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Engine Designs vs. Prop Speeds

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Rick Gaylord
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#1 Posted: 1/3/2011 02:09:44

I've dabbled with internal combustion engines most of my life.  I've only become familiar with GA piston engines in recent years.

As I understand it, propellers operate most efficiently at low RPM.  As tip speeds approach the sound barrier, I'm told, efficiency drops off (and noise increases).

I also have heard that piston engines seem to have the opposite characteristics i.e. they run more efficiently at higher RPM than propellers seem to work. 

So if I understand these basic concepts correctly, the best combination for a crankshaft mounted propeller on a piston engine is for that engine to be designed with compromised efficiency and require High Octane Fuel to prevent pre-detonation.

Can anyone please explain to me why most four stroke aircraft engines have the propeller attached directly to the crankshaft rather than to a geared down (2:1) camshaft designed to support the load of the ( more efficient ) propeller

I presume this idea might increase the engine weight somewhat BUT, would that weight increase be greater than the weight of a gear reduction unit such as ones currently used on thousands of Rotax and auto conversion engines?... engines designed to operate at greatly improved efficiency using less expensive (lower octane) fuel.

Wouldn't the power to weight efficiency be improved to at least justify the heavier camshaft and reduction gears/belts to drive it?...  allowing redesigned propellers to offer more efficient thrust at lower RPM?

Please forgive my naivety but so far, I've never discussed this with anyone who can provide the answers to these questions or the reasons for the current (engine design) conventions.

Respectfully,

The Frustrated Engineer, RG.



Techsponge
Neal Miller
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#2 Posted: 1/3/2011 10:33:39

Lot more experts here than me...

But many aircraft have had gearboxes, esp WW2 aircraft.

Better prop and prop control technology address some of the issues.

Reasons gearboxes are not more prevalent are combination of reasons.

Probably more legal than anything.  It costs a certified manufacturer a king's ransom to get something approved and the company knows in advance it will be sued somehow for the new gizmo in future accidents.



Terrifying Flying Service. If you fly with us once, you'll never fly with anyone else.
Bill Berson
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#3 Posted: 1/3/2011 11:09:20

The 2 to 1 reduction might cause a resonance. An odd  gear value is often used to prevent this. The reduction drive can have many issues, too much to address here.

It is a good thought, but may not be needed for most airplane where a direct drive works well. A slow flying, high drag airplane might need a reduction drive, it depends on the aircraft.



Dana Hague
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#4 Posted: 1/3/2011 12:12:03

Part of the answer is tradition. Older aircraft engines turned slow enough that a reduction drive wasn't necessary, or at least not worth the added cost, weight, or complexity. Most new aircraft engines are simply updated versions of the old ones... but many of the new crop of engines (e.g. Rotax 912/914) turn faster and have redrives. And, of course, nearly all ultralights with 2 stroke engines have redrives. 



Mike Muetzel
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#5 Posted: 1/3/2011 12:30:57

Propeller attached directly to the crankshaft rather than to a geared down (2:1) camshaft designed to support the load of the ( more efficient ) propeller has been done at least once for a light experimental aircraft engine, sometime in my memory.  What isn't in my memory are the name of the engine or the name of the company, and I can't find info anywhere about that engine.  So I guess the answer to your question "Can anyone please explain to me why" is that one way sold well, the other way didn't.



Dave Prizio
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#6 Posted: 1/3/2011 13:16:27

In the 1960s-'70s Lycoming and Continetal made some geared engines for GA airplanes for some time and still support some geared models with parts to some extent, but geared engines proved costly to build and maintain. Ultimately it was their lack of reliability that doomed them to be replaced by turbines more than cost considerations.

Rotax has been very successful with geared engines in more recent times. Their lower horsepower ratings along with better metals and technology have made it possible to produce more reliable and economical geared engines than were possible 50 years ago. It is worth noting that the TBO on the popular 912 engine has only very recently increased to the point where it is competitive with the Lycoming O-235 and Continental O-200 engines.  As more of the Rotax engines make into flight school airplanes, it will be interesting to see how they hold up. If these more complex geared engines hold up well in that environment, Continental and Lycoming may be more inclined to revisit the geared engine concept in the  future.



Dana Hague
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#7 Posted: 1/3/2011 18:01:58

I suspect one reason Rotax has been more successful with geared engines is the higher engine rpm. The higher crankshaft speed means lower toqrue lets you build a lighter (or for the same weight, stronger) redrive for the same horsepower. There are also more power pulses per crankshaft revolution, which reduces fatigue.

Of course Rotax is in the 100HP class, too, while Continental and others were putting redrives on their larger engines.
 



