Question about power and props

[V1670]

Re: Question about power and props

Thanks,Big Jim. Did a little research on the "Sky Prop" and wouldn'cha know it, it took me back to this forum circa 2011. Big Jim gave a really good synopsis of the prop and Bucky had a really nice photo album of the Sky Prop, Tiger Claw and various other experiments tried on Fury and Super Chief.

Thanks again guys. Just love the history and thinkin'-outside-the-box of this sport. I remember when "NOBODY'S gonna break 430"..and then 450...and then.... As the saying goes; "the sky's are the limit"

[MIKE NIXON]

Re: Question about power and props

When you consider the forward speed of the prop at 500 MPH the pitch has to be high. The tip speed is a vector of the RPM and the forward speed. We usualy run the math in feet per second and convert to a % of mach #. Mach varies with temp so that is a factor. As the tip approches Mach the HP/thrust is converted to noise and heat and the aircraft quits going faster. Add a little more RPM and you will slow down. The tips on Strega are shaped to allow higher speeds before the problem starts. The problem is the tip breaking the speed of sound and creating a sonic boom. As the rpm increases the shock wave blanks out more of the prop as the point of generation moves inboard. Like all multiple curves, with more rpm the engine can make more HP but the with the prop starting to loose thrust the net curve flattens and then drops off. That is why we can make some noise and go faster but then a little more noise and we slow down. On a cold day its worse which is one of many reasons we go faster on warm days.
MN

[Big_Jim]

Re: Question about power and props

That is why we can make some noise and go faster but then a little more noise and we slow down. MN

And a perfect example of this is when Skip was flying Tsunami in the '91 Gold race. He was about 1/4 lap back on lap 6 and pulled the power back from 'destruct power', which wasn't getting him anywhere....only to pick up about 15-20 mph and (though a lot of people disclaim it) run the first two 500 mph laps.

If you saw it, it was impressive.

[1:1 Scale]

Re: Question about power and props

Have any of the teams played with changing the twist of the prop blades? The twist of the blade from root to tip is only optimized (from what I understand/remember) for a single pitch angle. You can increase the total pitch on a constant speed prop, but the angle of attack along the length of the blade won't be consistent, and therefore efficiency suffers.

[MIKE NIXON]

Re: Question about power and props

Actualy we were just waiting for our check for $1,000,000 to clear before we get serious about prop design.

The optimal prop has to be designed for a specific RPM, HP and speed to get the twist, cord and diameter correct. If you get it right you go faster than you thought and then you make more changes etc.
The blades are forged alum and to make changes is very expensive and time consuming. They have to be heat treated to remove the hardness and then you can only make a few degrees of change and then the alum work hardens and you start over.

If you make the perfect racing prop you will need 2 miles to get the plane off the ground. That was one of the reasons the early records were set with seaplanes that had fixed pitch props and miles of water to take off on.

Like most of what we do it is all a mix of balance this and that to get the best average of all the factors.
MN

[440_Magnum]

Re: Question about power and props

I've got the answer: blades made of "Terminator" metal so that the twist, profile, chord, and pitch can *all* change as conditions demand....

Unless the turbo encabulator fails, then the blades revert to liquid and disappear in a cloud of metallic droplets....

[FlyKidChris]

Re: Question about power and props

Actualy we were just waiting for our check for $1,000,000 to clear before we get serious about prop design.

MN

Only a million dollars? Damn, you work cheap.

Mike, thanks for your posts here. I know that I'm not the only fan that appreciates them.

[hm66sk]

Re: Question about power and props

Excellent read.

[J.A.W.]

Re: Question about power and props

What diameter is the largest prop currently being raced?
Is it as big as the largest diameter stock fighter props?

Interesting the opposite end of continuum approaches taken by the trans-sonic turbo-shaft guys..
Ex-Junkers guys with the mighty Tu-95 Bear hi-torque/low[ish] revs, - but big diameter prop
VS the fast spinning, short/fat bladed XF-84 'Thunderschreech'.