Twin-VW engine Push-Pull design idea (The "Beetlemaster")

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Vigilant1, Nov 16, 2014.

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  1. Nov 16, 2014 #1

    Vigilant1

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    [FONT=&amp]I've been intrigued for a long time about the idea of an inline (push-pull) VW-powered two-seat homebuilt. The layout would be like the Cessna Skymaster (aka Cessna Model 337, USAF O-2, etc) or its little brother, the Powers-Bashforth Minimaster.
    [/FONT][FONT=&amp]
    Pops mentioned in an earlier thread that he'd sketched out a VW inline twin at one point in the past, so that helped me think the idea might not be crazy.

    The goal would be to have twin-engine reliability (esp useful for operations over rough terrain, water, at night, etc--places where a forced landing would be especially hazardous). A "plus" would be relatively high cruise speeds from approx 140 HP continuous (or over 70 HP at 10K' MSL for some respectable TAS cruise speeds). The excess power could also be used for climb, too, but not for critical situations where the climb is needed for obstacle avoidance (because the climb rate won't be there if either engine quits--so it would have twice the risk of "critical engine failure" as a single engine airplane).

    The main challenge would be achieving safe single engine performance.

    Some back of the envelope estimates of weights/dimensions:
    Empty weight: An empty Sonex weighs about 650 lbs, an empty Thatcher CX5 weighs about 721 lbs. Add 180 lbs for a second VW engine, 10 lbs for engine mount, 10 lbs for another prop, maybe 15 additional pounds for the dual tail booms and tail (vs conventional empennage), 30 lbs for more wing (see below) = approx 895 to 966 lbs empty. Say 930 lbs.

    Max gross weight (what we'd need to have some utility): 930 lbs (empty weight) + 400 lbs (two people) + 50 lbs (baggage) + 150 lbs fuel (ideally tanks large enough for 300 lbs fuel in for solo flight) = 1520 lbs For comparison, that's 150 lbs less than a C-152, but with about 40 more HP (both engines) and more wing (see below).

    Wingspan: It's no trick to get a good rate of climb with an 80 HP engine, even if the plane is relatively heavy--it just takes a lot of wing (especially, span). A fully loaded Dimona H36 motorglider weighs 1698 lbs and the single 80 HP Limbach gives a climb of 532 ft/min: But it has a wingspan of 52 feet and a wing area of 154 sq ft. A little closer to "conventional" airplanes, the Thatcher CX5 (80-85 HP VW) claims a 1000 FPM rate of climb at 1320 lbs and with a wingspan of a wingspan of 28' (area of 126 sq ft). So, if, for emergencies, we want to have approx 300 FPM ROC on a single engine at our 1520 LB target max TO weight (approx midway between the Dimona and the CX5) it looks like we'd need something like a 35-40' wingspan a wing area of about 135-140 sq ft. So, about the span of a C-172, but less wing chord.

    Layout: A tandem seating arrangement has a lot to recommend it for this concept. There would be less "blanking" of the rear prop by the fuselage. Also, if the "mission" of this plane is to use that second engine to generate some impressive cruise speed numbers, then minimizing frontal area is a big plus. On the other hand, if the plane is just to be used to have get two people over inhospitable terrain with a bit more safety, then a side-by-side configuration mights work (with careful attention to the shape of the rear fuselage to help get air to that rear prop). High wing/low wing--probably not much difference (though gravity feed from a high wing is a very reliable thing, would work great here).[/FONT]

    [FONT=&amp]Other:
    Props: Controllable pitch props would be very handy in order to facilitate safe single-engine operation (fine pitch, low airspeed, let the engine go to 3400 RPM to get all the power available) as well as high cruise speeds at reduced RPMs in normal operations. The Ivoprop adjustable pitch units look "reasonably priced" (i.e. about 1/3rd as much as each engine), but I don't know anything about their reliability. The Hoffman props used on the Grobs and Dimonas would be ideal, since they can be feathered (which would significantly reduce the drag from a dead engine), but I'll bet they are very expensive. The Revmaster R2300 engines, with that big front bearing, can take props weighing up to 22 lbs (IIRC), and are even still plumbed to provide oil pressure for a controllable-pitch prop.
    [/FONT]
    [FONT=&amp]I wonder how a person, having built such an airplane, could economically get checked out for twin-engine, centerline thrust.

