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Seeking apples-to-apples data on construction methods

Posted By:
Mike Whaley
Homebuilder or Craftsman
10
Posts
2
#1 Posted: 9/17/2009 00:36:08

Hi all,

 

I am interested in building an LSA-legal, Corvair-powered replica of a somewhat unusual warbird (OV-10 Bronco), and in the course of my preliminary design efforts I have run into an issue that it seems should have been answered long ago: what construction methods are best for building the thing? Since it's a replica, one of the major driving factors is that the shape and dimensions will be fixed, and ability to make it look "convincingly" like the real thing is a consideration. But there's little data (that I've found) that lets one directly compare construction methods for the same external shape with the same loads applied on it.

 

There's a wide variety of possibilities, and what I would LOVE to find is some kind of data that would compare a variety of properly-designed structures to each other... structures with the shape, exterior dimensions, and loading held identical between each one, varying only in construction method. This would allow one to compare relative weights, costs, and building complexities. I've even designed a simplified test structure in CAD for this purpose, but I know the design learning curve is steep and doing this properly means one has to be well-qualified in EACH method being evaluated (the results are meaningless unless each "sample" is designed to EXACTLY meet the specified loads only... ANY method can be built "strong enough" if you just overdo it!)

 

The lack of this data has been frustrating. Trying to compare wood vs metal vs composite vs X for a structure designed to do the identical job...  there's GOT to be some homebuilder someplace who's already considered all these questions and established this data already!! Right?


Hiring a bunch of qualified structural design engineers with vast experience in various construction methods is bound to be both expensive and complicated, unless folks could be found with both the qualifications and personal interest to do this for free. That being said, it might be a worthwhile project, so long as the results were "open sourced" for the benefit of all. Eventually I have to learn some of this in real detail, BUT I think the only way to keep my task within reach is to get this design comparison data to select one desired construction method, before going fully down the path of detailed design. The learning curve is steep and I don't want to climb the same hill ten different ways all at once!

 

The methods I've thought about so far include:

* Wood - I'm by far most comfortable with this one, though weight may be an issue. For realism the outer surface needs to be solid - thin ply or something else, maybe even a non-structural panel or shell.

* Aluminum - I'd only be interested in pulled rivets, ala Zenith and Sonex. Not much experience with it, but it appears to be easy enough... and it would "look" realistic for an OV-10, a really "dirty" aircraft with many round rivets and such.

* Composites, specifically Fold-A-Plane (like Personal Cruiser) - I really like the concept, and since most surfaces are flat or gently curved except at the corners, should be suitable. I did work for a fiberglass homebuilt manufacturer years ago, and know I do NOT want to deal with composites too much, though the FAP concept appears much less difficult and messy overall.

 

* Aluminum angle (like Texas Parasol / Chuckbird) - I like the general idea, but am unaware of it being used on other (bigger) designs. Would need outer covering of some sort. the corners on the OV-10 are not sharp angles so would need formers, shells, skin, etc. added to obtain the final shape.

* Aluminum tubing (like Kolb tail boom, Aventura, Sky Ranger) - would require some kind of lightweight structure to give it the proper shape.

 

* Wood or metal load-bearing structure, covered by an exterior composite frame.


* About steel tubing - Yes, it's the old standby, BUT: I don't weld, don't really WANT to (I worked for homebuilt bipe manuf. until last year, I know a bit about tube structures and what they take to build and I just don't think it's something I'd really enjoy much), and know it would cost a bundle to have someone else do it for me. To me, this is something that's just going to be used for certain sub-assemblies (engine mount & landing gear) unless it's proven it's the only way to achieve the final goal.

 

Given the type of plane, I think fabric covering is unlikely to look right... although the Loehle 5151s "work" in an aesthetic sense, so maybe it's possible.

 

 

 

As always, cost, ease of construction, general durability / maintainability, crash safety, etc. are always important factors as well. Given that I am seeking to create a 2-seat LSA with a 100 HP pusher and it's a draggy airframe, I suspect weight reduction will rule the day in the end.

 

Anyway, if anyone has any advice to offer on how to best determine this, I'm all ears! I hope I don't sound too naive... but having been near homebuilts for most my life, and working in the industry long enough to see some of what works and what doesn't, I have come to believe strongly that half the battle is just deciding that you really can do it if you try. So I figured what the heck, a little here and a little there and eventually, you'll get there, even on an ambitious idea like this!

