Rotor sweep?

Hot Wings

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My interest is in building an ultralight tractor gyro. Designing something that flies well and is a legal US part 103 is hard enough for a fixed wing. To get good performance with part 103 restrictions means optimizing as much of the aircraft as possible - within a defined budget.

To that end I'm starting with the rotor since it is the defining feature of a gyro. There has been much discussion here regarding 'tail heavy' rotor blades and the related pitch change as a result. A variable collective seems to have some advantages but given the weight limit of part 103, and the complexity added, this doesn't seem to be an option. However with modern composite technology we can, or should be able to, design a rotor blade that has about any desired aeroelastic response we desire. In other words a passive collective should be possible.

In the various threads where there has been debate about rotor speed not changing because the rotor blades balance aft of the 25% chord, but it being alleged that the rotors do in fact balance at 25% how much of the blade twist might be due to undetected sweep effect?

The sketch below shows how even if the blade has been strung so that the line between the same point on each tip crosses the rotor axis a small amount of sweep, either for or aft, would go undetected and result in a large torque about the aerodynamic axis. It seems to me that this sweep could be used to our advantage?
Scale 1"=1'

rotor sweep.JPG
 

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Vance

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Good morning Leon,

I admire your willingness to take on a challenging project.

What advantage do you feel you could get from the “sweep”?

Not all the posters on The Rotary Wing Forum are knowledgeable so I feel it is important to carefully filter the information you find here.

The book “Aerodynamics of the Helicopter” by Gessow & Myers has lots of information on rotor systems. You may find more reliable information there.
 

gyrojake

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I always thought that this type of sweep ( lead, lag ) was only a powered rotor idiosyncrasy
 

Hot Wings

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Vance;n1143643 said:
Not all the posters on The Rotary Wing Forum are knowledgeable so I feel it is important to carefully filter the information you find here.
True- and there are some pretty knowledgeable individuals here as well. I'm pretty good at sorting out the good bits of grain from the mouse droppings.

I've done a fair bit of research relating to swept flying wings as part of my fixed wing part 103 project and know how important aeroelastic tailoring can be. It just seems from what I've read that this is an area that hasn't had much interest in the gyro world. The improvement in performance possible is not as inexpensive or simple as just adding power. For a typical aircraft that works most of the time but when dealing with artificially restricted aircraft, such as part 103 and LSA, optimization of the air-frame and aerodynamics becomes more important.

I understand that I'm reaching for the fruit near the top of the tree.
 

Vance

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I did not know what Aeroelastic Tailoring was so I looked up the NASA report and I am having a hard time seeing how it relates to rotor "sweep".

https://ntrs.nasa.gov/archive/nasa/c...0140006404.pdf

The rotors on the gyroplane I fly have a pivot and a soft bushing that allows the two blade semi rigid rotor to better manage lead/lag forces.

I have not been able to imagine how that could be used to improve rotor efficiency.

I don't want to distract you from your mission.

I am simply curious.
 

Hot Wings

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Vance;n1143647 said:
I
The rotors on the gyroplane I fly have a pivot and a soft bushing that allows the two blade semi rigid rotor to better manage lead/lag forces.
.
The rubber bushing on your rotor would pretty well eliminate any possibility of inadvertent sweep.

Imagine a classic Bensen bolted block attachment but the mating ends got milled 1/2 degree off from 90 and assembled in the most adverse way. An almost imperceptible error with rather large effect. Sketch represents 1 degree error. Even a 1/3 degree total error still means a sin(1/3) x rotor length sweep, or about a 7/8" sweep on a 24 ft rotor.
Bensen stub.JPG
 

Vance

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I have flown gyroplanes with the blades not straight and it appears to me it makes things shake without an appreciable change in performance.

How does the sweep change the rotor efficiency?
 