    Performance: I wonder what cruise speeds of such a pane might be. If it could be kept aerodynamically "clean", the 140HP (or so) available at SL might provide quite a push, but all the wing needed for acceptable SE climb might slow her down a bit (but, it should fairly leap off the ground with both props turning!)[/FONT]

    [FONT=&amp]Thoughts, nasty comments, corrections are solicited. Crazy idea after all??[/FONT]

    Mark
     
  2. Nov 16, 2014 #2

    akwrencher

    akwrencher

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    Sounds cool, sounds expensive (two engines....), sounds like if done right it would climb like a homesick angel :)
     
  3. Nov 16, 2014 #3

    Cabover

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    I’ve been thinking much the same of late! Great minds run in small circles!

    My thoughts were more in the way of a conventional looking Cub in the front of the rear door post and a Woody pusher behind the door post. The Double Ender with the front engine high might solve a few CG problems but initially I was thinking of keeping the rear prop more out of the front wash and the cooling air and exhaust would tend to have less interference problems with the front engine mounted in front of the pilot rather than above.

    I gave some thought to the lift and wing area. Yes, that is a problem. Here are my thoughts (No slide rule yet applied). Make the wing longer and a bit thinner than you otherwise might. Maybe 10% thick or, perhaps, one a bit closer to symmetrical rather than the USA35B that I tend to like. Use a leading edge device. There are a number too choose from (This discussion is always like walking thru a refinery with a lit match!). Fixed perhaps or leading edge flaps like those on the Butter Cup. These would get CL up a little and the drag as well which would kill the speed, but speed is not the point after losing an engine but simply maintaining altitude and limping off to whatever safe landing can be arranged.

    VW’s have a little problem with heat rejection at continuous high output. A supply of distilled water so the heads can be spray cooled for a reasonable time to get to a low power glide to a landing. Designer’s choice as to how much of the GW to devote to "Emergency cooling". Pilot’s choice on when to carry it.

    Rather than the VW’s I was thinking of Subaru’s, a pair of the smaller ones. Out of production but seem to be quite a few around. They tend to stand the high outputs for a bit longer but after too much time at high output I might want to have a look at the valves. Still, that would be a cheap recovery from an engine out where extreme measures to get down safely were required.

    A parting thought. If you can get a dead engine to stop the prop you can reduce the drag a lot. Not as much as a feathered prop but the drag goes WAY down!

    Cabover
     
    Last edited: Nov 16, 2014
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  4. Nov 16, 2014 #4

    cluttonfred

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    I, too, have kicked around ideas for a VW-powered twin, including inline engine arrangements. Some good reasons to do this would be:

    --Regular travel over water or inhospitable terrain (IF single engine performance will allow an acceptable rate of climb, otherwise why bother?)
    --Because you want to obtain or remain current in a multiengine license
    --Cool factor (always justifiable in my book)

    I don't think top speed or two-engine climb rate make a lot of sense as arguments--for the price of the twin-engine installation, you could go with a single-engine installation at the same or a little less total horsepower that would still give the same performance.

    Cabover is absolutely right about the prop--for this to be affordable, I'd stick with fixed-pitch (maybe ground adjustable) props, but some sort of manual prop brake would be a big help for single-engine operations. Maybe a little cable-actuated arm with a brake pad on it that can be pulled tight and locked against the back of each prop hub if needed?

    If I were tackling a twin, regardless of engine installation, I'd design for the worst possible engine-out situation right from the beginning. Let's say you are running two 80 hp big bore VW conversions. In the Skymaster, the single-engine performance is better with the front engine out, rear engine running than the reverse. So rear engine out is your worst case scenario. Your twin needs to be designed to have positive climb, say 400-500 fpm at sea level, at gross weight and with the drag of the stopped prop on the front engine only and at the maximum continuous power of about 76 hp. That means a very light airframe and a big wing.

    I think this is very doable, but the temptation to treat your design as a 160 hp airplane is dangerous. You really need to design it as a somewhat draggy (because of the stopped prop), overweight (because of the second engine) 76 hp airplane and then make sure your big wing an all the rest can handle the strain of 160 horses when they are all galloping along. Cabover is on the right track with Cub-type wing area in mind, but I'd want a much cleaner aerodynamic package than he describes. The result, if done right, would be akwrencher's homesick angel for take-off and climb with modest top and cruising speeds but with decent single engine performance.