Richard Brown
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#8 Posted: 1/3/2011 18:27:46

Geared Lycs and Conts were just as reliable as any other engine they built.  The problem was and is pilots.  Anyone that flies large radials today knows the problem.   The geared engines were improperly flown, mostly in decent and in traffic patterns.  Engines like the GO 480 were flown like an 0-200.  You should never ever allow the props on geared engines to be "uloaded" ie..windmill.. or the gears backlash creating stress and high wear.  Props should be kept loaded all the way to roundout, flare and touch down.   I think we can agree that an R1830-94 P&W is reliable.  They go well beyond TBO as long as the props are loaded and not run lean of peak.  Same with engines like GO 480s.



Who me? I was fishing on the day in question Mr FAA man. Nope must have been another bright yellow plane.
John McGinnis
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#9 Posted: 1/3/2011 19:20:12 Modified: 1/3/2011 19:25:10

Rick,

You are not being naive. You've merely rediscovered one of the most regrettable issues in all of aviation, and you've put your finger squarely on it. The relationships between prop design, blade count, diameter, tip speed, torque, and horsepower for propeller -driven aircraft are completely goofed up. We're stuck in a hysteresis where our engines come largely from our torque delivery requirements, which come from our props, which are designed (poorly) for existing violent, ancient engines, so the question of what to prioritize depends on what you have under control. Whole systems thinking is a 'luxury' the industry has not supported.

Really, none of the status quo is anywhere near where it could be. Your thinking is correct and headed in the right direction.

The observation you have joined many brilliant folks in making is glaring regardless of whether one looks at it from an engine perspective, an aerodynamic perspective, or from acoustic, structural, thermodynamic, or mechanical points of view.

Why is such a correctable cancer tolerated? A weak answer is that our aircraft engine opportunities have been rigidly slaved to issues of certification, even within the experimental segment, due to indirect coupling. The costs of certification for the installed base dictate our choices and our 'known dataset'. But the uglier truth is that we don't collectively know what to target yet.

I hope to help make some of the solutions obvious soon, when those of us on the same page unite to overpower  ignorance. It starts with re-examining the aerodynamic issue from top to bottom, where from quiet and efficient props we will learn that the powerplant issue is torque at low RPM: we want far more of it.

Due to their relative torque independence, electric motors are the perfect catalyst for establishing and overturning a huge number of myths associated with aircraft propeller design; myths that depend entirely upon the perpetual reign of ancient air-cooled aircraft engines. We are not alone, and the vanguard will be led by those paying attention to what guys like Jack Norris, Paul Lipps, Howard Handelman, the rest of the PADA, and the CAFE Foundation have to say.

You make one of the most important points that needs to be made when safety is priority one: attaching an aircraft propeller to an automobile crankshaft is a DUMB idea, done all the time. The gyroscopic forces acting on an aircraft prop are INSANE. One of these days I'd like to rig up a 'kinetic display' of gyroscopic prop forces for AirVenture visitors to wrestle with. It would have an equivalent angular momentum, a disk spinning at 2700 RPM, hooked to a non-rotating crankshaft for its handle, and visitors could finally see and understand what they are asking their auto-conversions to do. Bent crankshaft, anyone?

This finally brings us to the PSRU (Prop Speed Reduction Unit, prevailing jargon for gearbox).

What a can of worms. I'd rather take back everything I just said about bolting a prop onto an auto engine and stick a fat, efficient little thirteen blade unducted fan on the back of a direct-drive V-8 pusher than wade into the cold, deep waters of PSRU design unarmed.

As others have said (well), it gets complicated. Fast. Harmonic resonance issues and astonishing power pulses in both driven and driving directions are far too often not considered adequately even when the biggie (gyroscopic forces) is tamed, which is rarely. Lightweight solutions are costly by nature, and the market balks. Therefore the problem is that we tend to adopt either of two dumb ideas: no PSRU, or PSRU on a budget. There are better ideas coming.

The beauty of this organization is that it gets enough smart people frustrated enough to collectively fix things. Thank you for calling it as you see it.
GFC EAS 5.jpg


 



Tom Hackel
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#10 Posted: 1/3/2011 20:24:25

Limited by the "Norm" is by far our own downfall. Just mention the idea of building your own plane to a group of people outside of this type of community. The stares of "Your Crazy" are not subtle,,, and that is not why I'm crazy.

 

To the OP's first comment, most A/C engines are optimized for cruising rpm. Compromise of many factors determine the character of an engine, choosing what is of the most value to you is the task. Fuel type, efficiency, power, rpm, weight, size, cylinders, cooling, longevity, maintenance, build cost, popularity, profitability,on and on.



