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 30, 2014 #121

    Vigilant1

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    Yep, if designed to be "clean", I think good economy cruise fuel burn and climb as good/better than a fully-loaded (1750 Lbs) RV-9 with an O-320 is reasonable. But I don't think the Beetlemaster will match it's cruise speed if we use fixed pitch props. That RV-9 that goes 188 MPH at 8000 ft (75% power= 160 HP x .7 = 112 HP ) using a fixed-pitch prop has a prop pitched for high speed. Unlike the Beetlemaster (which must be able to create a lot of low-speed thrust from both engines to be able to climb at best climb speed when running on just 80 HP), the RV designer can assume all the engine power is available all the time, and so the prop can have a fairly "coarse" pitch and still produce acceptable thrust for TO and climb (because they have 160 HP to throw at the problem, efficiency isn't critical).

    Anyway, maybe there's a way to pick a FP prop that will produce acceptable low-speed thrust and enough high-speed thrust to still allow high cruise speeds. The Prince P-Tip props are popular among the Sonex set--they've proven reliable and people have seen improvements in both climb and cruise. They are designed to flex a bit, and vary their "bite" according to the prop loading. Price and weight is about like a Sensenich wood prop. I regarded it as hype at first, but now I think there is something to it.

    Another thught: If compound curves are required (with or without built-in CF structural members), then the molds are complex and a centralized production facility (making kits) makes a lot of sense. If 90% of the work can be 2D curves (wings, fuselage sides, fuselage bottom, Booms, tailfeathers, even gear leg fairings), and the compound curves were limited to the "usual suspects" (front cowl, rear cowl, wing and tail tips, wheel pants), then the kits can be a lot smaller and the ability to homebuild the panels (with built-in CF framing members) becomes more feasible. Of course, these 2D parts could still be made available as a kit. One attraction (meantioned in the "21st Century Volksplane" thread by Matt/Cluttonfred) is that a plane that >looks< easy to build gets the attention of a lot of potential builders for that reason alone. "Even I could build that!"

    I suppose molds for 3D parts could be rented out, with a suitable deposit. If a builder had already built his 2D parts, maybe he'd want to try this, too. Buying finished ones would be alot more practical, but there's no accounting for whta people might want to do (heck--building an airplane at all makes very little sense if we're only looking at the economics--well, except the Beetlemaster. You really >can't< buy a twin for the price this airplane might be built for).

    If mass appeal is a goal, then having an affordable pathway to a suitable twin engine rating will also be attractive. There's no economical way to do that today. Could a company formed to sell the Beetlemaster kits offer that multi-engine (centerline thrust) rating/Beetlemaster transition training in the company Beetlemaster? Transition training in E-AB aircraft is approved (and encouraged) by the FAA, big kit makers do it. But that's not a new rating.

    Hadn't thought of that.
    Going with Matt's eariler idea, I wonder if a simple/light cable-operated disc brake like on mountain bikes would be enough. Squeeze it in the cockpit, the handle ratchets as pressure is applied. If you want to give the engine a re-try, release the ratchet.
     
  2. Nov 30, 2014 #122

    autoreply

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    In the end, you might find that there's no reasonable way to get around variable-pitch props. It does add considerable cost, but even for a slow airframe (100 kts max cruise), I doubt FP props will give acceptable single-engine performance.
     
  3. Nov 30, 2014 #123

    cluttonfred

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    I am not sure that I agree. If the FP props are optimized to redline at climb speed (meaning you must throttle back in level flight) and you have some sort of brake to stop a dead prop, the cost and weight benefits might justify FP.

     
  4. Nov 30, 2014 #124

    autoreply

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    At the speeds discussed, your prop would be at redline in cruise without any engine power (nor running against compression), let alone higher speeds. Even for something overpowered like a Van's RV8, performance with a fixed pitch prop is acceptable at best. Save an Aircam (which is incredibly overpowered), pretty much all twins have (and need) variable-pitch props.
     