    PS--Not a push-me-pull-you type, but I have always thought that the wonderfully retro Grumman XF5F Skyrocket would make a great inspiration for a homebuilt light twin. The engines would balance the crew and rear fuselage weight, fuel could be placed in the inboard leading edge, the H-tail twin rudders would provide good low-speed and engine out rudder authority and you could place the engines very close together to minimize critical engine issues even without contrarotating props. And talk about cool factor! ;-)

    606px-The_latest_type_of_a_Grumman_Navy_fighter_-_NARA_-_195921.jpg Grumman_XF5F-1_Skyrocket.jpg
     
    Last edited: Nov 16, 2014
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  5. Nov 16, 2014 #5

    autoreply

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    We had a similar topic before, not that I can trace it back though.

    Some thoughts.

    Older motorgliders are heavy, not the least because they use glass. Carbon is way lighter.

    About 100 hp/tonne seems about the lower treshold for both conventional twins (N-1) and motorgliders. With push-pull you could go even lower. That yields about an 750-800 kg mtow (1750 lbs). No need not to use that fully.

    If done well, you can build very light. With attention to detail, 400-ish kg is well do-able (900 lbs). That gives you a payload close to 400 kg/900 lbs. Not that you HAVE to use all that, but it screams for 4 seats (or a 2+2 with massive luggage space). Note that a bigger cabin weights very little and only hurts cruise (a bit).


    Note that while not in concept, but in numbers the Dyn'Aero Twin-R is pretty close:
    Dyn'Aéro Twin-R - Wikipedia, the free encyclopedia
     
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  6. Nov 16, 2014 #6

    dino

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    Last edited: Nov 16, 2014
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  7. Nov 16, 2014 #7

    skier

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    At least in the US, legally, this wouldn't be required. You just get your MEL in any old twin and that covers centerline thrust aircraft. If you were to get your MEL in a 337 (or similar) you would end up with a centerline thrust restriction on the MEL and would need more training to have that restriction removed.
     
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  8. Nov 16, 2014 #8

    autoreply

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    And this one is handy for a comparison of some real parameters for other light twins:

    Twin-R
     
  9. Nov 16, 2014 #9

    Pops

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    You have been thinking this out. I was thinking about 34' wing span with 137 sq' for wing area. Empty weight of 900 lbs max, and lighter if I can by trying to save every ounce. You have a VW engine mount at 10 lbs, my VW engine mount weighs 3 lbs. I would run fixed pitch wood props, about 5 lbs each. Fuel load-- 20 gal for each engine for a total of 40 gal.@ 240 lbs.
    EW--- 900 lbs
    fuel ---240 lbs
    crew---450 lbs
    baggage--- 60 lbs
    --------------------
    Gross Weight--- 1650 lbs.

    Wing area --- 137 sq'
    Wing span--- 34'
    Wing loading @ gross-- 12.04
    Power loading @ gross---10.31 lbs per HP. (I love it)
    Power loading single place ( 225 lb pilot with baggage) --- 8.91 lbs per HP. ( I'm in love).

    Two place tandem with retract gear. Main gear leg will retract in side of fuselage and gear leg will bend and form the leading edge of the rear engine air intake and then another bend so the wheel will go into the bottom of the wing. Passenger setting under the CG of aircraft.

    Two Revmaster 2300, 80 HP engines.

    Type of construction?

    Dan



    Added--- To have a 900 lb or less EW you will have to watch every ounce and design everything for light weight. Carbon fiber cowls, Doors, faring, etc.

    This is close to my Falconar F-12 that I built in 1980. EW-937 lb, Wing span-- 27' 6", Power Modified Lyc- 290 putting out about 165 HP, Cruise 150 mph with a ROC of 1700 fpm at gross weight. With just me and 1/2 fuel-- 2200 fpm.
     
    Last edited: Nov 16, 2014
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  10. Nov 16, 2014 #10

    cluttonfred

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    I like the concept, Dan, though I would question the need for retractable gear in this application. With such low power available, you really need to have a plane designed for that worst-case scenario, which means it needs to be SLOWER than an equivalent 160 hp single. I really hope no one is thinking of a VW twin as a real IFR machine, rather a simple VFR machine for confidence over rough terrain and water. With that in mind, fixed gear and bare minimum instruments would make a lot more sense to shed all the weight possible.
     
  11. Nov 16, 2014 #11

    Himat

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  12. Nov 16, 2014 #12

    Vigilant1

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    It should climb well if both engines are humming—just a by-product of trying to get acceptrable single-engine climb. Two engines do add to the expense. Still, a new R2300 Revmaster is about $8400 assembled and test run by the factory, not bad from a $$/HP perspective.