Frank Giger
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#11 Posted: 1/4/2011 02:30:02

I know that with VW engines both direct drive and PSRU's are used (belt driven rather than geared), each option owing largely to the size of propellor desired and allowed.

For a 90 horsepower 1835cc VW, cruise RPM is around 3200 - nothing too crazy wild - so throwing a 60x28 wooden prop doesn't put the engine at too much risk.  To mitigate it further, the smart money is on spending some dough with the boys from Great Plains for a beefed up crankshaft and hub.

A PSRU will mean taking the direct stress off of the crankshaft, but it also usually means throwing a bigger prop (if I understand it properly).  Some designs just won't work with them, unless one likes prop strikes. 



Eric Marsh
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#12 Posted: 1/4/2011 13:17:01

What is the optimal propeller size? As big as will fit?

Are chains being used as reducers? They can be low drag, lightweight and inexpensive.



Rick Gaylord
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#13 Posted: 1/4/2011 13:37:49

John,

I didn't expect such a comprehensive response to my curiosity query! - WOW!  I appreciate all the thoughtful and useful replies.

I'm with you John, about the advent of Electric Power.  I think it's regrettable that (so-far) only the Chinese Government seems to find this emerging technology worthy of investment.  However, I am grateful for their demonstration of viability through the Yuneek, whole system approach.

Compared to internal combustion engines, electric motors seem to offer far superior torque delivery - and smoothly.  As I understand it, now we just have to work on the batteries and power regulation systems to meet current GA needs. 

For aviators who live and fly where the sun shines, I forsee a future with photovoltaic hangar roofs and aircraft skins instead of gas pumps.  I wonder, what can we do to speed that process?  P.O.P.? (Prod Our Politicians??)  

For transportation, we've been married to internal combustion engines all our lives.  But I agree, that marriage has been born out of ignorance.  I really appreciate EAA for its innovative thinkers who continually lead us to new paradigms.

BTW, I like the attachment depicting the futuristic aircraft.  Is that a working design?  A P.I.P? (Prototype In Progress?)

"Onward and upward", thanks!

Rick



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Dave Prizio
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#14 Posted: 1/4/2011 16:36:14

While l welcome your enthusiastic support of electric airplanes, a little more respect for the people who have spent the last 100 years building and refining vehicles powered by internal combustion engines might be in order. They may be many things both good and bad, but by and large 'ignorant' is not one of them. The internal combustion engine has proven to be a very flexible and efficient means of getting a small airplane from one place to another, and it will likely remain so for some time.

When dreaming about how the world might some day be we have the luxury of escaping the constraints of the current state of technology, the regulatory environment ,and economics. People who must produce reliable and efficient powerplants for today's airplanes have no such luxury available to them. Yet they, too, dream about how they can make their products better.

I am glad that the Chinese government is spending a lot of money on electric airplanes, and I am also glad that our government is not. I am sure that the Chinese will learn valuable lessons that can benefit us all in the future, but I am also sure that the best ideas will not come from any government program but from the fertile minds of our own EAA members and their friends.

Please retain your enthusiasm for what we all might achieve in the future, but never forget the hard work and imagination that got us this far.



John Eiswirth
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#15 Posted: 1/4/2011 22:26:34

The General Motors automotive engine I want to bolt a propeller to will be used to "Carve Air"!  It may not last forever, but looks like the most affordable option with HP & weight comparable to a 0-200 Continental.



John McGinnis
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#16 Posted: 1/5/2011 01:02:54

Dave,

I owe you and our readers an apology. A proverb says that 'in an abundance of words there does not fail to be transgression', and my big mouth was running mighty fast when I found the wrong tone and the wrong words last night. Those who know me will attest that I am not arrogant, just overly passionate at the moment and easily misunderstood.

Respect is key to learning, and frankly, it's the only reason I finally have something to say. I respect those who made this activity possible so much that I get really riled up by the facts of our GA progress since it was left to their successors four or five decades ago. My work builds on all that has come before, and I hope to honor those who played a role in fostering my perspective and understanding. I also have the utmost respect for you as an expert homebuilder and major EAA contributor.

In using the word ignorant, in reference solely to aerodynamic and propeller design issues, I had not considered all of the ugly definitions that might come to mind. Mirriam-Webster online says "destitute of knowledge or education". I meant this second definition found elsewhere: "lacking knowledge or information as to a particular subject or fact."

