Prop pitch speed, static rpms, and actual flight speed?
06-03-2011, 10:00 AM
#1
Prop pitch speed, static rpms, and actual flight speed?
I was just re-reading the December 2010 issue of R/C Sport Flyer where they reviewed the DA 120. It showed that they got 6100 rpms with a VOX 28 x 10, which translates to a pitch speed of 60 mph. Ok, so far so good.
Then the engine was placed in an Aeroworks Yak 54. With the DA-120 and the same prop, the airplane was consequently radar clocked at 103 mph. Now I know that an engine and propellor will "unload" in the air and the rpms will increase, but these numbers indicate that the engine is unloading in flight to over 10,000 rpms. Is that true, or am I missing something here? I find it hard to believe the engine is unloading to the tune of 4000 rpms!
With a Mejzlik 28 x 12 prop they got 121 mph, which is also an apparent discrepancy in rpm, pitch speed, and actual speed numbers. Besides being surprising that the Yak would go that fast with all that cowl drag....
They stated that the tests were repeated multiple times to insure accuracy, and that they were done in level flight.
Please, not looking for a lecture on static thrust, prop sizing and brand wars, flying with different props until the plane flies how you want it to, etc. Just would like to understand the physics involved here. Thanks!!
AV8TOR
Then the engine was placed in an Aeroworks Yak 54. With the DA-120 and the same prop, the airplane was consequently radar clocked at 103 mph. Now I know that an engine and propellor will "unload" in the air and the rpms will increase, but these numbers indicate that the engine is unloading in flight to over 10,000 rpms. Is that true, or am I missing something here? I find it hard to believe the engine is unloading to the tune of 4000 rpms!
With a Mejzlik 28 x 12 prop they got 121 mph, which is also an apparent discrepancy in rpm, pitch speed, and actual speed numbers. Besides being surprising that the Yak would go that fast with all that cowl drag....
They stated that the tests were repeated multiple times to insure accuracy, and that they were done in level flight.
Please, not looking for a lecture on static thrust, prop sizing and brand wars, flying with different props until the plane flies how you want it to, etc. Just would like to understand the physics involved here. Thanks!!
AV8TOR
06-03-2011, 11:22 AM
#3
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RE: Prop pitch speed, static rpms, and actual flight speed?
I'm certainly no aeronautical engineer but would suspect that a prop develops lift from its airfoil along with thrust from its measured pitch angle. The two combined forces could produce airspeeds in excess of what pitch measurement alone would suggest.
I know many models I have had (mostly pylon racers of some type or another) went faster than their pitch calculation would suggest. Yes, some were slower too! This is all just a guess on my part though and will welcome some real answers.
I know many models I have had (mostly pylon racers of some type or another) went faster than their pitch calculation would suggest. Yes, some were slower too! This is all just a guess on my part though and will welcome some real answers.
06-03-2011, 11:42 AM
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RE: Prop pitch speed, static rpms, and actual flight speed?
Always wondered about that....Our methanol GT80s turn about 11,500..Prop is 19-18 carbon...theoretical speed would be about 207...actual radar clocked speed is around 195...Must be more drag than we think there is...
Our first race is June 9-12 in Hearne, Texas...If you can be there you will see speeds approaching 240 mph with our Unlimiteds, and the 195 with the GT80 methanol engines...
Our first race is June 9-12 in Hearne, Texas...If you can be there you will see speeds approaching 240 mph with our Unlimiteds, and the 195 with the GT80 methanol engines...
06-03-2011, 02:19 PM
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RE: Prop pitch speed, static rpms, and actual flight speed?
In the normal world, pitch is defined as the advance that a screw makes in a solid medium.
In this world, one starts out on a wrong leg. Pitch should be defined using the angle of the neutral airfoil line. However, in our props, pitch is defined as the lower foil tangent line. Real life pitch is at least some 10% more.
Add to this fact that at pitch speed (=calc + 10%), the cambered foil still has considerable lift, depending on foil camber. This adds up in Clark-Y foils to about 3° extra foil incidence, or 40% lift. so (calc+10%) x 1.4 is about 1.5 times the calculated speed, in a sleek plane even more.
Numbers above are very coarsly collected, but they reflect the real life errors that are made in prop designations and prop aerodynamic properties.