    I'm not quite ready to give up on the "speed" argument yet.:) I'll concede the "great climb" rationale is not very important: You'd need to be able to operate safely with the single-engine climb available, but there are certain (admittedly fringe) applications where a high climb rate offers only efficiency/convenience, and this is where the good two-engine climb capabilities would be a plus. Examples include the ability to climb straight ahead and over that mountain/Class C airspace rather than go around/fly a climbing circle; getting up to pattern altitude faster for doing "circuit work", meeting noise restrictions on some departures ("remain on runway heading until XXXX MSL before making turn to XX deg"). But, there are many "regular" flying situations when having the ability to climb at a healthy rate is a safety feature in itself, and a nice by-product of this wing and the excess power available. As far as cost: a new (non-certified) O-360 costs about $27K, or 150 $/HP. Brand-new VW derivatives are about $100/HP, a used one is a lot less. Now, this isn't an entirely fair comparison (as the residual value of the Lyc is a lot higher), but the new VW-derived engines are economical to purchase. The real eye-opener is the difference in the price of replacement parts. So, even if the dual-engine redundancy >isn't< part of the equation, a twin VW can still make sense if the available HP over 75HP can be "nice to have" rather than "gotta have".

    The prop-brake could probably be implemented with very little trouble. One idea--IIRC, folks flying VW s and using PSRUs don't see the prop windmill when the engine dies (maybe given the ratio, there's not enough torque available from the prop at typical airspeeds to overcome the engine compression.) Just throwing the idea out there--a PSRU might accomplish our "prop brake" function and also allow us to turn a larger prop=good low-speed thrust, which might be handy in an engine-out situation. As a bonus, a slighty longer prop reduces some of the inefficiencies caused by the fuselage in a push-pull layout. Disadvantages: More weight, complexity, cost.

    Of course, this is just a thought experiment and it is subjective, but I'd be satisfied with single-engine climb performance of about 250 fpm (I recall I’d often see about 300 FPM in a heavy C-152 on warm days, and that was normal operations!). I found this to be interesting: the FAA sets >no< minimum SE climb gradient for certified twins weighing 6000lbs or less. It’s “legal” if they can’t even maintain altitude on one engine. For those more than 6000 lbs and registered after 1991, the minimum climb gradient is 1.5%. For a plane flying at 75 knots, that’s about 112 fpm. I’m not saying that is safe, but it is legal.
    I agree 100%. But I’m hoping, in keeping with all the above, that it might be possible to use the available HP when both engines are turning for good speed/efficiency. The wing is where this could be troublesome: The high lift needed for acceptable SE ops are a bit in conflict with a high twin-engine cruising speed. Some work-arounds:
    - Go high: Use that wing and the available power to get up where the air is thinner. The low fuel burn will give longer range between fuel stops and the higher TAS at altitude (at 12,000’ MSL, 125 KCAS = 150 KTAS—If we can get there, those are Cirrus SR22-like numbers).

    - Reflex the ailerons (flaps?) in cruise to reduce drag. This would seem to be a bit of a “kludge”, but it might work.
    - Choose an airfoil/wing planform optimized for cruise and then use high-lift devices, etc to get acceptable SE performance. I think this is where Cabover is headed with the LE devices, etc. I’m not an expert, but many of these I’m familiar with add a lot of drag for each increment of lift. Like in a typical GA aircraft: Even the first notch of flaps seldom increases your gliding range, and anything beyond that definitely doesn’t. So, for SE operations any "tricks" would need to offer a lot of cL gain with minimal added drag.


    Thanks for the input—keep it coming!
     
  13. Nov 16, 2014 #13

    Vigilant1

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    My thoughts, also without any benefit of actual numbers: The thin wing in a long span like this is probably going to call for a lift strut (or some very beefy spar caps). That will add some drag, which will only be a problem if we also want high cruise speeds. I am usually surprised how little additional drag is added by thickening the wing at our airspeeds. And, it's a good place to put fuel. Again, crunching the numbers would answer this.

    True, if they are pushed too hard, but all is well if the limits are respected and cooling air/baffling is given some thought. The cooling air supply to that year engine is will need to be well thought out: a scoop admitting slipstream air from the front engine would work fine for normal ops, but we need to plan for SE, slow, rear-engine only flight. Something requiring no pilot-actuated "special measures" would be ideal.

    Thanks.
     
