Single engine 300-400hp pusher

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KC135DELTA

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The cirrus VK-30 flew with a drive-shaft also.



It can also be noted that it was capable of 260knots on naturally aspirated 300hp :shock:
 

autoreply

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It can also be noted that it was capable of 260knots on naturally aspirated 300hp :shock:
Do you have a source for that speed? According to this article it gets about 240 mph (208 kts) which seems about right for a non-turbo-ed aircraft with 300hp.

As for the driveshafts, the aircraft more than occasionally flown with it (BD-5, Mini-imp, Stemme, P-39) none of them use an engine, bigger than about 100hp, except for the P-39 of course. Would that be the practical limit to keep the problems solvable in experimental aviation?
 

KC135DELTA

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I think it is 240mph cruise and 260mph max not knts. Still very very fast. Anyhow,I was informed the resonance is much easier to control on a CS prop, which would make sense on an application like this and make sense as to why every vk-30 has one.

Also it can be said the VK-30 is roughly the size I have in mind, just optimized.
 

wsimpso1

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P-39, P-61, Micro-Imp, Cirrus VK-30, Lesher Teal, Sierra Sue.

Oops, only the Sierra Sue is new.
 

lr27

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If I remember correctly, the Taylor Aircar was a certified aircraft and had a long driveshaft. I also recall it had some kind of vibration damper device in the shaft involving metal balls that spun out to the outside of a disc shaped container. Or something like that.

I'm probably missing something, but if you have a long driveshaft can't you just make it compliant enough so that the resonant rpm is passed very quickly on startup? How about a torque converter, a la automotive automatic transmission?

Seems like for quiet you'd want a pretty significant distance between the fins and the prop.

Cirrus has pretty long gear, doesn't it?

I wonder how they handled the drivetrain on the Vmax Probe? It ended tragically, but it seems like they did a lot of homework. I'm guessing that it shows the minimum distance from tail to prop that would have low drag, since they had Bruce Carmichael consulting on the project. (Assuming they followed his advice.)
VmaxProbe
 

wsimpso1

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Molt Taylor did the MicroImp, MiniImp, and the Aircar the same way, using a Dodge Flexidyne, which is a maintenance headache. Much written about elsewhere. Not a great solution...

Billski
 

bmcj

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If I remember correctly, the Taylor Aircar was a certified aircraft and had a long driveshaft. I also recall it had some kind of vibration damper device in the shaft involving metal balls that spun out to the outside of a disc shaped container. Or something like that.
You are referring to a Flexidyne, or "dry fluid" coupling.

Bruce :)

EDIT: Oops, Billski beat me to the punch.
 

lr27

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Well, what about the compliant shaft solution then? What if resonant frequency is lower than idle?
 

Dan Thomas

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Well, what about the compliant shaft solution then? What if resonant frequency is lower than idle?
Any shaft as soft as that is going to wind up like a rubber band when under load, and then, probably, suffer a spiral fracture.

Dan
 

wsimpso1

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Actually, if you can make the shaft strong enough to carry torque while being torsionally soft enough to get resonant frequency below idle, you have to watch out for shaft critical speed being within the operating range. This is the speed where the frequency of the shaft's bending modes are within the speed range of the system, and the shaft flails. This is why helo tail rotor shafts have many support bearings along their length...

The whole issue of shaft drive is one that has to be done correctly. You have to do torsional modeling of the systems, iterate the design it in model form, drive your torsional modes out of the operating mode by adjusting shaft stiffness and inertias, support the shaft in enough places, apply appropriate tuned absorbers or order absorbers, and so on. It has the capability to turn into a tour de force of engineering design and test effort.

It is done, but it is not simple nor straightforward. Helos and tiltrotors and turboprops all fly, and they have dealt with all of these issues.

Billski
 

KC135DELTA

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Keep in mind as it stands now the shaft will only be 50-70" long. The engine choice and cg will have to be dealt with prior to the shaft.

On another subject. What are the qualms on running a really large turbo or two sequential turbos in order to get a 35,000ft~ critical altitude? Also for the sake of simplicity could the cabin be pressurized off this? I seem to remember a guy building a turbo ls1 setup with a critical altitude of 30,000ft that did this.
 

Starman

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If you are running an 8 cylinder engine then just having a few degrees of flex in the drive system will guarantee that resonant frequencies will be below idle speed. If your resonances are below idle speed you are guranteed to not have torsional resonance problems unless possibly you get a hard starting engine but even then it is doubtful.