Different approach:
Tests have shown that planes fly at pitch speeds because the prop still provides substantial lift due to foil camber. Props in flight seem to have at least some 1200 rpm extra. (for props at 6000 rpm static). That is about 20% extra rpm. IMHO, you can add 10% to that calculated speed to get balance between prop pull and plane drag. So static pitch speed of 60MPH would be (60 x 1.2) x 1.1 = 79.2MpH, which is about the real life observed plane performance. (give or take a little)
Please be aware, that the pitch noted on our props is too small. Joe Zinger uses a better pitch figure because his foil is so very flat. That is why you cannot compare JZ pitch with other props pitches. compare a JZ 9" pitch with a Menz 8" pitch and you will be close. That is the reason why some seem to turn away from JZ props. Add one or two numbers in pitch, and they perform just as well, or better.
In this world, one starts out on a wrong leg. Pitch should be defined using the angle of the neutral airfoil line. However, in our props, pitch is defined as the lower foil tangent line. Real life pitch is at least some 10% more.
Add to this fact that at pitch speed (=calc + 10%), the cambered foil still has considerable lift, depending on foil camber. This adds up in Clark-Y foils to about 3° extra foil incidence, or 40% lift. so (calc+10%) x 1.4 is about 1.5 times the calculated speed, in a sleek plane even more.
Numbers above are very coarsly collected, but they reflect the real life errors that are made in prop designations and prop aerodynamic properties.
Different approach:
Tests have shown that planes fly at pitch speeds because the prop still provides substantial lift due to foil camber. Props in flight seem to have at least some 1200 rpm extra. (for props at 6000 rpm static). That is about 20% extra rpm. IMHO, you can add 10% to that calculated speed to get balance between prop pull and plane drag. So static pitch speed of 60MPH would be (60 x 1.2) x 1.1 = 79.2MpH, which is about the real life observed plane performance. (give or take a little)
Please be aware, that the pitch noted on our props is too small. Joe Zinger uses a better pitch figure because his foil is so very flat. That is why you cannot compare JZ pitch with other props pitches. compare a JZ 9" pitch with a Menz 8" pitch and you will be close. That is the reason why some seem to turn away from JZ props. Add one or two numbers in pitch, and they perform just as well, or better.
06-03-2011, 03:03 PM
#6
RE: Prop pitch speed, static rpms, and actual flight speed?
ORIGINAL: av8tor1977
I was just re-reading the December 2010 issue of R/C Sport Flyer where they reviewed the DA 120. It showed that they got 6100 rpms with a VOX 28 x 10, which translates to a pitch speed of 60 mph. Ok, so far so good.
Then the engine was placed in an Aeroworks Yak 54. With the DA-120 and the same prop, the airplane was consequently radar clocked at 103 mph. Now I know that an engine and propellor will "unload" in the air and the rpms will increase, but these numbers indicate that the engine is unloading in flight to over 10,000 rpms. Is that true, or am I missing something here? I find it hard to believe the engine is unloading to the tune of 4000 rpms!
With a Mejzlik 28 x 12 prop they got 121 mph, which is also an apparent discrepancy in rpm, pitch speed, and actual speed numbers. Besides being surprising that the Yak would go that fast with all that cowl drag....
They stated that the tests were repeated multiple times to insure accuracy, and that they were done in level flight.
Please, not looking for a lecture on static thrust, prop sizing and brand wars, flying with different props until the plane flies how you want it to, etc. Just would like to understand the physics involved here. Thanks!!
AV8TOR
I was just re-reading the December 2010 issue of R/C Sport Flyer where they reviewed the DA 120. It showed that they got 6100 rpms with a VOX 28 x 10, which translates to a pitch speed of 60 mph. Ok, so far so good.
Then the engine was placed in an Aeroworks Yak 54. With the DA-120 and the same prop, the airplane was consequently radar clocked at 103 mph. Now I know that an engine and propellor will "unload" in the air and the rpms will increase, but these numbers indicate that the engine is unloading in flight to over 10,000 rpms. Is that true, or am I missing something here? I find it hard to believe the engine is unloading to the tune of 4000 rpms!
With a Mejzlik 28 x 12 prop they got 121 mph, which is also an apparent discrepancy in rpm, pitch speed, and actual speed numbers. Besides being surprising that the Yak would go that fast with all that cowl drag....
They stated that the tests were repeated multiple times to insure accuracy, and that they were done in level flight.
Please, not looking for a lecture on static thrust, prop sizing and brand wars, flying with different props until the plane flies how you want it to, etc. Just would like to understand the physics involved here. Thanks!!
AV8TOR
