Let's make a helicopter blade
Iconoclast
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Active blade twist?
First of all I have a question. On a blade that does not have active blade twist there a built in twist to the blade. In other words if the blade has a 7-degree twist then with the blades in a static position with no collective pitch the root would be at 7-degrees and the tip at 0-degrees.
Does an active twist blade have this built in twist or, is all twist commanded to meet specific aerodynamic loading? It has been stated that the ideal active twist blade would have 0-degrees AOA to the relative wind. If there is no built in twist then with full collective pitch the root would measure for instance 18-degrees and the tip would measure 0-degrees which means that the active pitch range would be in excess of 18-degrees which seems to be excessive. If the blade has a built in twist and the twist can be altered then this same blade would have a root measurement of 18-degrees plus the built in setting of 7-degrees giving a total of 25-degrees of twist in order to maintain a 0-degree AOA at the tip. Hopefully I got the numbers correct.
Does an active twist blade have this built in twist or, is all twist commanded to meet specific aerodynamic loading? It has been stated that the ideal active twist blade would have 0-degrees AOA to the relative wind. If there is no built in twist then with full collective pitch the root would measure for instance 18-degrees and the tip would measure 0-degrees which means that the active pitch range would be in excess of 18-degrees which seems to be excessive. If the blade has a built in twist and the twist can be altered then this same blade would have a root measurement of 18-degrees plus the built in setting of 7-degrees giving a total of 25-degrees of twist in order to maintain a 0-degree AOA at the tip. Hopefully I got the numbers correct.
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Lu,
To my knowledge (Tmk
), no one has yet built a blade with full Active Blade Twist.
Tmk, the ABT methods that are currently being experimented with are slow to reposition and/or have only a few degrees of pitch change. Tmk, there aren't any blades with ABT in actual helicopters; if you exclude Kaman's long existing servo-flap rotor. Tmk, a tip flap control was being considered for Northrop Grumman's UCAR tailless concept featuring twin intermeshing rotors but this project was canceled in December.
Your question therefor becomes a hypothetical one.
The " 0-degrees AOA to the relative wind", which you mention, may be Mart's personal feeling as to what would be optimal angle at the tip. Perhaps he would be the best person to answer your specific question.
The preliminary and crude calculations for the UniCopter show that for a full ATB (including reverse velocity) the blades must be able to twist from -16.8º to +10.8º. More information on this is at;
UniCopter ~ Control - Flight - Independent Root & Tip (IRAT) ~ Blade Pitch Settings
Dave
To my knowledge (Tmk
), no one has yet built a blade with full Active Blade Twist.Tmk, the ABT methods that are currently being experimented with are slow to reposition and/or have only a few degrees of pitch change. Tmk, there aren't any blades with ABT in actual helicopters; if you exclude Kaman's long existing servo-flap rotor. Tmk, a tip flap control was being considered for Northrop Grumman's UCAR tailless concept featuring twin intermeshing rotors but this project was canceled in December.
Your question therefor becomes a hypothetical one.
The " 0-degrees AOA to the relative wind", which you mention, may be Mart's personal feeling as to what would be optimal angle at the tip. Perhaps he would be the best person to answer your specific question.
The preliminary and crude calculations for the UniCopter show that for a full ATB (including reverse velocity) the blades must be able to twist from -16.8º to +10.8º. More information on this is at;
UniCopter ~ Control - Flight - Independent Root & Tip (IRAT) ~ Blade Pitch Settings
Dave
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"The " 0-degrees AOA to the relative wind", which you mention, may be Mart's personal feeling as to what would be optimal angle at the tip."
The easiest thing is to look at the history of fixed wing development. The Supermarine Spitfire had elliptical wings that enabled it to fly efficiently under all conditions. Concerns about manufacturing costs and tip stall in tight turns led the P51 Mustang to have straight wings, but with inbuilt twist so that the tips were flat (ie 0 degrees AOA). This lead to the P51 having an elliptical lift pressure distribution at cruise, but it moved away from this when pulling g for a turn or at higher speeds. Lets not forget all the Wright flyers which had wing warping.
My view is thus, to cover the largest range of speeds and manouvres would require straight wings, but with variable twist to keep tips flat rel airstream. The easiest way to accomplish this would be powerful tip servos, set at 0 degrees AOA, and a torsionally flexible wing. By altering wing root AOA you would get the most efficient and manouvreable fixed wing imaginable.
A helicopter blade is basically just a wing rotating about an axis. So once again for the least drag (or torque in this case) the lift pressure distribution should be elliptical (Prouty mentions this somewhere). The complication comes from the need to get this distribution from a wing that is flying slower at the root than tip. The best solution is thus to have the planform choord distribution proportional to 1/radius, in addition to the elliptical washout.
To cover the full range of flight conditions in any helicopter, this is why i suggest a powerful tip trim tab. This forces an elliptical lift distribution in all flight conditions. The nest result is low torque requirements.
Mart
The easiest thing is to look at the history of fixed wing development. The Supermarine Spitfire had elliptical wings that enabled it to fly efficiently under all conditions. Concerns about manufacturing costs and tip stall in tight turns led the P51 Mustang to have straight wings, but with inbuilt twist so that the tips were flat (ie 0 degrees AOA). This lead to the P51 having an elliptical lift pressure distribution at cruise, but it moved away from this when pulling g for a turn or at higher speeds. Lets not forget all the Wright flyers which had wing warping.
My view is thus, to cover the largest range of speeds and manouvres would require straight wings, but with variable twist to keep tips flat rel airstream. The easiest way to accomplish this would be powerful tip servos, set at 0 degrees AOA, and a torsionally flexible wing. By altering wing root AOA you would get the most efficient and manouvreable fixed wing imaginable.
