reduction drive with prop extension

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wsimpso1

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Looking over the rest of thread, AR asked another insightful question. I have it on good authority that the reason the Starship and Piaggio Avanti are not faster than they are is prop efficiency is lowered by being pusher props in the wake of the foils ahead of them. Downwash and some velocity variation and transient flow all contribute. Instead of the prop blades running in nearly constant AOA (tractor configuration), they have a constantly varying AOA and velocity profile. Result is that the blades are both significantly off of optimal angle and in transient (not steady state) flow for much of the rotation. All of the above significantly lower prop efficiency.

Then if you run the prop at angle to the input flow, you add the whole P-factor variable too. So for your most important flight mode, the prop really should be normal to the input flow and in clean air. Burt Rutan wisely went to tractor props for Catbird and Boomerang - just one of the contributors to those airplanes' amazing performance. For pushers, the lowered efficiency is part of the trade-offs.

Billski
 

wsimpso1

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PT Airco does have a point. While I am sure that the builders of airplane using the various geared two-stroke engines did not do huge analysis and test programs, I am sure that Rotax and friends did more than a little bit of engineering, testing and development. In the end they found combinations that are robust with props of certain sizes and weights. I am also pretty certain that they will tell you that you are on your own if you want to do other gear ratios, use long prop extensions or shaft systems, or props bigger than certain diameters or weights.

Using a well developed and tested powerplant intended for pusher aircraft is a far cry from mating a gearbox and long shaft to an automobile engine all and hanging it all off of the back end of an airplane.

Billski
 

Jay Kempf

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You have one other pulse input in a tractor engine. Nose up in climb attitude you get P factor giving one per rev per prop blade. How to simulate that is interesting. I suppose that you could build a flow straightener ahead of the prop and then run the system at 15 degrees to the straightened flow. You would probably have to drive the flow straightener with another engine-prop to accelerate the inflow to the prop to flight level and make sure that the majority of the air the prop is ingesting is driven to the prop at the desired angle.

Billski

That's an interesting point. To test the bad part of the flight modes even if you have designed for the perfect flow at another speed/AOA/trim you still have to look at whether the system is working in other less optimum modes. With at tractor that is say breathing hard like it would be in climb but speed limited like it would be in climb by fighting gravity you have a large thrust vector for the relative airspeed. I always thought that a prop would induce a relatively normal input flow at that point. Never considered that it would have a slightly skewed input flow. The truth normally if you look at the whole influx field is that all kinds of things are happening like partial p factor and unevenly loaded blades and that the resultant at the prop hub is a composite of all kinds of flow angles. I am guessing that the flow right at the prop does go normal it would have to but it would induce a downwardly curved inflow field meanline. And if you looked at those meanlines across the entire prop on the XY plane they are changing across the width of the prop diameter. Simulating that would be a whole study unto itself in a wind tunnel and I am not sure you would learn much from it. But interesting. I can't imagine the overall effect is a big fraction of prop efficiency during climb.
 

Richard Schubert

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PT Airco does have a point. While I am sure that the builders of airplane using the various geared two-stroke engines did not do huge analysis and test programs, I am sure that Rotax and friends did more than a little bit of engineering, testing and development. In the end they found combinations that are robust with props of certain sizes and weights. I am also pretty certain that they will tell you that you are on your own if you want to do other gear ratios, use long prop extensions or shaft systems, or props bigger than certain diameters or weights.

Using a well developed and tested powerplant intended for pusher aircraft is a far cry from mating a gearbox and long shaft to an automobile engine all and hanging it all off of the back end of an airplane.

Billski

This plane had some problems, but the gear drive didn't seem to be one of them:
Just sayin...
VolumeIII.jpg
 

BBerson

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BBerson,

The impact test requires accelerometers and/or a microphone to get the vibe data. In the automotive world, we usually do both. Sampling rate must be at minimum 4x the highest frequency you might be concerned with. Is this an even firing engine or an odd firing twin? Many V Twins use one crank throw, so for a 90 degree v-twin, the firing pulses come in at 90-610 or 270-450.
Billski
It is a 90 degree V twin. The number two cylinder fires 270 degrees after number one.
The problem for me is that even if I obtain some vibration information .... What would I do with it? The engine will obviously have some vibration at various rpm's. I just need to know if the crankshaft is strong enough.
thanks,
BB
 

wsimpso1

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BBerson,

The data will only show you what the frequencies are. Rest of the system work still remains. Big thing I will point out about motorcycle engines is that the crank is beefy enough for it to survive loading from the primary drive and the rest of motorcycle operating loads. And then, most motorcycles only have to live a very small part of their existence at high power. Even road racers spend most of their time decelerating, cornering at modest power, and accelerating under traction control. Road machines spend much of their time at very low power and torque. Now none of this is meant to imply that they are not capable of continuous high power with an airplane powertrain hung from it, but this is where you get to either design and test or do analysis or both.

Billski
 

Mac790

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hmm
A fatigue crack developed in the engine crankshaft as a result of corrosion pitting and the absence of a case-hardened layer on the fillet radius of the number six connecting rod journal. The fatigue failure of this section of the engine crankshaft resulted in a complete loss of power.
Like I said I took it from 2/3 years old thread, maybe not the best example, but someone probably could find a better one.

Anyway thanks for the link.

Seb
 
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