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How to make a propeller driven aircraft go really fast ?

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plncraze

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Tsunami is a good example of smaller. It was supposed to be really fast with a hopped up engine. Sport Aviation had an article on air racers which talked about piston swept area as a comparison. More piston or more stroke or something like that. You are fighting the drag rise that comes with more speed and dealing with an airframe that can carry the fuel and the power plant to fight it.
 

Dan Thomas

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So far, no one has mentioned the hard limit of propeller tip speeds. You have to take the static tip speed of the prop at redline RPM (or whatever RPM you're using at the max speed) and square it. Then you take the forward speed of the airplane and square that. Add the two results and find the square root, and you have the propeller's tip speed at the max forward speed. A squared plus B squared = C squared.

Now, propellers do not do well anywhere near the speed of sound. Remember that the airflow over an airfoil accelerates over the top of it, and that happens to a propeller blade, too, so nearing the speed of sound a shock wave forms at the prop tip as that accelerated airlow gets to sonic speed, and drag increases hugely and noise gets awesome. Just listen to a Cessna 185 with a seaplane prop on it during takeoff. That constant-speed prop is at redline (2850 RPM) and the tips of the 86-inch prop are doing 729 MPH plus the small factor of the forward speed of 80 MPH added to give a tip speed of 733 MPH. Speed of sound at sea level and standard temperature is 760 MPH. Some 185 pilots find that they can dial the prop back and leave the throttle full and actually get better climb and less noise, since the drag on the prop has decreased and more HP is going into performance instead of noise.

Now, some airplane doing 500 MPH, like an old warbird all hopped up and spinning a 12-foot prop, has to have a low RPM and huge pitch. 1200 RPM, maybe. And the picture of the Bearcat with that massive prop shows what such a prop has to look like to absorb all that horsepower at such low RPMs.

Now you know why jet engines rule for speed. And why helicopters are relatively slow.
 

rbarnes

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So far, no one has mentioned the hard limit of propeller tip speeds. You have to take the static tip speed of the prop at redline RPM (or whatever RPM you're using at the max speed) and square it. Then you take the forward speed of the airplane and square that. Add the two results and find the square root, and you have the propeller's tip speed at the max forward speed. A squared plus B squared = C squared.

Now, propellers do not do well anywhere near the speed of sound. Remember that the airflow over an airfoil accelerates over the top of it, and that happens to a propeller blade, too, so nearing the speed of sound a shock wave forms at the prop tip as that accelerated airlow gets to sonic speed, and drag increases hugely and noise gets awesome. Just listen to a Cessna 185 with a seaplane prop on it during takeoff. That constant-speed prop is at redline (2850 RPM) and the tips of the 86-inch prop are doing 729 MPH plus the small factor of the forward speed of 80 MPH added to give a tip speed of 733 MPH. Speed of sound at sea level and standard temperature is 760 MPH. Some 185 pilots find that they can dial the prop back and leave the throttle full and actually get better climb and less noise, since the drag on the prop has decreased and more HP is going into performance instead of noise.

Now, some airplane doing 500 MPH, like an old warbird all hopped up and spinning a 12-foot prop, has to have a low RPM and huge pitch. 1200 RPM, maybe. And the picture of the Bearcat with that massive prop shows what such a prop has to look like to absorb all that horsepower at such low RPMs.

Now you know why jet engines rule for speed. And why helicopters are relatively slow.
and then there was the Thunder-Screech

 

rv6ejguy

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All the fast Unlimited Reno racers use low gearing to keep the props as efficient as possible. Dago Red used a .38 gear ratio so the prop was only turning 1350rpm at full power.
 

Dan Thomas

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and then there was the Thunder-Screech

Yes, I forgot to mention that airplane. It was about the loudest prop airplane ever due to supersonic prop tips. Pilots that flew it once refused to fly it again. It could be heard ten miles away when being run up on the ground. That's loud.
 

Speedboat100

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By your logic, shouldn't ultralights be absolute speed demons?
I am making one...let's see.

Part 103 is restricted speedwise by the rules.

---------

Tu-95 flies high and has swept wings....and very slim fuselage.

Also Mossie is doing pretty good at 415 mph using 0.42 hp/kg.
 
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cblink.007

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Thunderscreech was nasty. Supposedly it was so vile that it induced sickness & seizures in some personnel who were unfortunate enough to be nearby.

I've been up inside a Tu-95 running up at full song. Probably the loudest environment I have ever been in, thanks to her transonic props.

But, on topic of high speed prop driven birds. I am inclined to agree on the low drag airframe with a high bite subsonic prop. Too bad there is not enough interest to develop a specialized super high speed prop....it would be cool to try to beat all the old records!
 

Speedboat100

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Thunderscreech was nasty. Supposedly it was so vile that it induced sickness & seizures in some personnel who were unfortunate enough to be nearby.

I've been up inside a Tu-95 running up at full song. Probably the loudest environment I have ever been in, thanks to her transonic props.

But, on topic of high speed prop driven birds. I am inclined to agree on the low drag airframe with a high bite subsonic prop. Too bad there is not enough interest to develop a specialized super high speed prop....it would be cool to try to beat all the old records!

Yes curious thing it is actually flying 950 km/h ( 590 mph ) at same power to weight ratio as Dh Mosquito does 415 mph. So the ( relative ) reduced drag is enermous with swept wings and adequate huge propellers ( and flying very high in the thin air ).

 

henryk

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a specialized super high speed prop...

=4 place flying car, 43-750 km/h, <65 km/h (road) <30 m/s (climbe).
 

BJC

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=4 place flying car, 43-750 km/h, <65 km/h (road) <30 m/s (climbe).
= doesn’t exist.


BJC
 

Saville

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I can't find the article right now but I read an article a few years back that talked about how one thermal engineer transformed the unlimited racers, giving them a boost in speed.

I think the engineer's name is Pete Law and it may have been these articles or one very like them:

 

mcrae0104

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I think the engineer's name is Pete Law and it may have been these articles or one very like them:
Pete can be seen every year running from pit to pit at Reno diagnosing carburetor issues and helping crews troubleshoot their problems and stretch their speed. He's nice enough to stop and chat with you if you run into him. Truly a nice guy. A gentleman and a scholar in the world of aviation.
 
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