Push/Pull twin Sport Plane

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Joined
Mar 9, 2015
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23
Location
Carmel CA
10/8/21

Ahoy, Airplane Homebuilders,

Below, I offer my concept of a ‘Push-Pull’ twin airplane. Seldom seen, for various reasons.

My thinking on this subject is that the rear engine presents both CG and aerodynamic base drag challenges, and that a suitable drag-fix extension shaft is a further mechanical challenge.

I submit that my solution solves these problems, and results in a seriously fast low cost sport plane.

My credentials are just fine, but more than 60 years out of date, while retired in Carmel CA. Accordingly, I offer concepts, with little significant engineering, but with some detailed design. If enough of you like the concepts, the engineering will follow, to make this airplane happen.

The airplane: Side by side, push-pull, twin boom, retractable gear. Rear prop shaft. All easy -- yuhh!!!

Fuselage: Steel truss work. Aerodynamic shape by some suitable method. A composite shell, easily removeable, would be nice to live with.

Surfaces: Pretty much alum 6061, pull riveted. Some members may want carbon fiber. Eventually?

Landing gear: Steel weldment, with air bags. Will be ‘bouncy’, like a spring gear. Add shocks? Much lighter than a spring gear. A bit more work, but good fairing of a fixed gear is not easy, and still not as good as ‘not there’. Low-cost. I have done a fair amount of detailed design of the landing gear. The landing gear which I offer is NOT high tech. It is a ‘Blacksmith’s wet dream’.

Actuators: I suggest CO2 or nitrogen, regulated to whatever the highest pressure that components can handle. PLC control.

Engines: AeroMomentum AM15 @147hp. Near 300 hp in a little clean airplane like this will be fast as a sport plane!! I chose the AM15 because it is a fine value. The airframe is sized to accept the forthcoming AM2.0, and with 520hp available, will be awesome, and ‘hotted-up’, will likely do well in the Reno Racer’s Sport Class.

My plans were that there be a drive shaft of aluminum, or carbon fiber, coupled with Guibo’s. The best way to do it is to de-mount the PSRU, relocate it, and couple it with a shaft, but I have not yet asked Mark Kettering for his opinion. The short shaft will look a lot like the one in your car, with a critical speed well above the max rpm of the engine.

The aft engine with the extension shaft, plus a spinner, will nicely solve the base drag problem.

Bail out: Not with that spinning prop back there. A rear prop brake is indicated, and a forward prop brake may be appropriate. Controllable pitch/feathering props would be nice, but likely not seriously needed in what is intended to be a quite high performance but fairly low cost sport plane.

How to make this happen???

Form a committee ?????? There is a famed expression that says. ‘A camel is a horse designed by a committee’.

There clearly is enough competence among the membership to do the engineering, detailed design, and component fabrication. IF there is enough interest, there will also be members to devise an organization to bring this airplane to life. NOT me, at 93 years!! Interested members can offer their skills in the various disciplines. If this is successful, there may by some money to be made. ‘Not rich, but less poor’.

IF IF there is interest, my suggestion is to build one prototype. Then, with success, IF there is continued interest, that a limited number of ‘beta’ versions be built, with the possibility of forming a company to produce kits.

I suggest the following disciplines be included, with a manager of each.

Fuselage

Engines, forward, and aft

Rear shaft/hub

Surfaces and flight controls

Landing gear

Actuators and PLC electronic controls



My son is ‘untouched’ by any technical education, but is able to set-up, and manage a web site. He has further skills in marketing. He is willing to do this early ‘heavy-lifting. As the project takes shape, Zoom meetings will keep everyone informed, plus an agreed on engineering system -- probably EAA Solidworks. Try for an annual meeting at Oshkosh. Please note my use of ACAD. I have never become proficient at the preferable system.



My credentials include a career as a TailHook Naval Aviator, MS aero, Navy Design Officer of the A6-E Intruder (400+ built), Skipper, NavPro Bethpage, buying all of their Grumman Navy airplanes, including the first 70 F-14 TomCat Fighters, (I REALLY know how to spend money)!!



My final Navy job was as the F-14 Deputy Program Manager. The Deputy job was a lot of fun, since my Boss spent every day building briefings and doing budget drills, leaving me available to run the program.