 

Thanks,

Mike

 



#2 Posted: 9/18/2009 01:12:39

Mike-

When designing, you start with what you 'know'. LSA defines a maximum weight and speed. You need to decide on a payload specification(single pilot? two-seater?) I work in SolidWorks (3-D CAD). As you define what you 'know', some decisions are made for you. Engineering is all about decisions, based on desired outcome. Weight, speed, power-if you know the max weight, then you can decide on lift generating area. I am an EAA'er, but to tell you the truth, I don't know the definition of an LSA. I have a two-seat trike under construction (since 1993), and now I must certify it as an E-LSA since it is a two-seater. I am also building a Cri-Cri. I am a big fan of aluminum. A combination of construction techniques can be utilized as well. Start sketching on a 'napkin'. I run through a spec list in Excel, sketch basic concepts on paper, and then use SolidWorks to move into functional decision criteria narrowing. A back burner project is an 'ultralight' tiltrotor(certainly won't be ultralight, but it will single seat and way smaller than homebuilt helicopters). See attached SolidWorks concept 'sketch'.

An OV-10 is a twin, and using two corvair engines will eat up alot of weight, and will require sufficient fuel. An LSA can be a twin? I would be interested in a direct discussion with you. Where are you located? I am in Kalispell, Montana. You can call me at 509-307-9963, or less preferrably email: alpineglobalprivate@gmail.com. Check out my websites:

www.AlpineWorldwide.com

www.BrilliantDesignOnline.com

You might want to start an open Blog or a Yahoo group for your project. I am a member of CNC, DIYGasTurbines, Cricridrawings, and homebuilt helicopter groups and many brilliant people put their expertise and opinions forward.

-Christian von Delius



Files Attachment(s):
concept Ultralight turbine tiltrotor-conversion.jpg (151857 bytes)
Jeff Point
Vintage Aircraft Association MemberYoung Eagles Pilot or VolunteerHomebuilder or CraftsmanAirVenture Volunteer
94
Posts
65
#3 Posted: 9/18/2009 02:27:58

Mike,

Sounds like quite an ambitious undertaking, and I wish you well.  For a project like this, if you have not already, you might consider another forum in addition to this one, www.homebuiltairplanes.com.  This kind of project is right up their alley, and you will get a lot of good insight and discussion.



Joe Norris
Vintage Aircraft Association MemberYoung Eagles Pilot or VolunteerHomebuilder or CraftsmanAirVenture Volunteer
328
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137
#4 Posted: 9/18/2009 11:55:53

Mike,

The definition of a light-sport aircraft (found in FAR part 1) specifically requires that the aircraft have a single reciprocating engine.  This being the case, your OV-10 replica won't meet LSA no matter what it's made out of or what it weighs.  You can certainly build such a replica under amateur-built rules, but it won't meet LSA.

Joe



Lasciate ogne speranza, voi ch'intrate
Reggie Smalls
Homebuilder or Craftsman
126
Posts
49
#5 Posted: 9/18/2009 12:26:19

Is it allowable to have a single reciprocating engine that drives two propellers?



Joe Norris
Vintage Aircraft Association MemberYoung Eagles Pilot or VolunteerHomebuilder or CraftsmanAirVenture Volunteer
328
Posts
137
#6 Posted: 9/18/2009 13:35:00
Reggie Smalls wrote:

 

Is it allowable to have a single reciprocating engine that drives two propellers?

 

This has been a matter of some discussion but I have not yet seen an official answer to that question.  The contention is that since the definition calls out "A fixed or ground-adjustable propeller if a powered aircraft other than a powered glider" and does not include the plural of propeller, then only a single propeller can be used.  However, it does not specifically state a "single" propeller.  I think I see a huddle of lawyers at FAA....

If we get a firm answer, we'll post it.

Cheers!

Joe



Lasciate ogne speranza, voi ch'intrate
Kent Misegades
Young Eagles Pilot or VolunteerHomebuilder or CraftsmanAirVenture Volunteer
24
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7
#7 Posted: 9/18/2009 19:42:33

Joe is correct - only one engine allowed in LSA.  You had better check all the other restriction for LSA before going further.  I'd suggest EAB - Experimental Amateur Built - for such an undertaking.  I'd also stick with aluminum as this will allow you to scale down from the original Rockwell design, if you ever get your hands on drawings, that is.

The OV-10 was a remarkable aircraft for what it did, COIN.  You'll need a multi-engine rating to fly the plane; now you're talking serious money for the training.  If you have never designed and built and airplane before though, you might want to start with a single engine plane before trying a multi. 

But consider the guys that are building a 4-engined, single-seat B-17!   If you really want to do this, go for it.

Kent Misegades, Cary, NC

 

 

 



kmisegades
Ried Jacobsen
194
Posts
26
#8 Posted: 9/18/2009 20:20:32

Joe and Kent are correct, two props on one engine is  contrary to the intent of LSA.  LSA was estaablished to provide a simple, low tech, and somewhat low budget means to make flying more aavailable for hobbiests.  What you are attempting would be more suitable for the EAB designation.

