Propeller questions

[hitech]

How did he break your plane?

He did a high speed taxi off the runway, threw a ditch, onto taxi way  with me in the back seat.

Ended with bent main gear, and rear 2 bulk heads bent from when the tail wheel hooked the taxi way.

Was a bad day flying.

The flight home from Dayton that trip, I lost a mechanical fuel pump on climb out and starter didn't engage with out much help on the fuel stop. DFW approach was nice that day and let me fly right over the center of DFW at 9k. (i didn't feel like being low with only electric fuel pump working).

HiTech

[Ardy123]

He did a high speed taxi off the runway, threw a ditch, onto taxi way  with me in the back seat.

Ended with bent main gear, and rear 2 bulk heads bent from when the tail wheel hooked the taxi way.

Was a bad day flying.

The flight home from Dayton that trip, I lost a mechanical fuel pump on climb out and starter didn't engage with out much help on the fuel stop. DFW approach was nice that day and let me fly right over the center of DFW at 9k. (i didn't feel like being low with only electric fuel pump working).

HiTech

that sucks.... I'm glad you made it back to the airstrip, I'm guessing once the fuel pump stopped, your engine died.

[Oldman731]

that sucks.... I'm glad you made it back to the airstrip, I'm guessing once the fuel pump stopped, your engine died.

He had an electric pump as backup, probably.

- oldman

[Tupac]

That stinks HT. Glad you got everything back in one piece.

[SgtPappy]

I've always wondered about that as well. Specially with the F4U-4 swinging a four bladed prop. Wouldn't the earlier corsair marks have benefited from a four bladed prop?

I think the simple explanation is that there wasn't enough horsepower. Yes, 2000 hp is a lot, but the blades they used were massive and they could have opted for shorter by more blades (which is eventually what they did; the F4U-4's prop diameter is 13' 2" whereas F4U-1's have 13' 4"), but when they did their calculations, I suppose they came to the conclusion that 3 blades would be better. Then the R-2800-18W became available and it spun 4 blades.

[clerick]

I remember some of the experiments with swept blade props - NASA did some testing on that as well back in the 80s but it never seemed to go anywhere. 

I remember seeing the C-130 "Hurricane Hunters" when i was stationed at Keesler AFB and they have a swept prop.
http://upload.wikimedia.org/wikipedia/commons/6/69/Lockheed_Martin_WC-130J.jpg

I also read that the AF is testing a similar configuration but with eight blades instead of the traditional four bladed paddle props.

[Shiva]

Generally speaking, the fewer blades and the larger the diameter the better, but then you run into all the other design problems previously mentioned--keeping tip speeds subsonic, ground clearance, power absorption, etc., you sometimes need to add blades, make them shorter, or have thinner or thicker chords.

Or make other design decisions with the airframe, which gave us the F4U Corsair -- the inverted gull wing was necessary to keep the landing gear short enough so that making them strong enough for carrier landings didn't impose a weight penalty with the propellor size they had already decided on. That it was aerodynamically superior was a bonus.

[EskimoJoe]

Hm, I'm curious here. Why would one not want the tip of the prop to breach the
speed of sound? Would it have something to do with the rest of the prop not
spinning as fast? What would happen if it did?

[clerick]

Hm, I'm curious here. Why would one not want the tip of the prop to breach the
speed of sound? Would it have something to do with the rest of the prop not
spinning as fast? What would happen if it did?

I assume that it is a drag issue. I'm just guessing here but, I would assume that a shockwave starts to built at the tip and would cause some serious turbulence and drag.  Anyone?

[Stoney]

Hm, I'm curious here. Why would one not want the tip of the prop to breach the
speed of sound? Would it have something to do with the rest of the prop not
spinning as fast? What would happen if it did?

Supersonic prop tip speeds do occur.  T-6 Texan props are one example and if you've ever been to Reno and heard the T-6 class race, you know that they have a very loud prop noise.  Some Cessna 180 owners have the pitch on their props so high (in order to generate a lot of static thrust that shortens takeoff distance) that the prop tips will go supersonic resulting in a lot of noise on takeoff.  The Russian "Bear" bomber reportedly has very loud prop noise as a result of its props going supersonic.

