Effectivness of propeller near mach 1,0?

[niklas]

I compared the top speeds of the 109 and 190, engine RPM and prop gear ratio. I came to the conclusion that german AC only were only able to fly faster when they adjusted the prop gear ratio. Together with the topspeed, the tip of the propeller never exceeded Mach 1.0.

I.E when they changed the prop gear ratio of the 109G from 1:1,685 (db605 A) to 1: 2,06 (db605 AC), they reached 550kmh at sealevel and 685 kmh in 7,4km.

or while a bmw801 run with 2700rpm, 3,3m prop diameter and a prop gear ratio of 1:1,85, the jumo213A runned with  3250rpm, 3,5m diameter and a gear ratio of 1:2,4.

In all cases the propeller rpm and gear ratio was choosen in a way that the tip of the propeller stays below Mach 0,9-0,95 when the aircraft reaches the topspeed.

How efficient is a gear ratio where the propeller tip exceeds mach 1,0 ?? Is a increase in topspeed possible when the propeller tip reaches Mach 0,95-1,0 ??


niklas

[Jimdandy]

Ok now we're talking! This is the reason a prop driven plane cannot pull itself past the speed of sound. The same thing happens to the prop as the prop tip reaches the speed of sound as the wing. The prop cavitates. If you look at all of the prop ratios they are all that way. For higher alt planes they even turn the props slower.

[This message has been edited by Jimdandy (edited 01-12-2001).]

[niklas]

jimdandy, does this mean when you add the speed of the aircraft to the speed of the prop-tip (pythagoras) than the result must be lower than mach 1,0??

Can someone help me? I need the propeller diameter, engine RPM and (especially) prop gear ratio of the following aircrafts:

Spit9
P38L
P51D

thx!
niklas

[Jimdandy]

I'm a mechanical engineer and it's been almost 10 years since I took compressible flow so forgive me if I don't get it quite right. I work with pumps and piping not airplanes   . Here's may best quick explanation.

The prop is just a pump. When a pump tries to pull water at to high of a suction head it will cavitate. Cavitation is the formation of air pockets in the water stream. This will result in vibration and loss of efficiency. If the pump is run in that condition for to long it will fail. As the prop tip approaches Mach 1 shock waves are formed at the prop tip. These shock waves form and detach and cause vibration and loss of prop efficiency. If you keep driving the prop faster the whole blade will form a shock wave. This will result in total loss of thrust. The prop would more than likely rip off its mount before it could be spun that fast do to the high cyclic loading. Now if you point the nose of the plane down you get gravity working with you. The prop doesn't have to generate the thrust anymore. So if you dive the plane you can shove it past the speed of sound with out the prop. This would rip the wings off of WWII planes because the wing was not optimized for trans sonic and super sonic flight. Don't forget that the plan is traveling at  right angle to the velocity vector of the air on the prop. Look at the flow of air on the prop in the plane of the prop. Think of the prop as a wing because that's all it is. It is a wing mounted so that the lift it generates pulls the plane forward. I hope this helps.

[This message has been edited by Jimdandy (edited 01-13-2001).]

[niklas]

thx, your info sounds logical.

But now i have a problem.

From existing maxspeed numbers, prop gear ratios, rpm and speed of sound i looked for the mach number at which the prop tip is moving.

I got the following result for Sealevel:
 

well all AC far away from mach 1,0 , some more, some less.

And for the individual rated altitude:
 


Wow, the tip of the F4u-1d is moving araound at Mach 1,07!! Almost 0,5feet of the propeller tip is moving around with supersonic speed. Same for the F6F and F4U. ALL OTHERS have Machnumbers slightly below 1,0.

Strange isnīt it ...  

niklas

[This message has been edited by niklas (edited 01-13-2001).]

[This message has been edited by niklas (edited 01-13-2001).]

[F4UDOA]

Niklas,

Why is the F4U-1D higher than the F4U-4??

[Jimdandy]

I'm not sure why that is. From every thing I know your not supposed to push the prop tip beyond Mach 1. It should be less efficient. Like I said I was digging out of the 10 year old garbage in my head.    They may have done something to the shape of the prop to increase the critical Mach number but beyond the speed of sound? Just looking at it in two dimensions, the velocity contribution of the plane to the prop should only be added as drag. I'm not up to trying to play with the rotational flow right now. I think I may have also steered you wrong on the slower prop speed at higher alts. I think they are slowing the prop down at higher alt to "bite" the air. The air is so thin that the prop can't move enough mass to generate thrust. A high speed prop cavitates. A "slower" prop will pull more of the thin air. If I remember right the P-51H had a slower prop than the D for better high alt performance. I'm pretty sure that the B-29 had "slower" props for the same reason. There are people on here that can answer this question. I wish they would read it.

[wells]

The 'tip' is not a point anywhere on the blade, so the fact that the tip might be above mach 1.0 really makes no difference to the propeller.  If you have a point, say at least 1 chord width in from the tip that is exceeding some critical mach, then you'd start to see some losses.  

[Jimdandy]

Thanks wells. It has been a long time since I've really even thought about this stuff.  I have a question for you. Wouldn't you start forming an attached local shock wave on the on the actual prop tip as the Free Stream Mach Number (M inf.) is approached (0.8<M inf.<1.0 Transonic Flow). This would cause pressure loss and a large rise in drag thus reducing the efficiency of the prop?
Slap me into shape wells  

[niklas]

F4udoa, they changed the prop gear ratio for the -4 from 2:1 to 20:9

Does really nobody knows the propgear ratio of a P38, P47, P51 or spit??