours in gliders about 25 years ago, under the ORD TCA.  Aside from ceiling limitations, I loved the experience.  Some day I need to make time to complete my glider rating!

Ried,

Well as an Eastwood fan myself I'm glad you chose that quote.

Readers following cutting edge prop design are likely familiar with Paul Lipps' brilliant and provocative work first published in Contact magazine four or five years ago. EAA re-ran one of the articles recently and it's easy to see a lot of resemblance between our props. In fact, I first learned of eLIPPSe propellers when struggling to explain my prop design theory to our local Velocity builders about that time. They nodded along, then pulled out the magazine and asked, "Do you mean, like this?"

Besides being blown away I was greatly encouraged to learn that the test environment was suitably brutal (Reno) and that the results have been consistently spectacular. In talking with Paul and reading about his props it's clear our design intents diverge in a number of minor ways. The areas of similarity are the wide root chords at proper helix angles and the super-skinny tips. Everything in between, and our tools for making sense of things, are different. To these trends I say, get used to it folks. What works is what works, and we've been doing it wrong for way too long.

I cannot recommend any of the aircraft or modification strategies you've considered given my perspective, but in order to advocate a design using the ingredients in this thread, I'd have to nail down a payload and a mission, and/or select an engine or motor to optimize for. Preferably the latter.

Things in this field are happening very, very quickly and I'd like to encourage you and our readers to dream big, now! I can't see how there won't be an explosion of new aircraft using the patent-pending technology that started this mess, and its application to homebuilt aircraft of all kinds should be inexpensive and well supported. In other words, by the time we talk our way through what you want, others will want one too, and if there are enough of such folks plans for such an aircraft could certainly result.

One thing I'd like to ask is why the tandem seating arrangement remains a driving consideration in your criteria. As pointed out previously, our most efficient future aircraft are more 'killer whale' than 'tuna'. The exceptional width and stubby length of a minimum drag fuselage (powered minimum drag) would allow at least two seats, or two seats behind a solo front seat, as one possible semi-tandem arrangement. (Synergy uses a shared stick side-by side control arrangement behind a solo front pilot. Dual tandem is also possible that way.)

John-

I have mounted my pocket protector and put on my super poindexter black nerd glasses complete with bandaid and read your posts with great interest! Wow-excellent nerd candy! That prop is so alien cool. I am just fired up to see an aircraft that incorporates all this advanced technology and modern thinking to blow away the competition. Keep us posted.

-Christian

'-)

You wondered  "One thing I'd like to ask is why the tandem seating arrangement remains a driving consideration in your criteria."

Why not?

No great reason, other than I have nostalgic memories of the view out of a SGS 2-32 glider.

I understand your reluctance for "compromise" modifications of existing designs.  If I were a designer, I would have the same reluctance.  Who know what kind of a fruitcake I might be.  And what would be the ramifications if I cause injury to someone and someone sued. (it does happen on too many occasions, IMHO)

Yes, that is one of the applications that I recall from your photo.  It seemed intriging at the time.  It still does from both a noise/efficiency/simplicity viewpoint.

To whet you appetite, let me suggest this posible combination, eLIPPSe prop, Suzuki/Geo metro engine, minus the speed reduction system, for 75-80 HP.

1300 lbs goss weight.

Not a perfect choice, but possible good enough for what I am looking for?

For a glider configuration there are many very light air cooled motorcycle engines that you may want to concider. Your run time will be limited compaired to a conventional plane and it would save a great deal of wieght compaired to the water cooled small car engine. (suzuki 1200 bandit, honda cb 750, 900, 1100, 1200, older bmw flat twins, easy 45 to 75 hp) Well then the rotax heard of power plants from small to $$$$.

 Did anyone else see the JET Glider at OshKosh this year...... Very impessive assist motor!

Hey the sound is all behind you, that's why your putting the motor in the back right? Huh......

You'r not the first person to mention the Beemer motercycle engines.  I have heard previous comments that they are very smooth, reliable, and not very noisy.  And the technology is pretty well proven.  Some where I have a PDF I should post froom a 1930's era journal about an Englishman mounting a retractable 250cc single cylinder motorcycle engine on his single seat glider.

I did not make it to OSH this year, so did not see the jet-assist glider.  John McGinnis would appreciate your suggestion of mounting the engine facing the rear, if you have followed his earlier posts on this topic.

I just got back from a business trip, so a ton of stuff to sort through and get done.  I might be a couple of days before I repost.

