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Thread: Ideal push-pull configuration?

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    Moderator autoreply's Avatar
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    Ideal push-pull configuration?

    So, here's the deal.

    I want a push-pull twin with tandem seating and significant luggage space, including a retractable gear. Long range (1500NM), ultrahigh cruise (225 kts or so) is the design goal plus reasonable take-off performance (1500 ft roll, 2500 ft over obstacle sounds good). Engines should be the Rotax, Jabiru or (most likely) the BMW R1200S block, all around 80 kgs (175 lbs) and 100HP, including everything except prop. Mid-wing, to make aerodynamics both optimal and easier. And not to forget, removable wings with a maximum (naked) span of around 6' 5", so trailerable. Why a twin? More safety for crossing large bodies of water (like the Atlantic and Mediterranean), cheaper cost (a bolted BMW is about a quarter of the price of a Rotax, let alone a 200HP continental or so) and also, it's different which is also good ;-)

    What about the best configuration. 3 are common and realistic:

    The canard with a front engine added. The Defiant is the best example.


    I don't particulary like the idea of a canard. CG range is narrow since the wing has to shift forward (100 kgs extra in the front) and the canard has to be really big anyway because of the limited distance between main and canard. A big canard means very low visibility. Flaps can't be added so either a big wing or very long T/O length.

    Second one is the twin boom as used on the Cessna 336/7 and the Adam A500.


    Spar location is a bit of an issue (midwing), so the passenger probably has to sit just in front of the spar. Also the tailboom adds quite a lot of drag. The (RG) landing gear can either be stowed in the fuselage or in the booms. Stowing the LG in the booms is an option, though they have to be pretty big (and draggy) to stow even the smallest tires, let alone some 6" grass-capable ones. One thing I particularly don't like about this is the added complexity, double controlls running tru the wing, landing gear assembly going tru the wing. Also propellor diameter is a major issue since the booms can't be too wide apart (trailerable) which also includes the problem of an exploding prop cutting of the tail and fatigue because the interference between prop and boom. Wet wing tanks is almost impossible due to all stuff running via the center wing.
    Another issue is the massive turbulent airflow over the elevator when the aft engine quits, exactly then when you need attitude control the most..

    Third is the push-pull with controls at the tail like on the DO335.


    I personally like this one most. Really simple construction (simple is good and safe) with a complete wet center wing and simple plain flaps. Landing gear needs to be rather long, but stowing it in the fuselage (Extra-400 or F-16 style) seems a good option, also making a great crash absorber in case it does fail.. Turbulent airflow entering the rear prop is an issue since deflecting an elevator/rudder provides a lot of turbulence. An alternative is a T-tail. I probably can do without a longer driveshaft since especially the BMW is rather slim and light and it can thus be positioned and the rearmost point. Avoiding the driveshaft is a major advantage (vibrations!) A major issue is the point of gravity, there has to be some heavy stuff behind the spar, like a landing gear, an emergency chute and tail surfaces. Even though the tail will be rather long, comparable to the "Pfeil" Because of prop clearance the landing gear has to be rather long. We needed weight in the back anyway
    This configuration also leaves a lot of room behind the main spar to stow luggage (light and volumious).

    So, which points did I miss and why is my preference for the Dornier arrangement wrong? Why would you prefer another push-pull configuration?
    Last edited by autoreply; September 5th, 2009 at 03:00 PM.

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    Moderator Topaz's Avatar
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    Re: Ideal push-pull configuration?

    The one definite advantage to the Defiant or Adam-style arrangement is that you don't have to design a shaft-drive system for the rear engine and propeller. I'll agree that the Dornier 335 is probably the better design aerodynamically (and structurally lighter and simpler than the Adam 500), but search the forums here for "Shaft" or "Torsional Resonance" and you'll start to see the challenges of shaft drive. It can be done - and it has been done sucessfully - but it's not for the faint of heart.
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    Re: Ideal push-pull configuration?

    Quote Originally Posted by Topaz View Post
    The one definite advantage to the Defiant or Adam-style arrangement is that you don't have to design a shaft-drive system for the rear engine and propeller. I'll agree that the Dornier 335 is probably the better design aerodynamically (and structurally lighter and simpler than the Adam 500), but search the forums here for "Shaft" or "Torsional Resonance" and you'll start to see the challenges of shaft drive. It can be done - and it has been done sucessfully - but it's not for the faint of heart.
    In fact I already did that. It's so complicated (see the Stemme) that one should avoid that. Simply putting the engine in the very back, like on a long-ez is in my opinion the solution.