A helicopter blade is basically just a wing rotating about an axis. So once again for the least drag (or torque in this case) the lift pressure distribution should be elliptical (Prouty mentions this somewhere). The complication comes from the need to get this distribution from a wing that is flying slower at the root than tip. The best solution is thus to have the planform choord distribution proportional to 1/radius, in addition to the elliptical washout.
To cover the full range of flight conditions in any helicopter, this is why i suggest a powerful tip trim tab. This forces an elliptical lift distribution in all flight conditions. The nest result is low torque requirements.
Mart
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Mart,
The following graph was begged, borrowed, stolen from an article by Prouty in the Winter 2004 issue of AHS's Vertiflite.
It plots the Ideal distribution and the distribution from his example helicopter (single rotor) at 210 knots. There are three observations of interest, IMO.
~ 1/ The outer elements of the rotor blade contribute the largest portion of lift. The somewhat "elliptical lift pressure distribution at cruise" from a rotor is due to the summing of the lift from the front and the back halves of the rotor disk.
~ 2/ The helicopter's distribution of circulation would be closer to the ideal, if the rotor wake was from two counterrotating main rotors.
` 3/ The helicopter's distribution of circulation would be even closer to the ideal, if the craft had Reverse Velocity Utilization. This is because there will be less of a dip in the plot at -0.2 of span.
Dave
The following graph was begged, borrowed, stolen from an article by Prouty in the Winter 2004 issue of AHS's Vertiflite.
It plots the Ideal distribution and the distribution from his example helicopter (single rotor) at 210 knots. There are three observations of interest, IMO.
~ 1/ The outer elements of the rotor blade contribute the largest portion of lift. The somewhat "elliptical lift pressure distribution at cruise" from a rotor is due to the summing of the lift from the front and the back halves of the rotor disk.
~ 2/ The helicopter's distribution of circulation would be closer to the ideal, if the rotor wake was from two counterrotating main rotors.
` 3/ The helicopter's distribution of circulation would be even closer to the ideal, if the craft had Reverse Velocity Utilization. This is because there will be less of a dip in the plot at -0.2 of span.
Dave
Last edited by Dave_Jackson; 3rd May 2005 at 19:13.
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Aha - I thought this diagram might appear! I'll answer each point in turn.
1/ Agreed, but bear in mind these downwash velocities come from Prouty's blade sweep integration rotor charts, not taking into acount the tip loss vortex. The effect i'm talking about is in addition to the effect shown, since its purpose is to minimise tip loss power.
Downwash from a circular disk is elliptical (in forward flight) assuming constant downwash velocity throught the disk. Trouble is you never get that, since you want the fewest blades to keep the profile drag to a minimum. By using an elliptical downwash velocity across radius you minimise tip losses, and still virtually provides the overall elliptical lift pattern shown.
2/ Agreed, but this does also raise the important point about how to combine the velocity from overlapping blades. Both Prouty and Stepniewski treat overlap regions as having the same downwash velocity as a single rotor. If you have differing downwash velocity, such as when a retreating blade overlaps an advancing blade, i believe that averaging the velocities is a more accurate estimate since the lower downwash blade interferes with the higher downwash blade. My belief in feathering retreating blade is to minimise this interference.
3/ True, but the higher pressure air from below would stillleak through the zero velocity circle. An advancing blade below it, with reverse velocity blade twist for feathering, stops this
Mart
1/ Agreed, but bear in mind these downwash velocities come from Prouty's blade sweep integration rotor charts, not taking into acount the tip loss vortex. The effect i'm talking about is in addition to the effect shown, since its purpose is to minimise tip loss power.
Downwash from a circular disk is elliptical (in forward flight) assuming constant downwash velocity throught the disk. Trouble is you never get that, since you want the fewest blades to keep the profile drag to a minimum. By using an elliptical downwash velocity across radius you minimise tip losses, and still virtually provides the overall elliptical lift pattern shown.
2/ Agreed, but this does also raise the important point about how to combine the velocity from overlapping blades. Both Prouty and Stepniewski treat overlap regions as having the same downwash velocity as a single rotor. If you have differing downwash velocity, such as when a retreating blade overlaps an advancing blade, i believe that averaging the velocities is a more accurate estimate since the lower downwash blade interferes with the higher downwash blade. My belief in feathering retreating blade is to minimise this interference.
3/ True, but the higher pressure air from below would stillleak through the zero velocity circle. An advancing blade below it, with reverse velocity blade twist for feathering, stops this
Mart
Last edited by Graviman; 4th May 2005 at 19:08.
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Mart,
This thread is galloping off in too many directions.
1/ Tip loss on a helicopter is only 2-3%, so don't get overly concerned about it.
2/ Stepniewski will be your best source for information on twin main rotors.
3/ The utilization of reverse velocity is a relatively new subject. You may want to get a better grasp of it by searching the net. Understand this weird rotor and you've pass the test.
Dave
This thread is galloping off in too many directions.
1/ Tip loss on a helicopter is only 2-3%, so don't get overly concerned about it.
2/ Stepniewski will be your best source for information on twin main rotors.
3/ The utilization of reverse velocity is a relatively new subject. You may want to get a better grasp of it by searching the net. Understand this weird rotor and you've pass the test.
Dave
Last edited by Dave_Jackson; 3rd May 2005 at 22:19.
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Well OK, but i only suggest the tip tab as an interim solution, as a very cost effective initial solution. The retreating blade can easilly incorperate reverse twist, to follow overall downwash,as long as it is feathered. I think the biggest concern with cyclically altering blade twist has to be the risk of eigenmodes becoming divergent...
Mart
Mart
Last edited by Graviman; 5th May 2005 at 04:32.
