Prop tip speed on large piston engines?

[ben87r]

I have read allot about redrives on smaller A/C but what about large piston engines? Do they still run direct drive meaning lowrpm engines or is there a gear box type used??

 

Met a lame last week who spent 25 years at Mareeba war birds on Dc3, p40, Nanchang's,t28 t6 and he was trying to explain to me but i didn't get what he was saying

 

 

[Deskpilot]

In a nut shell.........

 

Tip speed near the sound barrier......very bad, very noisy and prop close to disintegration.

 

No matter what the size of the engine, tip speed must be kept to reasonable level.

 

Redrives just match the available power to the prop size and pitch for maximum efficiency.

 

Remember, for a given rpm, the tip speed increases as the prop diameter increases.

 

 

[ben87r]

So are you saying that a larger engine might not necessarily have a larger prop size??

 

 

[facthunter]

Tip speed is a problem with some direct drive engines like the bigger "Flat" american engines and large dia props. The effects are noise, (C-185's etc) loss of efficiency, erosion of leading edges in rain.

 

Metal and finely dimensionally controlled props can run a higher tip speed than wooden

 

Virtually all higher HP engines run a reduction gear, even though the bigger engines do not do high revs. As an Example the twin row P&W R-2000 has a reduction ratio of about 2/1 and the cruise RPM's are from 1800 to 2050 so your prop RPM is around 1,000 rpm. Nev

 

 

[ben87r]

Virtually all higher HP engines run a reduction gear, even though the bigger engines do not do high revs. As an Example the twin row P&W R-2000 has a reduction ratio of about 2/1 and the cruise RPM's are from 1800 to 2050 so your prop RPM is around 1,000 rpm. Nev

Thanks Nev, that is what i was looking for

 

 

[spacesailor]

For a Hummelbird try to get a scimmiter prop, a bit like a veriable prop with finer pitch at take-off then courser at cruise

 

Bryan

 

 

[ben87r]

Cheers Bryan, are you using 54x28?

 

 

[spacesailor]

Trying to make a semi-scimmiter prop, made a couple of 48 X 36" that seem allright, but 54" will cut grass on take off (heavy fuel( and landing( bounce when learning)))

 

Have 5" wheels on this one but going to 6" on the tail-dragger.

 

Bryan

 

 

[Louis Moore]

A lot of radial engine do indeed have reduction gears, most people just don't think they do because they associate such a thing with a large looking bumpily piece of milled metal sitting just before the prop. With a radial they tend to be tucked neatly on the inside!

 

Tip speed is a problem with some direct drive engines like the bigger "Flat" american engines and large dia props. The effects are noise, (C-185's etc) loss of efficiency, erosion of leading edges in rain.

My favourite would have to be a later model 210, listen to that harsh cracking whip as they take off!

 

 

[Yenn]

I don't think many of the flat opposed yanky engines are geared. They get their power from bigger capacity rather than high revs. Those that are geared usually feature a "G" in their nomenclature. Direct drive may have to use more than 2 blades to absorb the power.