  5. Nov 30, 2014 #125

    Pops

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    I love everyone adding their ideas. Two or more heads are better than one :)

    I think company supplied rating at a low cost would be the way to go. Very easy to learn and not much time involved.



    Dan
     
  6. Nov 30, 2014 #126

    Vigilant1

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    Yes, I think this is the way it would have to be if using FP. Both engines with props more closely optimized for climb (= lower airspeed, fine pitch) in order to assure adequate SE performance. At cruise, they'd be very lightly loaded and hit redline. Despite the RPMs, at least they wouldn't use much fuel.

    Lonnie Prince claims his FP P-Tip props vary in pitch about 4 " in pitch between climb and cruise. That's not whole lot (about 12 MPH change in the prop's target pitch at 3200 RPM), but it is something, and it comes without any complexity or weight.

    I thought there were some props with auto-pitch changing based on loading?
     
  7. Nov 30, 2014 #127

    autoreply

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    Careful with opinions. Aeronautical engineering doesn't care much about them.

    Try it for yourself; take a plane with a variable pitch prop, put it so fine that you can reach redline on the ground, take off and try until which speed you can go in level flight, adjusting the engine to not exceed redline. Don't be surprised if you won't be able to surpass 100 mph. There's a reason all twins and most not-terribly-slow singles have variable-pitch props....
     
  8. Nov 30, 2014 #128

    Pops

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    My daughter used to have a 1963, Lyc- 360 powered Cessna 172 with a fixed pitch prop ( 76" X 60"). A friend of mine had a 1966, Lyc-360 powered Cessna 172 with a variable pitch prop . I have flight time in both aircraft. On take off length , the Variable pitch prop C-172 would not beat the fixed pitch prop over a few feet. Maybe 2 aircraft lengths. ROC of climb for the variable pitch prop C-172 was maybe 25-50 fpm better. Cruise speed at 75%, the fixed pitch C-172 was about 2 mph slower. Both aircraft with full fuel tanks and similar equipped radios.

    Dan
     
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  9. Nov 30, 2014 #129

    Jan Carlsson

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    Jarno is right here, can't have a redlined climb prop to perform at high speed. when the speed is up and red line, the alpha on the blades goes to zero and no thrust is performed. on the other hand a higher pitched prop at take off will have a higher alpha and will make a lot of thrust.
    even a IF1 is at 80% rpm and power, static or take off.
    more normal planes 85-95% rpm.
    Engine power isn't every thing but thrust power is. So it might be better to go to a bigger prop and lower rpm.

    I have not run any numbers in this project.

    But one thing come to mind, it have to be a balance in cost between engines and fuselage, cheep engines and expensive airframe, contra more expensive engines and cheaper design airframe. it would also be a good idea to keep the design open for other engines, like Rotax 100 HP or turbo 115 HP.

    CF spar and wood wings will probably keep weight and cost down.
     
  10. Nov 30, 2014 #130

    Vigilant1

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    We're saying much the same thing. For our "critical case" (SE climb) the FP prop would need to be optimized for that flight condition (= Best SE climb speed for that airframe--probably approx 70 knots based on aircraft of similar wing/span loading). With that done, the prop will not be useful (because it will be underpitched) at significantly higher speeds. It will run out of AoA. I'm not sure setting the prop to reach redline at zero airspeed is of great relevance, but setting to redline at best SE climb airspeed might be.

    Regards the opinions (estimates, "probably" etc): When I'm making an estimate, I try to make it clear that's what it is. We're all making estimates. Until we get an HBA wind tunnel, we'll need to use them, and some reasoning and extensions from known situations. If the estimates are based on an understanding of the basic principles (aerodynamic, structural, etc) and on performance of analogous aircraft/situations and some formuli when they apply, that's the best situation.
     
  11. Nov 30, 2014 #131

    autoreply

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    Not really. Any prop optimized for good climb will severely limit any decent cruise. It's not for nothing that all twins have moved to variable pitch props.
    The difference is that with "opinions", experience really counts. I'm rarely far off (hard, measurable data points), while some "opinions" on this thread couldn't be further from the truth/reality.
    I hate to invoke that card, because it's a call on authority, but lacking a good understanding of those basic principles, many opinions are without ground and completely incorrect.
    Treating that in the simple democratic fashion, the experienced - factually correct - will always loose out, because providing the exhaustive reasoning and calculations behind those arguments is too time-consuming.
     