  14. Nov 16, 2014 #14

    Vigilant1

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    Well, looks like my wingspan/area estimates aren't much off from yours. Your allowable passenger/cargo/fuel weights (750 lb total) would make for much more utility than my max useful load of 600 lbs.
    So, with your MTOW and wing, what did you figure that single-engine climb performance would look like? Guess as to cruise with both engines?

    As you point out (and as autoreply's Twin-R link shows), empty weight is very important. And if we want to go fast with the extra power, drag reduction is key. Composite wing (CF) with a metal fuselage as a compromise for ease-of-build?

    Maybe there are other ways to save weight and costs with two engines. Do I need a generator on each engine if I have one and a battery? Do I require redundant ignition systems on a twin-engined aircraft (not much weight, but saves money).
     
  15. Nov 16, 2014 #15

    Vigilant1

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    Leaving the retracts aside for the moment, this layout might be a very good platform for IFR work. Modern instruments don't weigh much at all, and there's a lot to be said, at least in the recip world, of the value of having a second engine when actually operating in IMC.

    I would worry about the weight of the retracts. But, hey, keep the adjustable props, and add the retracts and this beastie can be used to log "complex" time, multi-engine time, etc.
     
  16. Nov 16, 2014 #16

    Vigilant1

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    I like the "just" in there. Cha -ching! :)
    Or, get the MEL in the "Beetlemaster" after it was built?
     
  17. Nov 16, 2014 #17

    Vigilant1

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    So, Dan's 1650# MTOW and my proposed 1520# look acceptable, if other things are done right.

    I'm guessing that getting to that 900 lb empty weight will be the trick, at least if we want to do it at home (no autoclaves, etc), it might require "Colomban-like" construction techniques (lots of tiny pieces and goop). Maybe that's what it takes.

    I think the most practical approach to choosing a MTOW will likely be: engineer enough strength for the highest likely allowable weight based on SE climb ability (e.g. maybe 1800 lbs total), then build the plane and de-rate the MTOW as required based on actual observed SE climb performance. If that ends up being a 1700 lb MTOW, that's okay--with an empty weight of anything under 1000 lbs, that's still a very useful 2 place airplane--with some extra strength. Keep the empty weight lower and it can carry more.
    Thanks!
     
    Last edited: Nov 16, 2014
  18. Nov 16, 2014 #18

    autoreply

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    Climb speed at sea level (and 15C) is useless. At hot&high is where it matters. Personally, for such a config; 250 fpm @ 10000 ft would be the goal.

    That's where longer wings really help.
     
  19. Nov 16, 2014 #19

    Vigilant1

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    At 10,000' MSL and 0F OAT, a perfectly functioning C-172 at max gross wt climbs at just 285 fpm according to the factory (wink, wink). So, your goal seems good under those conditions with both engines turning. Single engine (= about 50 HP available at that altitude), it is probably unobtainable unless weights are kept very low and the wing very long.
     
  20. Nov 16, 2014 #20

    Cabover

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    As long as we are discussing twins, how about carry it just a little further?

    I like Cubs as a starting point but take off from there to about any layout of your favorites. I rather like the Woody’s pusher/CUB (absent the turtle deck structure) tail but a twin boom/Mix Master will work for this.

    I have been given twin cylinder engines over the years. Some got scrapped, some got used on other projects. Some are quite light, others boat anchors. The original Quickie used one with a little power improvement from about 18 to about 22HP, if memory serves. There is a new crop of V-twins. Kohler has one or two in the lineup that don’t look too bad. A little work on the exhaust and maybe cam and what might happen to power?

    One of the Kohler’s comes in at about 35hp if memory serves. So make it a 4 engine! 2 in the push pull locations, one each in nacelles with the boom trailing out the rear of the fairing. Worst case 25% loss of power will not create the yaw problem it does in a twin and the arm is never as long as a conventional 4 engine layout loosing an outboard engine.

    The other approach would be to use a simple centrifugal clutch and two engines drive one prop shaft(PSRU). A simple chain drive or cog belt drive and most of the parts are off the shelf. All four engines are then CL thrust, two front and two back. Granted, one engine driving the prop built for two will not spin up like it should but that’s still something around 60% or 70% of full power. That beats 50%. And the engines are inexpensive(free to me often) and in current production. The parts are reasonably priced, unlike fire breathing LiCon parts which are all electroplated with 100% pure unobtanium. So, ‘nuff power ta git ya home?

    Cabover
     
    Last edited: Nov 17, 2014

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