    It does induce the problem of CG way too forward, but that can be solved another way. It does also induce the problem of attaching the fin/stabilator somewhere else. However, to prevent the propellor from oscillating too much you already had to put them about 3' in front of the prop, so you can simply mount them to the fuselage. Or am I wrong

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    Registered User Mac790's Avatar
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    Re: Ideal push-pull configuration?

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    Last edited by Mac790; October 6th, 2010 at 05:07 PM. Reason: ...
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    Registered User wsimpso1's Avatar
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    Re: Ideal push-pull configuration?

    I can offer a designed experiment approach to covering the design space. You appear to have several variables:

    Fuselage length - Short, Long;
    Layout - Conventional, Canard, Three-Surface;
    Booms - Yes and No.

    That gives 2*3*2 = 12 permutations, but a canard layout never needs booms or a long fuselage, and a long fuselage never needs booms, so you are down to seven basic configurations. They are:

    Fuselage Layout Booms
    Short Conv Yes 337/Adam
    Short 3 Surf Yes
    Short Conv No
    Long Conv No Dornier
    Short Can No Defiant
    Short 3 Surf No Avanti
    Long 3 Surf No

    Now think on the ones unidentified...

    I can set up a little spreadsheet with the basics of this if you want. Just PM your addy to me.

    I have never heard anyone complain about visibility from the Defiant, as you are basically looking out at the thickness of the canard.

    The Dornier type ends up with a driveshaft to the aft prop, which is operating in the wash of both the wing and the tail, so it will require some careful analysis and design for dynamics and resonance...

    In restricting yourself to push-pull twins, you have skipped what I suspect is the single best configuration of a propellor driven twin, and that is the Boomerang, with a large and small fuselage, and set the wing and tail areas around the CG based upon optimizing for both single engine cases. Baggage can go in both fuselages...

    Billski

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    Moderator autoreply's Avatar
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    Thumbs up Re: Ideal push-pull configuration?

    Your input is highly appreciated

    Quote Originally Posted by wsimpso1 View Post
    I can offer a designed experiment approach to covering the design space. You appear to have several variables:

    Fuselage length - Short, Long;
    Layout - Conventional, Canard, Three-Surface;
    Booms - Yes and No.

    That gives 2*3*2 = 12 permutations, but a canard layout never needs booms or a long fuselage, and a long fuselage never needs booms, so you are down to seven basic configurations. They are:

    Fuselage Layout Booms
    Short Conv Yes 337/Adam
    Short 3 Surf Yes
    Short Conv No
    Long Conv No Dornier
    Short Can No Defiant
    Short 3 Surf No Avanti
    Long 3 Surf No

    Now think on the ones unidentified...

    I can set up a little spreadsheet with the basics of this if you want. Just PM your addy to me.

    I have never heard anyone complain about visibility from the Defiant, as you are basically looking out at the thickness of the canard.
    Let's say that I don't like canards that much, so I'd prefer another configuration. The CG issue is a real one. Since wing and canard are so close you need to have a big surface for a reasonable moment coefficient and what about the verticals? Also, a canard is much more limited in CG travel. With a tandem configuration that's unfavorable.
    The Dornier type ends up with a driveshaft to the aft prop, which is operating in the wash of both the wing and the tail, so it will require some careful analysis and design for dynamics and resonance...
    Why does it need a driveshaft? Why not simply copy/pasta a long-ez aft section and put a fin/stabilizer just in front of it? I don't see why that shouldn't work?
    In restricting yourself to push-pull twins, you have skipped what I suspect is the single best configuration of a propellor driven twin, and that is the Boomerang, with a large and small fuselage, and set the wing and tail areas around the CG based upon optimizing for both single engine cases. Baggage can go in both fuselages...
    Actually, I agree. The single advantage of the Dornier configuration is simplicity, one fuselage and one wing, no complicated intersections, thus limiting complexity of the design (another reason to ditch 3-surface craft).
    Another reason is less rational, I like the idea that nothing happens with a push-pull craft when an engine fails. Also, sucking off the boundary layer of that fuselage has a real advantage. The Boomerang still has some asymetric effects and I don't consider designing such a craft and - besides that - it's not a push-pull airplane

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    Registered User BBerson's Avatar
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    Re: Ideal push-pull configuration?

    Twin engines operating independently but with coaxial shafts is another option for center line thrust.

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    Moderator Topaz's Avatar
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    Re: Ideal push-pull configuration?