Enjoy /s/ Bob Belter [email protected]
 

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Vigilant1

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BJC
Reporting as ordered! 😉

Hello, Bob, and thanks for the post.
I like the idea of a centerline thrust twin a lot. I've frittered away err, spent a lot of time with others here batting around two such ideas:
1) The Beetlemaster two-seat (Autoreply says 2+2) aircraft concept. Inexpensive VW engines, safe single engine climb on 75hp, and spritely climb with very good cruise speeds anticipated even with fixed pitch props. I had lots of great input from other fans of the idea: Pops (VW expert who has plenty of Skymaster time), Autoreply, Jan Carlson, etc. Much more here: Beetlemaster thread.
2) A single seat Micromaster aircraft concept using 28-30 hp industrial engines. Again, the rough numbers indicate safe single engine performance is possible along with quite respectable cruise speeds (given the modest installed power). Lots of valuable contributions by others in the extensive thread: The MicroMaster thread.

Again it is a great idea. More comments a bit later.
Mark
 
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Vigilant1

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Bob, some comments and questions:

1) The use case that will largely bound your design is achieving safe single engine climb. This will largely determine the MTOW/wing span tradeoff.
1) Fixed pitch props or adjustable? This proves to be a pretty fundamental choice in the rest of the design. Adjustable pitch props drive up cost and complexity a lot, but can provide for good high speed prop efficiency/thrust while also allowing for very good single engine thrust at slower climb speeds (Vx/Vy). Because that later situation is so important to the overall design (see above), adjustable props can allow for a significantly heavier twin. If you can feather them, that also reduces drag when SE and net thrust is critical Still, an aerodynamically clean design with enough span and fixed pitch props can produce good numbers (single engine or both). Rutan's Defiant did it, and I think the Beetlemaster and Micromaster could do it, too (see those threads for the math and performance estimates (rough). But, it is important to be realistic. Fixed pitch prop performance falls off a LOT at airspeeds significantly above the prop's design speed (and less drastically below it).
Because of design compromises to permit safe single engine flight as well as other things, an inline twin of 200 total installed HP will have a lower top speed and/or lower MTOW than a well designed single engine plane with 200 HP. But, the twin's performance with one engine out will be considerably better. 😉

Retractable gear: it certainly looks nice. When Pops was mulling over his own Beetlemaster concept years before we discussed it here, he was leaning toward retracts. My own choice would be include them only if:
1) They would significantly enhance performance at cruise speed compared to well faired fixed gear. That typically happens at airspeeds above 180-200 kts.
AND
2) The increased empty weight still allowed me safe single engine climb with a lot of useful load. Again, the SE case is always in play.
AND
3) I was convinced I could design very reliable retracts that could also absorb enough crash loads to enhance survivability as well as fixed gear can do.
That's a tall order.

I don't think I'd mess with a driveshaft. With the rear engine running, Cessna found that the aerodynamics of the rather blunt rear cowl were cleaned up a lot. Incorporating a long driveshaft is not a trivial thing, as Molt Taylor would tell us.

I like the Aeromomentum engine choice and the push-pull config. The twin boon layout looks great, but it is worth noting that the Cessna had to work hard to get smooth, slop-free, low friction tail controls.

Looking forward to hearing more! Got any (provisional) numbers to go with the sketches?

Mark
 
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Riggerrob

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I too have been struggling with how to combine two seats side-by-side with a pusher prop. ... but without a long drive shaft. Most of my sketches include a Schumann wing to move the center of lift aft. Schumann wings have straight or slightly swept trailing edges with all of the taper in the leading edges. Look at recent Formula 1 racers or competitive sailplanes.
Another idea is installing a shorter wing center section chord to allow for a (relatively) gentle transition from a 4 foot wide cockpit bulkhead to zero at the trailing edge. I have even played with sketches of very short chord wing center-sections with strut-bracing between the rear spar and center pod?????
 

wsimpso1

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Well, some like the centerline thrust concept, others prefer the Boomerang concept. What the Defiant can do on 15 gph the Boomerang did on on about 6 gph. But if you want to build a centerline twin, more power to you.

As for CG with an engine at each end of a short fuselage, there really are several topics:

Do the best you can on weight of booms and tailplanes, put the pilot's eyes where you want them for seeing outside the airplane, then manipulate the engine positions to finish getting enough static margin, but not huge amounts.

Mark Kettering is a smart guy and most likely knows how much you can shove the prop forward off his hub.

