Untwisted Autototation>
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Untwisted Autototation>
Has anyone had experience doing autorotations in a machine that has blades that are parallel with no twist. If so are there any notable diferences compared to twisted blades. Thanks Bug
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Hi Bug,
Quite all the gyros use untwisted blades and autorotate fairly well
(www.rotaryforum.com)
the question could be "what is the better for autorotation : flat or twisted ?"
cheers
Victor
Quite all the gyros use untwisted blades and autorotate fairly well
(www.rotaryforum.com)the question could be "what is the better for autorotation : flat or twisted ?"
cheers
Victor
If I remember right Prouty's Helicopter Aerodynamics a twist inverse of that of a normal helicopter would be best for the gyrocopter or for autorotation. Seems logic to me, because the rotor would be optimised for the air comming from below.
Bug,
The Rotorways that I fly have blades that have no washout. The autorotation characteristics are good - and mere mortals like me can't tell any significant difference in auto characteristics.
Blade inertia and aerofoil section play a huge part in the auto characteristics.
J
The Rotorways that I fly have blades that have no washout. The autorotation characteristics are good - and mere mortals like me can't tell any significant difference in auto characteristics.
Blade inertia and aerofoil section play a huge part in the auto characteristics.
J
Bug,
Providing the rotor is sized correctly, an untwisted rotor will autorotate acceptably. To understand the detailed aerodynamics of the particular geometry you have in mind I think you will need to do some blade element analysis. Have a look at Leishman's book.
However, with a low cost homebuilt helicopter, I assume that you are considering non-twisted blades for manufacturing simplicity. If this is the case then there are two points that are far more important than the aerodynamics of untwisted blades in autorotation.
Firstly, twisted blades will create the same thrust on about 90% of the power of an equivalent untwisted blade. Therefore, by not twisting the blades you are creating a serious rod for your own back in terms of the amount, size and weight of the power you will need to install to get the machine of the ground.
Secondly, providing the rotor is acceptably designed then the aircraft will autorotate...providing you can get it into autorotation before rotor stall in the first place. If you are trying to design a machine with 'good-autorotation characteristics' then my suggestion is to focus on installing adequate rotor inertia, to give the pilot plenty of time to enter autorotation in the first place and then the required energy to tidy it up at the bottom. This involves the use of tip weights and a sufficiently strong rotor system to carry the additional radial loads.
PM me if you need some more help.
CRAN
Providing the rotor is sized correctly, an untwisted rotor will autorotate acceptably. To understand the detailed aerodynamics of the particular geometry you have in mind I think you will need to do some blade element analysis. Have a look at Leishman's book.
However, with a low cost homebuilt helicopter, I assume that you are considering non-twisted blades for manufacturing simplicity. If this is the case then there are two points that are far more important than the aerodynamics of untwisted blades in autorotation.
Firstly, twisted blades will create the same thrust on about 90% of the power of an equivalent untwisted blade. Therefore, by not twisting the blades you are creating a serious rod for your own back in terms of the amount, size and weight of the power you will need to install to get the machine of the ground.
Secondly, providing the rotor is acceptably designed then the aircraft will autorotate...providing you can get it into autorotation before rotor stall in the first place. If you are trying to design a machine with 'good-autorotation characteristics' then my suggestion is to focus on installing adequate rotor inertia, to give the pilot plenty of time to enter autorotation in the first place and then the required energy to tidy it up at the bottom. This involves the use of tip weights and a sufficiently strong rotor system to carry the additional radial loads.
PM me if you need some more help.
CRAN
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- -12-deg (or greater negativity) twist is best for hover.
- -6-deg (+/-) twist is best for fast forward flight.
- +1-deg (or greater) twist is best for autorotation.
- Independent Root and Tip Control is best for everything.
Edit
I goofed
The signs, which preceed the values, have now been corrected.
Twist is the change in the pitch of a blade when viewing from the root out to the tip. Negative is downward and positive is upward.
Last edited by Dave_Jackson; 17th Jan 2005 at 06:06.
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Dave ,
MIMS (Maybe i miss something)
How can you reduce the reverse flow if you twist positively at the root ? doing so, the airfoil gererates a reverse thrust, less drag, more reverse flow.
BTW (by the way) , the common pitch used in gyro blades is from 1 to 3 degrees...
or DOIMSA ?? (do i miss something again)
TY (thank you)
MIMS (Maybe i miss something)
How can you reduce the reverse flow if you twist positively at the root ? doing so, the airfoil gererates a reverse thrust, less drag, more reverse flow.