As many have more tactfully pointed out, as an industry we are certainly lacking knowledge of certain facts that have the potential to completely change everything. It was shocking to discover that many whom I respect greatly were also not as aware as they could have been about things I learned... sometimes from them! That doesn't make my task easy.

Thank you for holding me accountable for what I say. When I read Rick's words as those of an outside observer, they touched a nerve with me and my previous sixteen hours of airplane-sanding angst were uncorked on you without warning. For the record, I am not an engine guy. My passion is obvious if you've read my Beyond Streamlining posts. Synergy is coming together but I am as frustrated by societal apathy as anyone who ever tried to do something 'impossible' in a short time with no money. Some of the take-no-prisoners attitude my mission requires leaked out on us.

Rick, the rendering is of a design used for a baseline comparison of Synergy versus an advanced aircraft of conventional design. I'm not a fan of it per se.

 -John

 

A400M Picture... when you have a lot of torque, look at what happens...

 
A-400-2-728033.jpg



Frank Giger
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#17 Posted: 1/5/2011 04:19:49

Aw, don't worry about it.  We mostly go through life ignorant of just about everything - and just when we start to get a handle on being educated in a well rounded manner we die from old age.

Now that we're going astray of the original topic, the choice between fuel and electric comes down to fuel weight.

Pound for pound nothing so far can match petroleum in the energy versus weight or volume contest.  Let's say one is burning four gallons of gas an hour and is in the air for an afternoon (four hours).  That's 96 pounds of gas - sixteen gallons.

The tank itself is minimal in weight.  It's just a shell for the energy it stores and feeds to the engine.  One can vary the weight of the aircraft based on the amount of energy (gasoline) one puts in - something very important to our friends who live at high altitudes or with a lot of baggage to put in the plane.

Electricity doesn't have the luxury of this variable weight.  The batteries are a fixed weight regardless of charge.  And all batteries have a useful life that require replacement; the costs will make the griping of today about the high price of aviation seem quaint!

Once storage for electricity can come to parity with gasoline in this area we'll see a gradual move towards it in aviation.

Relying on sunshine to keep the plane flying might be okay for the IFR guys above the clouds, but us VFR guys will have to start treating cloud cover as a source of engine failure.

But it's not all doom and gloom.  Somebody out there is going to come up with the Next Big Leap in electrical storage, akin to the transistor or the internal combustion engine, and chances are it'll be an EAA member working in his garage that marries that technology to an aircraft.



Neal Miller
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#18 Posted: 1/5/2011 05:14:59
Eric Marsh wrote:

 

What is the optimal propeller size? As big as will fit?

Are chains being used as reducers? They can be low drag, lightweight and inexpensive.

 

Not to my knowledge on modern stuff.

I seem to recall seeing chain prop drives on some of the early a/c.  Most were leather belt driven though. 

This was still the heyday of the belt drive in most industries.  One big engine in the basement and belts and pulleys throughout the building, amazing systems. Potentially extremely dangerous though.

Modern chain drive is certainly possible, esp since chain materials and technology has come so far.

 

So far as alternate motors, the gm Volt idea interests me, except skip the batteries for now, and fuel w/diesel/jet A.

I'm no physicist, so I don't know how good the total efficiency is.

But putting a small diesel motor driving an electric motor driving a prop is certainly doable...just blueskying



Terrifying Flying Service. If you fly with us once, you'll never fly with anyone else.
Eric Marsh
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#19 Posted: 1/5/2011 10:52:48

A chain system would seem to offer some merits. Chains stretch just enough to offer some dampening of the power pulses. Ratios can easily be changed. Of course if the chain broke that would be a bad thing but a system with more than one rows of chain would offer redundancy.

My drag bike transfers 300 hp through a 530 chain. Yes, it is necessary to replace them occassionally and it's not like I'm putting a lot of miles on them. But a double row of such chains would probably hold up pretty well with less than 200 horsepower. A triple row would even be better.

It just seems to me like something worth considering.



Robert Dingley
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#20 Posted: 1/5/2011 17:18:30

Rick Gaylord asked:

 " Can anyone please explain to me why most four stroke aircraft engines have the propeller attached directly to the crankshaft rather than to a geared down (2:1) camshaft designed to support the load of the ( more efficient ) propeller? "

I read of an engine that was put in limited production by Teledyne Continental Motors about 35 - 40 years ago. It had the out put shaft connected to the cam shaft. ( two to one reduction) It is called the TIARA. As Bill Berson pointed out, resonance was a factor, but TCM got it to run. I recall that it was installed in some ag planes and homebuilts. Six cylinder about 300 HP. I've never seen one. Anyone recall it?

Bob

 



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