I seriously suspect that unless you are a powertrain vibe geek, this product would be more wisely tackled with both engine/box/props being set up nominally, and balance achieved by sliding the engines fore and aft until you like the package of humans, bags, fuel, engines, and flying surfaces. The shaft drive with remote gearboxes has been done, but it has also humbled good engineers...

The thought of messing with a shaft drive aft is scary at best. The shaft, sized to get critical speed out of range high and carry engine torque plus some margin will be a torsional spring in series with the giubo AM uses, further dropping first natural frequency, but not a huge amount. By itself, this will be beneficial - Mark admits that his current min idle rpm is set by encroachment of frequencies upon first natural frequency. There are a pack of other issues that may be significant:

While engine mounting is probably easy, the gearbox must be separately mounted to the airframe. The gearbox is nominally attached to the engine, and the reaction torque is engine torque times torque ratio of the gear set, so this torque is substantial.
  • Numerous vibratory inputs to the gearbox may require an elastic mount to isolate the vibrations from the airframe, similar to the same mounts for the engine.
    • Since the gearbox is much lighter than the engine, this mount scheme will have to carry the same torques as the engine mounts would, while probably needing lower spring rates than the engine mounts;
    • If instead you go for hard mounting of the gearbox to airframe, understand that all vibratory inputs to the gearbox will be transmitted to the airframe, so it would then need to be sturdy indeed.
  • Axial and angular movement of the engine and perhaps the gearbox must be allowed for. Movement will come from minor position errors, vibration of both engine and gearbox, torque on both, steady state and cyclic prop thrust and moments on the gearbox, and others. This will require compliance in both axial and angular dimensions at both ends of the shaft connecting the engine and gearbox. The giubo may or may not have enough compliance - your research task;
  • The shaft may be able to work fine with the giubo at the engine end with a spline connection at the gearbox, or it may not... If lash in the spline end is problematic (firing order impact loading), you will need to find a lashless coupling that can carry engine torque plus vibe while giving enough axial and angular compliance to make it all work. Not sure if such an animal exists, that will be your research too.
Good Luck on this.

Billski
 

Urquiola

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Aug 23, 2013
Messages
319
Location
Madrid, Spain
10/8/21

Ahoy, Airplane Homebuilders,

Below, I offer my concept of a ‘Push-Pull’ twin airplane. Seldom seen, for various reasons.

My thinking on this subject is that the rear engine presents both CG and aerodynamic base drag challenges, and that a suitable drag-fix extension shaft is a further mechanical challenge.

I submit that my solution solves these problems, and results in a seriously fast low cost sport plane.

My credentials are just fine, but more than 60 years out of date, while retired in Carmel CA. Accordingly, I offer concepts, with little significant engineering, but with some detailed design. If enough of you like the concepts, the engineering will follow, to make this airplane happen.

The airplane: Side by side, push-pull, twin boom, retractable gear. Rear prop shaft. All easy -- yuhh!!!

Fuselage: Steel truss work. Aerodynamic shape by some suitable method. A composite shell, easily removeable, would be nice to live with.

Surfaces: Pretty much alum 6061, pull riveted. Some members may want carbon fiber. Eventually?

Landing gear: Steel weldment, with air bags. Will be ‘bouncy’, like a spring gear. Add shocks? Much lighter than a spring gear. A bit more work, but good fairing of a fixed gear is not easy, and still not as good as ‘not there’. Low-cost. I have done a fair amount of detailed design of the landing gear. The landing gear which I offer is NOT high tech. It is a ‘Blacksmith’s wet dream’.

Actuators: I suggest CO2 or nitrogen, regulated to whatever the highest pressure that components can handle. PLC control.

Engines: AeroMomentum AM15 @147hp. Near 300 hp in a little clean airplane like this will be fast as a sport plane!! I chose the AM15 because it is a fine value. The airframe is sized to accept the forthcoming AM2.0, and with 520hp available, will be awesome, and ‘hotted-up’, will likely do well in the Reno Racer’s Sport Class.

My plans were that there be a drive shaft of aluminum, or carbon fiber, coupled with Guibo’s. The best way to do it is to de-mount the PSRU, relocate it, and couple it with a shaft, but I have not yet asked Mark Kettering for his opinion. The short shaft will look a lot like the one in your car, with a critical speed well above the max rpm of the engine.

The aft engine with the extension shaft, plus a spinner, will nicely solve the base drag problem.