  12. Nov 30, 2014 #132

    Vigilant1

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    I don't think I'm communicating well. You can have an "opinion"/assessment/conclusion different from mine, but you'll have to pick a different issue, because we agree on this.:) I've (tried) to say the same thing.

    And from the OP:
     
  13. Nov 30, 2014 #133

    Pops

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    I made that last post to illustrate that a higher drag airframe will make less of a difference with a Variable Pitch prop than a lower drag airframe. If the BeetleMaster is not designed with a clean airframe, there will not be a lot of difference between the FP and Variable pitch prop, then a lighter FP prop might be the best choice.

    Dan
     
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  14. Dec 1, 2014 #134

    BBerson

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    I can't think of any successful Beetle powered pusher. Rutan gave up on the VW pusher.
    Might cool better with a fan. The rear engine on a Cessna 337 that I looked at a while back had a fan and all sorts of scoops, VG's. Must be harder to keep that rear engine cool.
    The VW crank isn't designed for any prop extension, needed on the rear engine for a smoother cowl.

    The twin engine push/pull configuration is higher drag than a single of same power, as I see it.
     
  15. Dec 1, 2014 #135

    Pops

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    I think you can use a prop extension on the Revmaster engine.

    Dan
     
  16. Dec 1, 2014 #136

    StarJar

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    May I politely point out that the Rutan Boomerang does not even have a Vmc speed, becuase the loss of an engine is such a non issue. Thats why i suggested it early on. It will fly circles around a Baron, and has a 12 hour range.
    If you plug all your numbers into it, it may work best of all. It's really no harder to design.
     
  17. Dec 1, 2014 #137

    Vigilant1

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    Keeping the rear engine cool will be a bigger challenge when the slipstream form the front engine isn't pushing air through the scoops. If that's the >only< time it is a problem, then it would be important to know if it was enough to cause engine problems short-term (burn/swallow a valve), or only if run hot like that for many hours (valve seat erosion, etc)
    A fan might be needed, maybe ejectors/augmenters as some have fitted to VariEzes/Cozy's to use the exhaust effluent to pull air through the engine.
    The Revmaster has some features that may be useful:
    -- The very large front bearing and their crank make a prop extension feasible. The engine is approved for a prop extension, and they make one. I don't know if it can be used with an (electric) VP prop:

    [​IMG]



    - The cooling fins on the Revaster heads are beefier than those on a stock VW head, and the walls thicker to help move the heat from the exhaust port. So, that may belp somewhat with the rear engine--but there still needs to be a way to move the air through. At least, unlike a pusher, the plane won't be taxiing for minutes on the rear engine or taking off from brake release (zero V) on the rear engine alone. There should always be at least airspeed equal to Vs feeding it.

    - The Revmaster is plumbed to accept an oil-controlled VP prop. I don't know who makes one for a VW.
     
    Last edited: Dec 1, 2014
  18. Dec 1, 2014 #138

    Vigilant1

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    I was with you right until the "no harder to design" part. The forward-swept wing (and the potential twist/AoA increase at the tip that comes with it) would be a handful. Anyway, that's what experimental aviation is all about, maybe I'm all wet. And, though it makes no difference and is entirely subjective, I think it's horrendous to behold from any angle.
     
  19. Dec 1, 2014 #139

    BBerson

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    Well... with all that, the Revmaster engine isn't really a VW.
    Does the name Beetlemaster still apply?

    Maybe if Revmaster powered it should have a different name. Let's see... not Skymaster, not Beetlemaster, how about ..
    Revmaster.:gig:
     
  20. Dec 1, 2014 #140

    StarJar

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    OK, but I forgot to say the wing doesn't have to be bent. It can be perfectly straight. Is it any less ugly then? Maybe it would only be ugly for the first few flights.
     

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