Again, generally speaking, supersonic prop tips can create three conditions:  (1) Extremely loud noise.  (2) Flow over the propellor blade separates, greatly reducing prop efficiency and creating drag and (3) introduction of vibration/stresses on the propeller blades that can damage the propellor.  Generally speaking, the aircraft won't gain any advantages from the increased prop speed/diameter if the blade tips are supersonic, and therefore there's no reason to design a prop that will regularly have supersonic tip speeds.  There are exceptions, as I mentioned above.

Not the best example, but watch this 180 video and you can hear the prop tips going supersonic during the takeoff and climb out.

http://www.youtube.com/watch?v=_BI0c_0uupQ

[EDIT] Here's a video of the T-6 class at Reno.  A better example of supersonic tip speed noise.  http://www.youtube.com/watch?v=nen94IP8mNg

[colmbo]

Some Cessna 180 owners have the pitch on their props so high (in order to generate a lot of static thrust that shortens takeoff distance) that the prop tips will go supersonic resulting in a lot of noise on takeoff.

If the pitch was set "high" the prop would be limited in RPM keeping the tip speed low.  The 180/185 prop noise is more from the length of the 2-blade prop the engine turning up to 2850 RPM for takeoff power.

[EskimoJoe]

Supersonic prop tip speeds do occur.  T-6 Texan props are one example and if you've ever been to Reno and heard the T-6 class race, you know that they have a very loud prop noise.  Some Cessna 180 owners have the pitch on their props so high (in order to generate a lot of static thrust that shortens takeoff distance) that the prop tips will go supersonic resulting in a lot of noise on takeoff.  The Russian "Bear" bomber reportedly has very loud prop noise as a result of its props going supersonic.

Again, generally speaking, supersonic prop tips can create three conditions:  (1) Extremely loud noise.  (2) Flow over the propellor blade separates, greatly reducing prop efficiency and creating drag and (3) introduction of vibration/stresses on the propeller blades that can damage the propellor.  Generally speaking, the aircraft won't gain any advantages from the increased prop speed/diameter if the blade tips are supersonic, and therefore there's no reason to design a prop that will regularly have supersonic tip speeds.  There are exceptions, as I mentioned above.

Not the best example, but watch this 180 video and you can hear the prop tips going supersonic during the takeoff and climb out.

http://www.youtube.com/watch?v=_BI0c_0uupQ

[EDIT] Here's a video of the T-6 class at Reno.  A better example of supersonic tip speed noise.  http://www.youtube.com/watch?v=nen94IP8mNg

Good stuff, thanks!

[hitech]

Some Cessna 180 owners have the pitch on their props so high (in order to generate a lot of static thrust that shortens takeoff distance) that the prop tips will go supersonic resulting in a lot of noise on takeoff.  T

You sure you have this correct? Unless you could previously over rev the engine standing still, higher pitch would = less thrust.  

And are you sure pitch changes tip speed much, unless rpm is changed?

HiTech

[EagleDNY]

Again, generally speaking, supersonic prop tips can create three conditions:  (1) Extremely loud noise.  (2) Flow over the propellor blade separates, greatly reducing prop efficiency and creating drag and (3) introduction of vibration/stresses on the propeller blades that can damage the propeller.  Generally speaking, the aircraft won't gain any advantages from the increased prop speed/diameter if the blade tips are supersonic, and therefore there's no reason to design a prop that will regularly have supersonic tip speeds.  There are exceptions, as I mentioned above.

Yes, that's it in a nutshell.  You can make a prop go supersonic but there really isn't much advantage to it.  You take a heavy duty (or specially designed) prop and mate it to a big honkin' powerplant and the tips go supersonic - which creates a huge noise problem (and shockwave hazard - do a quick search on the XF-84H trials and see what I mean about that).  Here's an article on it if you want some supplemental reading:

http://www.me.wustl.edu/~aiaa/November_1996.pdf

[Stoney]

You sure you have this correct? Unless you could previously over rev the engine standing still, higher pitch would = less thrust.  

And are you sure pitch changes tip speed much, unless rpm is changed?

HiTech

Sorry, poor terminology on my part, but an A&P friend of mine once told me that a lot of the 180 community purposefully generates those types of tip speeds to get those short take-off runs.

[Edit] Dale, do I remember correctly that you had a 180 at some point?