    Quote Originally Posted by wsimpso1 View Post
    ...In restricting yourself to push-pull twins, you have skipped what I suspect is the single best configuration of a propellor driven twin, and that is the Boomerang, with a large and small fuselage, and set the wing and tail areas around the CG based upon optimizing for both single engine cases. Baggage can go in both fuselages...
    Amen to that. I've read both Rutan's design justification for the configuration and Raymer's commentaries on it, and it's really a clever concept. Almost no engine-out asymmetry at all. Supposedly you can put full-throttle on either engine with the other out and barely feel the difference. In fact, it's apparently hard to tell which engine is out without looking out the window for a stopped propeller.

    Quote Originally Posted by autoreply View Post
    Why does it need a driveshaft? Why not simply copy/pasta a long-ez aft section and put a fin/stabilizer just in front of it? I don't see why that shouldn't work?

    Two reasons:
    1. If you simply tack that 'power pack' on the tail end of a Do335-like airframe, the CG will be much too far aft. Probably about the trailing edge of the wing, as a matter of fact.
    2. If you move the powertrain (and propeller, and tails) forward enough to bring the CG into the right position on the wing (or, conversely, move the wing aft to achieve the same result), the airplane becomes extremely short-coupled. In fact, you may not be able to move the rear power-pack forward enough to accomplish this without ending up with what is, essentially, the Adam 500 fuselage pod configuration. At which point you'll need to move the tails aft, and that puts them on booms...
    The rear engine on the Do335 is right under that 'slot' (it's actually the exhaust stacks) cutting through the registration markings and national markings on the forward part of the aft fuselage. They did that for a reason, and the reason was CG position.

    Quote Originally Posted by wsimpso1 View Post
    ... and - besides that - it's not a push-pull airplane
    Ah, well if you simply want a push-pull twin, then there's nothing wrong with that. We each have our own tastes.

    A configuration that you've missed is the 'Scissor wing'. Rutan's SpaceShipOne was one-such airplane:





    and another was the (unbuilt) Blohm und Voss P.215 and it's day-fighter relatives:



    I think it's pretty easy to see how that configuration could be adapted to a 'short pod' push-pull twin. It's challenging from a stability and control design aspect, as well as structurally, but it meets your requirements.
    Last edited by Topaz; September 5th, 2009 at 06:34 PM.
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    Moderator autoreply's Avatar
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    Re: Ideal push-pull configuration?

    Regarding the hollow shaft, good idea, but the risks are comparable to a single-engine. If only one driveshaft or bearing fails you don't have any propulsion left.

    Quote Originally Posted by Topaz View Post
    Two reasons:
    1. If you simply tack that 'power pack' on the tail end of a Do335-like airframe, the CG will be much too far aft. Probably about the trailing edge of the wing, as a matter of fact.
    2. If you move the powertrain (and propeller, and tails) forward enough to bring the CG into the right position on the wing (or, conversely, move the wing aft to achieve the same result), the airplane becomes extremely short-coupled. In fact, you may not be able to move the rear power-pack forward enough to accomplish this without ending up with what is, essentially, the Adam 500 fuselage pod configuration. At which point you'll need to move the tails aft, and that puts them on booms...
    The rear engine on the Do335 is right under that 'slot' (it's actually the exhaust stacks) cutting through the registration markings and national markings on the forward part of the aft fuselage. They did that for a reason, and the reason was CG position.
    Well, the current BMW is "slightly" lighter. Let's assume the fuselage is symmetrical in the spanwise direction. Without pilot/pax that would place halfway fuselage around the quarter-chord. Let's say the pilot's shoulders are in line with the leading edge, so barely any moment. Landing gear will have a less or more neutral moment with the fin and stabilator adding a significant amount.

    You can then put the passenger behind the wing. True, visibility isn't great then, but regarding cg your problems are solved as far as I can see.
    Short-coupling shouldn't be a problem then. Moment arm to the tail is between 2.5 and 4 mac's on most experimentals. That's between 3 and 5m behind quarter chord, so about 2.2 and 4m behind the trailing edge. Seems somewhere in between (about 10") is achievable, "the egg" did it with about half of that
    Another option is to add a fixed canard (no "elevator"), just aft of the tractor engine. That does provide predictable lift while (because of the ordinary tail) deep stall is still impossible. It also allows the wing to stay aft.

    A configuration that you've missed is the 'Scissor wing'. Rutan's SpaceShipOne was one-such airplane:

    I think it's pretty easy to see how that configuration could be adapted to a 'short pod' push-pull twin. It's challenging from a stability and control design aspect, as well as structurally, but it meets your requirements.
    Thnx for the idea, that's a good one, definitely. My major concern is strength (it's a pretty complicated structure to analyse for flutter and torsion) and especially complexity since you still have all those controls going straight tru the center section, putting overboard about 50% of usable fuel and making the wing much more complicated and much heavier..

