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#11
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prop rpm question
Bob Fry wrote:
I wish to leave the engine out of the discussion, but let's continue... "KB" == Kyle Boatright writes: KB If we assume the plane in question is a C-152, Close enough, it's an Aircoupe with a C90. But let's look just at the prop. Why does a prop produce so much more thrust, much more than double, when it's turned at only twice the rate? KB Another way to look at it is that your prop has an advance KB rate. Let's say it the advance rate is 4 feet per KB revolution. Yep, 48" pitch. KB At 1,000 rpm, and no drag on the airplane (rolling KB or aerodynamic), the airplane would have a terminal velocity KB of 4,000 fpm, or about 48 mph. Of course, there is rolling and KB aerodynamic drag, and there is prop drag too, so the engine KB can only drag the plane along at, say, 30 mph, assuming a flat KB smooth runway. KB At 2,000 rpm, with no drag, the terminal velocity would be KB 8,000 fpm, or about 85 mph. Hmmmm...so prop thrust is indeed only twice at double the rpm?...ideally speaking of course. The idealized (no viscosity etc.) math seems to say that it is linear, but intuitive feel says not. Is aerodynamic drag of an airfoil linear with speed? Is airfoil lift linear with speed? Since a prop is just an airfoil going in a circle, why would you expect it to be linear? Matt |
#12
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prop rpm question
Kershner states that the maximum thrust force occurs when the plane is
standing still (at a fixed throttle setting, I guess), and decreases as you go faster. There are some fast homebuilts for which this won't be true. They have fixed-pitch props with very high pitches, and the blade is largely stalled at the start of the takeoff roll, making accelleration dismal indeed. The pilots report that the airplane seems to come alive at some point before liftoff when the prop blades finally get to work. My own Jodel has an efficient wooden prop and I often note a small RPM drop as the airplane accellerates through about 20 MPH. There's something happening with the airflow through the blades, probably to do with unstalling, or, perhaps (less likely) with leaving behind the larger prop vortex generated in the static condition. Dan |
#13
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prop rpm question
If you can find the engine performance plots you will see that the
percent of RPM and percent of power (HP or torque) are not at all the same thing. And it's torque that turns the propeller (not RPM). 1000 rpm might be near 1/2 RPM, but barely 10-20 percent max torque. At full power (torque), the prop can deliver x number of pounds thrust for any given airspeed. That's the most you'll get. Rolling off RPM also rolls one down the torque curve. And you are right, it's a very non-linear curve. Richard ps: also on the torque curve, note that max torque and max HP are usually NOT found at the same RPM... ta |
#14
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prop rpm question
Richard Lamb wrote:
If you can find the engine performance plots you will see that the percent of RPM and percent of power (HP or torque) are not at all the same thing. And it's torque that turns the propeller (not RPM). 1000 rpm might be near 1/2 RPM, but barely 10-20 percent max torque. At full power (torque), the prop can deliver x number of pounds thrust for any given airspeed. That's the most you'll get. Rolling off RPM also rolls one down the torque curve. And you are right, it's a very non-linear curve. Richard ps: also on the torque curve, note that max torque and max HP are usually NOT found at the same RPM... ta It's been a while since I saw so many errors in so little text. Matt |
#15
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prop rpm question
I think it has to do with how fast the prop can 'push' air versus how fast
the plane is moving through the air. Imagine you are in a small boat with an outboard motor. The motor is moving the boat at 15 mph. You decide you want to go faster so you pull out an oar. You can only move the oar at a speed of 15 mph so the net force you are applying to the water is zero. If you were going slower, you could apply force to the water, thus making the boat inch ahead a little faster. wrote in message ... I'm not an engineer, nor do I play one on TV. I do find it interesting that the speed or velocity of the aircraft is a factor in figuring thrust in both the propeller and the jet propulsion formulas. Perhaps someone else can explain in it terms that you and I can understand. TC |
#16
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prop rpm question
Matt Whiting wrote:
Richard Lamb wrote: If you can find the engine performance plots you will see that the percent of RPM and percent of power (HP or torque) are not at all the same thing. And it's torque that turns the propeller (not RPM). 1000 rpm might be near 1/2 RPM, but barely 10-20 percent max torque. At full power (torque), the prop can deliver x number of pounds thrust for any given airspeed. That's the most you'll get. Rolling off RPM also rolls one down the torque curve. And you are right, it's a very non-linear curve. Richard ps: also on the torque curve, note that max torque and max HP are usually NOT found at the same RPM... ta It's been a while since I saw so many errors in so little text. Matt That RPM and torque are NOT the same thing? Or that at full power will give deliver full thrust? Ot that the thrust delivered changes with airspeed? Or that it's very non linear? Very oversimplified, but go ahead and straighten me out, Matt. Richard |
#17
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prop rpm question
Richard Lamb wrote:
Matt Whiting wrote: Richard Lamb wrote: If you can find the engine performance plots you will see that the percent of RPM and percent of power (HP or torque) are not at all the same thing. And it's torque that turns the propeller (not RPM). 1000 rpm might be near 1/2 RPM, but barely 10-20 percent max torque. At full power (torque), the prop can deliver x number of pounds thrust for any given airspeed. That's the most you'll get. Rolling off RPM also rolls one down the torque curve. And you are right, it's a very non-linear curve. Richard ps: also on the torque curve, note that max torque and max HP are usually NOT found at the same RPM... ta It's been a while since I saw so many errors in so little text. Matt That RPM and torque are NOT the same thing? Or that at full power will give deliver full thrust? Ot that the thrust delivered changes with airspeed? Or that it's very non linear? Very oversimplified, but go ahead and straighten me out, Matt. Richard Yes, horsepower and torque are absolutely not the same thing. The following suggests that they are "At full power (torque)..." Rolling off RPM may or may not roll you down the torque curve. If you are running at an RPM above the torque peak, reducing RPM might actually increase the torque available. 1000 RPM isn't 1/2 RPM. It may be close to 1/2 of the maximum allowable RPM, which is what you hopefully intended to say. Matt |
#18
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prop rpm question
Do you really think including factual data is likely to resolve the
question? And do you know Dr Dan? |
#19
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prop rpm question
Thrust is a direct relation of diameter x pitch x rpm.
Get a copy of H.Glauert's book: The elements of aerofoil and airscrew theory. isbn 052127494 Also, thrust is a function of the third power of the prop diameter so changing the prop diameter on inch can have a major effect on thrust and vice versa. |
#20
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prop rpm question
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