BTW (by the way) , the common pitch used in gyro blades is from 1 to 3 degrees...
or DOIMSA ?? (do i miss something again)
TY (thank you)
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Dave has it right (as usual)
The twist on a powered rotor is mostly for hover and mostly to get the entire blade involved in the lift. The tip has less angle by about 10 to 16 degrees, so the tip (which operates at much higher speed) is producing less lift and the root is producing more than if there was no twist. Getting the whole blade working is a way to make the whole disk produce moving air, so the average downwash velocity can be lower, and the mass of air moved casn be greater. This makes the power needed less, so the engine can lift more payload. In other words, blade twist is like making the rotor act larger, and making the downwash more uniform and more efficient.
That is for powered flight. In autorotation, the blade is now in reverse flow, the so-called windmill brake state. In effect, each piece of blade is acting to slow the airstream down (the windmill is braking the flow). That means each piece is producing a negative downwash, and a reverse twist would be best. If the autorotation was purely vertical, the twist should be the exact reverse of the twist for a powered hover, but the autorotation is always made in forward flight, where the flow is much more complex, and twist much less useful. That is because the blade is going upstream and downstream, and varying its collective angle with cyclic, so the actual angle of attack is not an easy thing to optimize.
In short, the twist is nearly reversed when going from powered flight to autorotation.
The twist on a powered rotor is mostly for hover and mostly to get the entire blade involved in the lift. The tip has less angle by about 10 to 16 degrees, so the tip (which operates at much higher speed) is producing less lift and the root is producing more than if there was no twist. Getting the whole blade working is a way to make the whole disk produce moving air, so the average downwash velocity can be lower, and the mass of air moved casn be greater. This makes the power needed less, so the engine can lift more payload. In other words, blade twist is like making the rotor act larger, and making the downwash more uniform and more efficient.
That is for powered flight. In autorotation, the blade is now in reverse flow, the so-called windmill brake state. In effect, each piece of blade is acting to slow the airstream down (the windmill is braking the flow). That means each piece is producing a negative downwash, and a reverse twist would be best. If the autorotation was purely vertical, the twist should be the exact reverse of the twist for a powered hover, but the autorotation is always made in forward flight, where the flow is much more complex, and twist much less useful. That is because the blade is going upstream and downstream, and varying its collective angle with cyclic, so the actual angle of attack is not an easy thing to optimize.
In short, the twist is nearly reversed when going from powered flight to autorotation.
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Twist: (Washout)
zeeoo,
Blade twist is the difference in a blade's pitch between its root and tip. On a helicopter with twist, the tip has less pitch and the amount is referred to as a negative value, in degrees.
The blades on a gyrocopter have a small amount of positive twist, because they operate full-time in autorotation.
Dave
Blade twist is the difference in a blade's pitch between its root and tip. On a helicopter with twist, the tip has less pitch and the amount is referred to as a negative value, in degrees.
The blades on a gyrocopter have a small amount of positive twist, because they operate full-time in autorotation.
Dave
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Nick,
Thanks for explaining such complex things in such simple words.
If the explanation is very clear in the case of the powered rotor, it is still opaque in the case of a gyrocopter.
What should be the ideal blade design ?
Dave , TYFTE ( thank you for the explanation)
Thanks for explaining such complex things in such simple words.
If the explanation is very clear in the case of the powered rotor, it is still opaque in the case of a gyrocopter.
What should be the ideal blade design ?
Dave , TYFTE ( thank you for the explanation)
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Untwisted Autototation>
Thank you for your replies. In view of your various comments, would it seem reasonable to assume that the time lag between downflow in powered flight, and upflow during autorotation, is increased with a non twisted blade because of a cleaner transition due to the inboard area of a twisted blade producing more drag(which pulls down the mrrpm more quickly) whilst the rotor system is in the equilibrium state prior to the flow reversal and subsequent autorotation.
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One mans' opinion, but what about....?
To: NickLappos
Your explanation sounds reasonable but it addresses specific points in time (flightwise). But what about how the theory of blade twist is taught to pilots and mechanics at most factory schools including the service school at Sikorsky. I was taught along with thousands of “mechanics” , maintenance officers and pilots that have attended countless factory and military schools is that the twist is incorporated to minimize spanwise bending and the attendant stresses on the blades or the rotorheads. In otherwise it tends to equalize lift across the blade span. If I am wrong then you had better talk to the guys at your service school.
10-16 degrees negative twist?
Fire away, I'm in a protected bunker.
Your explanation sounds reasonable but it addresses specific points in time (flightwise). But what about how the theory of blade twist is taught to pilots and mechanics at most factory schools including the service school at Sikorsky. I was taught along with thousands of “mechanics” , maintenance officers and pilots that have attended countless factory and military schools is that the twist is incorporated to minimize spanwise bending and the attendant stresses on the blades or the rotorheads. In otherwise it tends to equalize lift across the blade span. If I am wrong then you had better talk to the guys at your service school.
10-16 degrees negative twist?
Fire away, I'm in a protected bunker.
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Lu,
Could you give your opinion on an ideal blade design for a gyrocopter please ?.
Bug,
are you working on a helico or gyro project, or is it "top secret" classified ?