A possibility with a long torsion bar is bending, and it can get more complicated to cure that, but best thing if you take that route is to copy the Porsche 928 drive shaft setup. I think maybe Corvette had a rear transaxle on some of their models, and if they did they would also have the long torsion bar drive shaft system to copy. If it was me I wouldn't copy one of those systems, I would buy a used one and be done with it!

The simplest solution is to use a short torsion bar on the end of a stiffer drive shaft, or inside the end of it. either that or a short torsion bar or clutch type plate that uses rubber bushings instead of springs, between the crankshaft and the gears.
 

autoreply

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Keep in mind as it stands now the shaft will only be 50-70" long. The engine choice and cg will have to be dealt with prior to the shaft.

On another subject. What are the qualms on running a really large turbo or two sequential turbos in order to get a 35,000ft~ critical altitude? Also for the sake of simplicity could the cabin be pressurized off this? I seem to remember a guy building a turbo ls1 setup with a critical altitude of 30,000ft that did this.
It's very complex and most likely also quite heavy. The Nasa has turbocharged a Rotax 914F (already fitted with a turbo normally) and I think the final weight was around 550 lbs (from 200 lbs originally). Cabin pressurization from a turbo is done in certified aircraft too, so that's certainly possible.

As for the drive shaft, during Ila06 or 08 (airshow in Berlin) a guy from Stemme told me that their (carbon) driveshaft is extremely stiff to avoid the problem of vibrations. While there've been some problems with it and it's lifetime is only 400 hours it seems to work, or would that only be because of the low rpm (the Rotax has a PSRU) and the low power?
 

Mac790

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wsimpso1 said:
MiniImp, and the Aircar the same way, using a Dodge Flexidyne, which is a maintenance headache. Much written about elsewhere. Not a great solution...
About a year ago, Orion wrote about an alternative to the Dodge Flexidine (he was talking about one of his current projects), I was looking for that post but I can't find it. Does anybody remember what was that?

Seb
 

KC135DELTA

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It's very complex and most likely also quite heavy. The Nasa has turbocharged a Rotax 914F (already fitted with a turbo normally) and I think the final weight was around 550 lbs (from 200 lbs originally). Cabin pressurization from a turbo is done in certified aircraft too, so that's certainly possible.

As for the drive shaft, during Ila06 or 08 (airshow in Berlin) a guy from Stemme told me that their (carbon) driveshaft is extremely stiff to avoid the problem of vibrations. While there've been some problems with it and it's lifetime is only 400 hours it seems to work, or would that only be because of the low rpm (the Rotax has a PSRU) and the low power?
That rotax is designed to run at 65,000ft. Way way higher than this. I think this can be achieved with a single inter-cooler as there are street cars running 40psi+ on the same setup all over the world.
 

wsimpso1

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Actually, I have said my piece. There is no magic scheme. You can not just do some particular thing and know it is safe.

Even with the prop hung on the crank flange, there are airplanes out there with yellow arcs on their tacho. That means that either the prop or the crank won't last if you run in the band. Put a PSRU and/or a driveshaft on it, and you will be best served to do the whole engineering job on it. Run any vibe mode in resonance with the engine making power, and things will break. Run any section of shaft at its critical speed, and things will break. No "maybes". No "sometimes". You had better believe that Sikorsky and Bell and Agusta and Boeing and GE and P&W and Rolls Royce and Garret all have rotational vibe geeks working for them, and vibe software and vibe measurement equipment...

Billski
 

Mac790

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About a year ago, Orion wrote about an alternative to the Dodge Flexidine (he was talking about one of his current projects), I was looking for that post but I can't find it. Does anybody remember what was that?

Seb

Seems it was about 3 years ago ;)

Orion said:
And of course then we have the key powerplant issue, that of the shaft drive and torsional feedback. Molt's use of the Flexidyne coupling did work but only to a limited extent. First, being filled with lead shot, the coupling is relatively heavy (60 pounds). As the engine spins the shot is distributed to the outside of the housing, locking against the inner swash plate, which in turn transfers the power to the drive shaft and prop. The idea is that during times of torsional feedback, the opposing torque moves the swash plate in such a way that the lead shot unlocks, allowing for a bit of relative motion, thus dampening out the reversing power pulses.

The concept does work but only for a limited time. The relative motion tends to work against the shot to a sufficient degree that the shot is eventually worn down to a fine lead powder. When spun up to operational rpm's, this powder then tends to get compressed radially sufficiently to actually lock up the system. As such, the shot has to be replaced periodically (I think every 20 hrs or so) in order to make sure the coupling still does what it needs to do.

Today I'd recommend getting away from that configuration to something a bit more modern - in past work we looked at product lines such as Vulcan couplings (visco-elastic rubber plates), as well as others that might fit the bill
Seb
 
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