The Lipps prop does provide, in some circumstances, new levels of efficiency. However, it is not a good all-around prop as Paul admits. It needs to be going fast to be good. I refer you all to Jack Norris's book  "Propellers Explained, the first and final explanation".  Jack's props can achieve efficiencies in the 90's in tractor configuration. How, then, can that be improved upon by 20%? A typical RV can produce a net propulsive efficiency of well over 80% (THP/BHP). I am not taking a position on tractor vs. pusher, merely pointing out how the math makes huge improvements impossible. To improve 80% by 20% you would need .8 x 1.2 = 96% and to improve 90% by 20% you would need .9 x 1.2 = 108%. The best glider wing has a L/D of about 60. That's an efficiency of 59/60 = 98.3% and it's not rotating and not dealing with all the other problems that prop blades must.

 Since a calculation of prop efficiency requires both THP and BHP, I developed some methods of determining THP in an actual, flying airplane. I originally published my findings in EAA's "Experimenter" in 2009 . I've since refined the methods. Please feel free to review my current, working page , including a computation of prop efficiency in a C-152. My work is based on concepts originally published by CAFE, particularly Jack Norris and Andy Bauer in Sport Aviation in 1995. However, since the ZT device is no longer feasible with many of the newer engines, I developed some approximation methods that, if carefully flown, can yield usable results. I invite your comments.

Howard,

Thank you for jumping in! I (and I'm sure many, many others) are very appreciative of your working up the spreadsheet tools and related detailed materials in your articles. I expect to assaulted with them frequently as they are very, very good. As is all of Jack's information. (I have 'both' books, but every time I get to the middle it stands me on my head.)

Much respect to you both.

There's a huge potential for concluding I'm out to rewrite the laws of physics here. Nothing could be further from the truth. I want them finally respected and applied!

Our reliance upon a frame of reference that implies 100% efficiency is the maximum possible not only creates confusion, it has kept us from understanding and using FACTS experimentally demonstrated over and over and over again (yet curiously, not undertaken in a decent aircraft embodiment!). As I covered in post #1, #20, and #43, 100% prop efficiency is only halfway there. To have much chance of seeing 'the other half', though, does require taking a stand on the side of wake propulsion.

We must not confuse the fact that NO PROP or impeller is ever going to get more than the mid nineties in terms of aerodynamic efficiency' with the discussion of open thermodynamic Propulsive Efficiency, in which using energy to lower drag causes overall propulsive efficiency to skyrocket WAY over 100%. (Goldschmied himself called it 'prop efficiency', adding to the same "well, that's just impossible!" argument he so vigorously tried to spotlight and overturn.)

While not showing the 198.5% propulsive efficiency results found in his 1987 paper , his presentation of the subject matter in this 1986 paper  introduces the concepts well and lends detail to topics we've briefly considered in earlier posts.

Returning now to how this relates to your prop comments, I'm glad you and Paul are the rare source of any 'less than ecstatic' opinion about his props. As to its behavior, I'd point out that a champion Reno racer with a fixed pitch prop will not have the same priorities as an all-around prop. Is it possible to improve upon Paul Lipps' present designs? I'm sure he thinks so. In a comparison, the difference in performance between a 'Norris type' 6-way optimum like the Whirlwind 200 and a previous eLIPPSe will be most noted in lower torque requirements at lower forward flight speeds. My props have yet to fly outside of their theoretical modeling and simulation, so it matters not the least what I say about them here.

My comments concerning your testing and approach to testing are to keep on it! Huge insights came from the original cold, hard facts and that will never likely change. There are statements in the foundation of your spreadsheet drag polar premise though, which is impacted by the information in this thread. Active drag reduction and synergistic configuration alter the fundamental relationships you've recognized as never changing and always the same, doing so in ways I've not fully explored. I hope you'll assist me to make sense of it in the future.

 Ried,

(in reply to post #53)

I love the view too. Anything I design will always prioritize that gorgeous sailplane view, so we're on the same page there.

I'm not worried about liabilities for liability sake. The reason I can't recommend the modifications are that the starting point isn't close enough to apply the lessons I'm sharing, and the amount of work involved in making them do so exceeds that of starting fresh. The starting point we're looking for exists only in a few short-runs and one-offs of designs that had a good wake propulsive configuration. So, to be candid, given the rarity of such efforts, I can't imagine an easier way to get the aircraft I picture for you than to design and build it.