Thanks
Could you give your opinion on an ideal blade design for a gyrocopter please ?.
Bug,
are you working on a helico or gyro project, or is it "top secret" classified ?
Thanks
Last edited by zeeoo; 14th Jan 2005 at 21:40.
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Bug,
unfortunately, or fortunately, i haven' break somthing (yet)
I am in the phase A : "Build something"
perhaps the phase B will be : "trash it" or "repair" , i hope it will be "fly it".
But i would be curious to read about your broken things..
cheers
Victor
unfortunately, or fortunately, i haven' break somthing (yet)
I am in the phase A : "Build something"
perhaps the phase B will be : "trash it" or "repair" , i hope it will be "fly it".
But i would be curious to read about your broken things..
cheers
Victor
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Lu,
Before Nick beats the
out of you, you have said exactly what Nick said.
Twist brings some of the thrust inward toward the mast and away from the tip. I.e. a reduced moment on the blade and therefore a reduced out-of-plane bend.
Dave
Before Nick beats the
out of you, you have said exactly what Nick said. Twist brings some of the thrust inward toward the mast and away from the tip. I.e. a reduced moment on the blade and therefore a reduced out-of-plane bend.
Dave
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Thanks, Dave! The problem with Lu's statment (as he transcribed from that little blue book he always quotes) is that he has it backwards. The twist evens out the lift across the blade's span, and makes the helicopter carry more weight for the power. Twist is secondarily a stress reducer at the tip, but a stress raiser inboard. The max twist on an aluminum blade is about 10 degrees because any more will cause the bending load to be too high. With twist, the inboard sections of the blade loses stiffness since the twist permits buckling which vastly reduces the strength of the blade.
The twist is used in hover primarily, and low speed flight. At higher speeds, a variable twist (recall those piezo-electric tabs) is best, where the lift distribution can be kept perfectly even while the blade whirls around the mast, going upwind and downwind.
The twist is used in hover primarily, and low speed flight. At higher speeds, a variable twist (recall those piezo-electric tabs) is best, where the lift distribution can be kept perfectly even while the blade whirls around the mast, going upwind and downwind.
Nick, Cran, Dave J et al,
What's the typical magnitude of hover efficiency gained by adopting 'washout' in a blade as opposed to a non-twisted blade.
It's a loaded question as the Rotorway needs all the 'lifting' help it can get and I was wondering whether a more efficient, twisted blade design would be a relatively inexpensive performance enhancement.
As it stands, the Rotorway autorotates extremely well with plenty of time to get the lever down and RRPM recovery easy. It might be worth sacrificing some of this for improved hover performance which, lets face it, is where most of these helicopters spend most of their time.
Could the required twist be provided dynamically by a fixed outboard tab on the existing blade (Aluminium extrusion leading edge with bonded/rivetted skins) or could you see structural / stress-related difficulties in such an application.
The current method for fine tuning the tracking is to bend the trailing by a few thou'. Therefore, I suppose simple blade twist could be established by bending the trailing edge to achieve reflex at the tip and downwards towards the root......
Your thoughts would be appreciated.
J
What's the typical magnitude of hover efficiency gained by adopting 'washout' in a blade as opposed to a non-twisted blade.
It's a loaded question as the Rotorway needs all the 'lifting' help it can get and I was wondering whether a more efficient, twisted blade design would be a relatively inexpensive performance enhancement.
As it stands, the Rotorway autorotates extremely well with plenty of time to get the lever down and RRPM recovery easy. It might be worth sacrificing some of this for improved hover performance which, lets face it, is where most of these helicopters spend most of their time.
Could the required twist be provided dynamically by a fixed outboard tab on the existing blade (Aluminium extrusion leading edge with bonded/rivetted skins) or could you see structural / stress-related difficulties in such an application.
The current method for fine tuning the tracking is to bend the trailing by a few thou'. Therefore, I suppose simple blade twist could be established by bending the trailing edge to achieve reflex at the tip and downwards towards the root......
Your thoughts would be appreciated.
J
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As Lu says Blade Twist or Washout is there to minimise longitudinal stresses on the blade.
I guess that the tip of the blade which is going much faster than the root will need less pitch on it to generate the same amount of lift as the inboard part of the blade which is slower and therefore needs more pitch.
I guess that the tip of the blade which is going much faster than the root will need less pitch on it to generate the same amount of lift as the inboard part of the blade which is slower and therefore needs more pitch.
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Untwisted Autototation>
Jellycopter. Bending the trailing edges as envisaged will not achieve the extra lift that you need. The blade chord has its centre of lift at the 25% fom the leading edge so as to provide maximum efficiency throughout the range of pitch change. Radically altering the setting of the entire trailing edge would degrade the lift coeficient that the blade already has. My guess is that it would also have a detrimental effect on the pitching moment causing vibrational feedback to your cyclic. Bug.