Even if you drive two props off one engine, if you have potential for assymetrical thrust you want training to handle potential problems of assymetrical thrust.

Have fun, but be careful!



#9 Posted: 9/18/2009 22:17:11

 

"I am seeking to create a 2-seat LSA with a 100 HP pusher and it's a draggy airframe, I suspect weight reduction will rule the day in the end"

Mike, you're a man after my own heart. As a young soldier in Vietnam who loved airplanes, I stood next to the first Bronco I had ever seen and about went nuts with excitement.

After discovering the possibilities of homebuilding, I also started thinking of building a replica. The Bronco looks like an easy build with its boxy fuselage and hershey bar wing, but I think building a close-to-scale copy would be easier, (although expensive) than trying to design an LSA within the severe weight restriction of 1320 lb even with a single pusher engine. .

Besides, it looks like someone is already doing something similar. I just found these on the web.

http://www.compositesworld.com/articles/plane-enthusiasts-plan-lsa-compliant-composite-aircraft.aspx

http://www.ionaircraft.com/index.html

 



Mike Whaley
Homebuilder or Craftsman
10
Posts
2
#10 Posted: 9/19/2009 16:27:39

Hi all,

 

Wow, I disappear for a day and look what happens! I appreciate the many good thoughts. I should clarify a few things...

 

I am very aware of the LSA restrictions and rules... I may not have made it clear, but my idea involves a single pusher Corvair engine centrally located at the rear of the fuselage (like a Kolb)... placed where the cargo door is on the real Bronco, and located high enough to provide the necessary ground clearance. I realize that having to conform to the scale layout will impact performance to some degree over a truly optimized design, however I find it hard to believe that the right combination of factors can't be determined to make this work under LSA rules... perhaps I'll have to "give" more on certain aspects than I'd like, but I am certain that it's possible to do this in a way that's worthwhile! Non-LSA is a non-starter for me, and I think the challenge of meeting LSA goals actually simplifies the problem somewhat, as it places some hard limits on the "mission creep" that kills so many good ideas over time. Do the best you can within the parameters, and in the spirit of homebuilding, use creativity and innovative as required to deal with any resulting shortcomings in realism or functionality that you lose in order to meet those parameters.

 

Here's an example... where the scale prop locations are, I will put only lightweight prop spinners, with no prop blades. I would like to mount the spinnners on some inexpensive ball bearings and use some kind of vanes or blades to cause them to spin... they will simply freewheel, driven only by the ambient airflow or at MOST a very tiny small electric motor. A few little clear plastic vanes (like those typically used for vortex generators on wings) should be plenty to get them turning, they only need spin fast enough to give the right impression that there is a prop there. As this is a purely cosmetic/realism feature, it can't be justified unless it can be implemented in a lightweight, safe, and reliable manner. Worst case: the thing blows a $3 bearing and stops spinning with no effect on anything but appearance. Cost: probably under $40 for the whole deal. Weight: 5 pounds. Realism: hopefully, greatly enhanced appearance. Cool factor: high!!! That's all secondary but I figure it'll be a neat feature that won't be too difficult to accomplish without impacting performance or safety... and it sure beats trying to do some kind of twin-engine beast with all the associated expense, complication, etc.

 

The two prop/one engine question crossed my mind too but the only way this would be likely to work (in an LSA context) appears to me to be either on a setup like the Cri-Cri (two props located very near a central structure to house an engine), or with a counter-rotating propeller unit (you're just tripling the headaches of prop optimization I think!). No idea what the FAA would think on this, but I don't want to go there anyway so I'll let others worry about it... would love to see a project like this, though.

 

I've already looked at rough sizing for the airframe, I think 3/4 scale (30 ft. wingspan) is in the neighborhood, and the wing will likely be slightly higher in aspect ratio than scale. It's the usual case where you have to get kind of close, then decide how much you're willing to fudge on scale outlines and such to get the numbers to work out. That part worries me less than the structural design questions, though... my background in aerodynamics is much better than my background in structures, not that I'm an expert on either.

 

I still haven't gotten an answer to the original question... is there pre-existing data on how to directly compare equivalent, properly optimized structures built by different methods?

 

Frederick, I'm the webmaster for the OV-10 Bronco Assn. and one of that group's founders, if that tells you anything... never was in the military but I'm an "adopted son" of the Bronco community if you will. I really like the Ion, thanks for the good info (hadn't seen that before). Close in configuration, but... it's still not actually a Bronco!

 

Thanks all,

Mike