My plate is full on the build side. It's pretty full on the design side as well, but preliminary aerodynamic configuration using modern 3-D solid modeling tools makes detail design and manufacturing easy and collaborative. 'Doing it anyway' is an economical way to get ambitious projects off to a more comprehensive start than they'd have otherwise.

In theory an advanced new design could be carried forward by its fanatic base, making a project into a directed open-source collaboration. Reality says this couldn't possibly happen in advance of the debut of a pioneering effort. Perhaps you'll like mine, perhaps not. Either way I'm pretty sure there will be takers for such offers in the future.

I've had trouble understanding your statements here:

Yes, that is one of the applications that I recall from your photo.  It seemed intriguing at the time.  It still does from both a noise/efficiency/simplicity viewpoint.

I'm not sure what you're talking about. If you are referring to the twin boom aircraft with the four bladed wake propeller, that aircraft is not a tandem two place motorglider. It's a somewhat impractical five place design study based upon the Synergy fuselage, illuminating how even advanced drag reduction initiatives run into trouble when combined in the absence of structural span efficiency. Since it reminds us of the Adam configuration, I'll just say it: there's a rough road to making the wings strong enough to prove inexpensive and practical (or carry fuel/landing gear). A page enumerating the deficiencies is shown; non-public aspects are obscured.

Replying to #57:

John, thanks for the kind words. I'm a crude amateur with some, hopefully, new approaches, but an amateur nonetheless.

Let me briefly identify a terminology problem.  A very generous and expert reviewer (Kevin Horton ) noted that I was using "propulsive efficiency" instead of prop efficiency.  He correctly noted that the term is commonly used to measure the energy input vs. the energy output. Since our engines are lucky to get beyond 33%, in vs. out, the numbers would be much different.  The term "propulsive efficiency" is being used to indicate how well a prop does in a given installation, taking into account the interactions between the "prop wash" and the airplane. It's also being used to compare theoretical BTU's in and BTU's out, to simplify it.  We know it's not the same thing but we need to be clear which one we are using.  I was, naively, using "propeller efficiency" until Jack Norris explained to me (with illustrations from his Luscombe) that it was really "propulsive" that I was attempting to measure. Since a prop is no more "active" than a winglet, we probably agree that it cannot contribute any energy of its own. Consequently, its efficiency measured by power-in vs power-out cannot be greater than 100%. Can it?

I agree that much can be done to alter induced drag.  If I were to lower the induced drag on my airplane, all other factors remaining equal, then the point at which induced and parasite are equal ( best L/D) would move to the left and (thus) down; I'd be able to fly at lower speeds but at even lower power.  The airplane would be more efficient.  But it is worth noting that at 200 mph in a typical RV, induced drag is around 10% of total drag.  Cutting it by 50%, for example, will only reduce total drag by 5%.  That's desirable, but not earth-shattering.

You suggest that the fundamental relationships can be changed.  Do you mean by this that the shape of the induced curve can be significantly altered (i.e. varies with the square of the velocity)?  I'd be very curious to see some rationale for that, if that is what you mean.

W. Kasper  did some remarkable work with vortices and coefficient of lift. You are probably familiar with it.  I never quite understood why his ideas were not used more.

I recall as a grade-school kid trying to explain to a friend that the (then new) jet engines were using some of the energy from the burned fuel to compress the intake air. His dad told me that was impossible - to get more energy from a system by using some of the energy in it. My wife's Audi's turbo engine says you can.  Therefore I believe you, up to a point.  Blown flaps work, too. Or as said by the Bard: "There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy."  I guess that goes for me, too. I'll study the posts you are pointing me to.

Thanks again!

 John, you wrote:

I've had trouble understanding your statements here:

Yes, that is one of the applications that I recall from your photo.  It seemed intriguing at the time.  It still does from both a noise/efficiency/simplicity viewpoint.

I'm not sure what you're talking about.

To be honest, it took me a few minutes to recollect my thoughts also!  I believe I was focusing on the propellor as shown on your #1 post, and the potential it might offer for reduced prop tip noise?  When I first saw the on-line video of the Yuneec aircraft flight, I was excited, but also disappointed with the level of prop noise.

I'm guessing the eLIPPSe prop might have a lower amount of prop tip noise?  I have no data to back this up, just a hunch.  On my list of things to do is contact Mr. Lipps and find out what he can and cannot do with his propellor.

Ried