Bail out: Not with that spinning prop back there. A rear prop brake is indicated, and a forward prop brake may be appropriate. Controllable pitch/feathering props would be nice, but likely not seriously needed in what is intended to be a quite high performance but fairly low cost sport plane.

How to make this happen???

Form a committee ?????? There is a famed expression that says. ‘A camel is a horse designed by a committee’.

There clearly is enough competence among the membership to do the engineering, detailed design, and component fabrication. IF there is enough interest, there will also be members to devise an organization to bring this airplane to life. NOT me, at 93 years!! Interested members can offer their skills in the various disciplines. If this is successful, there may by some money to be made. ‘Not rich, but less poor’.

IF IF there is interest, my suggestion is to build one prototype. Then, with success, IF there is continued interest, that a limited number of ‘beta’ versions be built, with the possibility of forming a company to produce kits.

I suggest the following disciplines be included, with a manager of each.

Fuselage

Engines, forward, and aft

Rear shaft/hub

Surfaces and flight controls

Landing gear

Actuators and PLC electronic controls



My son is ‘untouched’ by any technical education, but is able to set-up, and manage a web site. He has further skills in marketing. He is willing to do this early ‘heavy-lifting. As the project takes shape, Zoom meetings will keep everyone informed, plus an agreed on engineering system -- probably EAA Solidworks. Try for an annual meeting at Oshkosh. Please note my use of ACAD. I have never become proficient at the preferable system.



My credentials include a career as a TailHook Naval Aviator, MS aero, Navy Design Officer of the A6-E Intruder (400+ built), Skipper, NavPro Bethpage, buying all of their Grumman Navy airplanes, including the first 70 F-14 TomCat Fighters, (I REALLY know how to spend money)!!



My final Navy job was as the F-14 Deputy Program Manager. The Deputy job was a lot of fun, since my Boss spent every day building briefings and doing budget drills, leaving me available to run the program.



Enjoy /s/ Bob Belter [email protected]
Nice project! Why not both propellers in same line? The gap may induce some nose up or down couple if engines do not provide exactly same thrust. Will it?
Blessings +
 

Riggerrob

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Nice project! Why not both propellers in same line? The gap may induce some nose up or down couple if engines do not provide exactly same thrust. Will it?
Blessings +

This configuration only puts the two thrust lines a little out of phase ... not enough to create huge pitch changes with power. I also suspect that positioning props slightly out of phase will reduce some sympathetic vibrations.
Consider that some wind tunnels have 5 blades on out prop but 6 blades on the next prop ... again ... to reduce sympathetic vibrations.
 

Tiger Tim

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Model of Japanese push-pull twin-boom heavy-fighter (twin-radials)
I’m a fan of the Moskalev SAM-13, myself:
mir1.jpg

sam-13-2-gif.524492


Far as I know it’s just a smidge bigger than a KR-2 so a full-scale replica is a lot more attainable to us than, say, a Dornier 335 would be.
 

Riggerrob

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I’m a fan of the Moskalev SAM-13, myself:
mir1.jpg

sam-13-2-gif.524492


Far as I know it’s just a smidge bigger than a KR-2 so a full-scale replica is a lot more attainable to us than, say, a Dornier 335 would be.

Fokker built the similar D.XXIII push-me-pull-you fighter prototype in 1939. Powered by a pair of Walter Saggita I-SR, 530 hp. inverted V-12 engines, they hoped to top 425 mph. The prototype only flew 11 times before the German invasion in 1940.
 

Tiger Tim

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How to make this happen???

Form a committee ?????? There is a famed expression that says. ‘A camel is a horse designed by a committee’.

There clearly is enough competence among the membership
I think your heart is in the right place but any time someone’s tried to design by a committee of Internet strangers it just chases in circles. As it is pretty well every post on this thread between your initial one and this one has tried to pull in a different direction.

Seems to me you have a good background to jump off from and if you’re really serious about this IMO it’s best to at least start off doing it yourself. Take a look at what Eugene is doing on the forum with his ongoing Skyboy mods; not farming out the thinking but posing questions at each stage, taking in a wide breadth of responses, then distilling it down to what he thinks will suit him the best. That keeps him as the boss of the project and keeps it ultimately focused on his own vision while getting helpful guidance along the way. On top of that, he’s making good progress. You could do much the same but initially on paper, then once you are far enough along that the design shows some real promise you could vet appropriate builders to receive your drawings and notes.

That’s just my thoughts on it, others may vary.
 

Gregory Perkins

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Seahawk MK IV
 

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