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    Registered User wsimpso1's Avatar
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    Re: Ideal push-pull configuration?

    Well, if it just has to be a push-pull and you don't like canards, go for it. I try to leave matters of taste alone.

    A tandem two seat twin. Cool. I want sketches and renderings!

    Topaz covered things pretty well, but I have more to add:

    CG will require the aft engine to be fairly forward, and if you attach the tail around the aft engine cowling, tail volume requirements will force a huge set of tailplanes. And while control and stability goes with tail area times arm, damping goes with tail area times arm squared, which means even bigger tailplanes to get it to settle down quickly enough after a control input.

    Next, on the Long EZ and all of its derivatives, the external shape around the engine is just a cowling. Your idea would require attaching a pretty substantial set of tailplanes to the something else in the engine bay. Heavy, complicated, expensive...

    Now if you drive the prop remotely, the tail arm gets much larger, the tailplanes get smaller, the forces from the tailplanes gets smaller, the wing does not have to lift so much, the aft prop gets straighter cleaner airflow. Better all the way. But you do have to deal with the vibratory issue.

    For simplicity, I would still design a Boomerang style twin...

    Billski

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    Re: Ideal push-pull configuration?

    Quote Originally Posted by Topaz View Post

    A configuration that you've missed is the 'Scissor wing'. Rutan's SpaceShipOne was one-such airplane:





    and another was the (unbuilt) Blohm und Voss P.215 and it's day-fighter relatives:



    I think it's pretty easy to see how that configuration could be adapted to a 'short pod' push-pull twin. It's challenging from a stability and control design aspect, as well as structurally, but it meets your requirements.

    yeah from a stability point of view, as rutan found out, the spaceship one type outboard tail can be very tricky. In normal flight this type of outboard tail will see an upwash from the wing as opposed to a typical tail that sees a downwash from the wing. This means it is more efficient and can be made smaller. However, when the main wing enters a stall the upwash would reduce and you end up with a pitch-up making the stall worse than a natural pitch-down in stall that you see in tails exposed to a downwash.

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    Re: Ideal push-pull configuration?

    Quote Originally Posted by wsimpso1 View Post
    Well, if it just has to be a push-pull and you don't like canards, go for it. I try to leave matters of taste alone.
    It's not so much that I don't like canards, but they seem less suitable to the requirements. Decent take-off length is always a problem with canards and here in Europe runways tend to be quite a bit shorter, not to mention the lack of flaps. CG-range with a tandem is another one though that isn't strictly limited to canards. The third drawback is that there are still controlls running tru the wing. I'd rather not have that. Thus the preference for ABC (anything but canards )

    A tandem two seat twin. Cool. I want sketches and renderings!
    Up till now I have only some single-engine tractor drawings and some sail yachts (catia). I do however have some basic guestimates about the cg issue.





    Do those seam unreasonable? Sorry for the metric stuff, but already had that. A total length of almost 8 meters is rather long, but this is showing the aft engine needs to be as aft as possible to counteract the pilot-weight.
    I realize the pilot and passenger are stuffed together reasonably close, but it's done is almost every biplace glider. Especially with a slightly wider fuselage that should fit just fine (feet under elbows of front pilot)

    Next, on the Long EZ and all of its derivatives, the external shape around the engine is just a cowling. Your idea would require attaching a pretty substantial set of tailplanes to the something else in the engine bay. Heavy, complicated, expensive...
    Well, not if you attach them to the fuselage in front of the engine as above is done. In that case it as complicated as a conventional tail.
    Check out this (http://i25.tinypic.com/2zp4l6a.jpg) picture of the Adam. Remove the heavy tail and the red 0.25 chord point will probably shift about 0.5 m to the front. That puts your wing already in the middle and leaves plenty length for a decent tail, especially with a narrower chord and the relatively long fuselage of a tandem or not?

    Tonight I will probably make a very rough drawing of the above mentioned configuration.
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    Last edited by autoreply; September 6th, 2009 at 06:38 AM. Reason: Included xls sheet.

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    Registered User wsimpso1's Avatar
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    Re: Ideal push-pull configuration?

    Yeah, you can shove the aft engine and tail way aft and shove the people and stuff forward to balance it. that leaves a space that you really can not use for anything but very light baggage behind the back seat. Ultimately you have to be happy with how it works out. You are going the right way by reviewing CG early. During the process, you will do yourself a big favor to keep adding to and updating this as a weight and CG management tool.

    Your weights look optimistic... I am building vacuum bagged fiberglass sandwich structures for my fuselage, and I am around 0.70 lb/ft^2 or about 3.5 kg/m^2 (wetted areas) for the skin, bulkheads, etc. I guessed at about 12 m^2 for the fuselage skin area, or 42 kg already, and you still don't have firewalls, other bulkheads, or the rollover and canopy structure. Then there are cowlings, or are those already in the with the engine weight?
    For wings, if you are building wet wings to put fuel in them I get the skins at about the same 3.5 kg/m^2 for the forward 75% (wetted areas) and about 4.0kg/m^2 (wing areas) for solid foam control surfaces. An 7.5 m^2 wing will be 42 kg with out any spars, ribs, or control systems. On many airplanes, spars alone weigh more than the rest of the wing... As I said, these are vacuum bagged weights, open wet layups will be 10-20% more. If you are building in aluminum, you can usually get a little lighter.

    Remember to add in the rest of the heavy things, like avionics, battery, seats/upholstery, landing gear. They matter.

    Billski

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    Re: Ideal push-pull configuration?

    Quote Originally Posted by wsimpso1 View Post
    A tandem two seat twin. Cool. I want sketches and renderings!
    Tandem seating or tandem (centerline) engines?

    There was a twin engine version of the Aeronca Champ with two Continental O-200's on the wings with tandem seating. It was reportedly a terrible plane with a single engine service sealing somewhere below sea level. Oddly, I think the rear cockpit had a control stick while the front cockpit had a yoke, and the throttles were on the cabin roof. The props were non-feathering fixed pitch.

    Bruce

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    Moderator autoreply's Avatar
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    Re: Ideal push-pull configuration?

    Quote Originally Posted by wsimpso1 View Post
    Yeah, you can shove the aft engine and tail way aft and shove the people and stuff forward to balance it. that leaves a space that you really can not use for anything but very light baggage behind the back seat. Ultimately you have to be happy with how it works out. You are going the right way by reviewing CG early. During the process, you will do yourself a big favor to keep adding to and updating this as a weight and CG management tool.
    Actually I'm finding results that are contrary to what people say over here. Over here is mentioned that the aft engine should be forward to keep the cg correct. That's feels correct and history proves them right. Running the numbers however indicate that the aft engine should be placed aft as much as possible to compensate both pilot and front engine weight. Though in my case pilot weight is way higher (compared to engine/structural weight), is this the sole reason?

    Your weights look optimistic... I am building vacuum bagged fiberglass sandwich structures for my fuselage, and I am around 0.70 lb/ft^2 or about 3.5 kg/m^2 (wetted areas) for the skin, bulkheads, etc. I guessed at about 12 m^2 for the fuselage skin area, or 42 kg already, and you still don't have firewalls, other bulkheads, or the rollover and canopy structure. Then there are cowlings, or are those already in the with the engine weight?
    For wings, if you are building wet wings to put fuel in them I get the skins at about the same 3.5 kg/m^2 for the forward 75% (wetted areas) and about 4.0kg/m^2 (wing areas) for solid foam control surfaces. An 7.5 m^2 wing will be 42 kg with out any spars, ribs, or control systems. On many airplanes, spars alone weigh more than the rest of the wing... As I said, these are vacuum bagged weights, open wet layups will be 10-20% more. If you are building in aluminum, you can usually get a little lighter.

    Remember to add in the rest of the heavy things, like avionics, battery, seats/upholstery, landing gear. They matter.

    Billski
    You're completely right. Weights of the wing/fuselage are complete BS. I think your remarks about weight (wetter or projected surface) are extremely usefull. In the spreadsheat I limited myself to cg issues only, assuming both wing and fuselage have their cg in the quarterchord point. Landing gear is included, battery too (just shift it like a glider when needed) though unfortunately enough I didn't include seats and avionics. Maybe it's a good idea to add another 15 kg to the pilot/pax weight, just for those.
    Regarding the numbers, I'm going to open another thread for initial weight estimation. First however there's the decision where to put the damn tail


    The question that's bothering me: you're all mentioning the aft engine should be way forward, driveshaft, otherwise short-coupled and so on. Looking at both history and my "feeling" about it this is correct. Calculating this however shows it isn't. What am I doing wrong or what's the difference? The single idea I have is the much higher pilot weight compared to engine/overall weight.

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