Hot Wings

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Given a totally rigid teetering rotor, sweep would make no difference. With lead lag pivot, or rubber mounts, there can be no sweep, unless it's designed into the blade similar to the "V" shaped rotor tips

A straight rotor with forward sweep as it is flexed upward also has it's angle of attack increased at the tip, just like a forward swept wing of a fixed wing aircraft. This is a divergent interaction that prevented using forward sweep on aircraft until we learned how to design wings stiff enough to not flex and still be light enough to fly. Using composites we can move the torsional axis around so that the wing twists in the opposite direction. This is a self correcting interaction. Trying to do this with an isotropic material such as aluminum is pretty hard.

A rear sweep will tend to do the opposite. Even if the center of gravity of the blade is at, or in front of, the aerodynamic center (25ish%) the center of lift is behind that center and even with reflex will tend to washout the blade if that center of lift is behind the mechanical torsional axis of the blade.

Why my interest in this? Try to get Mu above .25 or so for a gyro that has a top speed of 60 mph. If i want to fly on low power (more power means weighs) at high altitude efficiency matters.

How - might - this increase efficiency? Airfoils have a set AOA where the L/D is lowest. The closer the airfoil is to that ideal lift coefficient the the less power needed. Sailplanes can change that ideal spot by reflexing their flaps/ailerons for different cruise speeds. Short of using some kind of computer controlled piezo operated Gurney flaps, twisting the rotor blade for different loads/speed is about all we have the technology to do today? Mechanical collective is just too heavy for part 103.
 

Hot Wings

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I don't mean to confuse - but that is the result too often. Keep in mind that I can be quite dyslexic at times.

Not all of my fixed wing aero theory knowledge may translate to rotor craft. I'm hoping others here will guide me down the correct path.
 

Hot Wings

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WaspAir;n1143674 said:
There is a wealth of academic research on these matters.
Thanks for the link! Looks very useful.
I did not have that paper. 440 pages! It is too bad that NASA is so much more verbose in their reports than the NACA was.
 

kolibri282

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Hi Leon,

this is a very interesting thread! Designing a rotor yourselfe means you certainly don't aim low in your endeavours...;-) For some more material on all sorts of stuff related to rotorcraft you might want to have a look at the "Technical Papers Books and Publications" section of the forum:
https://www.rotaryforum.com/forum/ro...d-publications

I am a mechanical engineer with a fairly decent background in Finite Element Methods and for your task it seems you would need something like this. Please keep us informed about the steps you want to take and the progress of your project!
 

Jean Claude

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Hot Wings;n1143669 said:
Airfoils have a set AOA where the L/D is lowest. The closer the airfoil is to that ideal lift coefficient the the less power needed. Sailplanes can change that ideal spot by reflexing their flaps/ailerons for different cruise speeds..
By necessity to balance the teeter, the A.o.A of a section of the blade varies from 2° to 8° during a half-turn . But unlike a glider wing, it is not allowed to change the Cm linked to the best adaptations.
 

Jean Claude

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Hot Wings;n1143661 said:
Imagine a classic Bensen bolted block attachment but the mating ends got milled 1/2 degree off from 90 and assembled in the most adverse way. An almost imperceptible error with rather large effect. Sketch represents 1 degree error. Even a 1/3 degree total error still means a sin(1/3) x rotor length sweep, or about a 7/8" sweep on a 24 ft rotor.
The relevant reference line should be the one that joins the hub to the application point of the centrifugal force of the blade.
Therefore,Leon, the sweep is neglectible

Sans titre.png
 

Hot Wings

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Jean Claude;n1143737 said:
The relevant reference line should be ]
Should be? There may be some miscommunication here due to the limitations of data exchange on a an internet forum - and some language translation error.

This isn't the way any of the rotor stringing procedures I've run across are done. Even if the CG was used as the alignment point it wouldn't make any difference aerodynamically if the aerodynamic axis is not aligned with the mechanical torsion axis.
Sure there is a tendency for a non ridgid rotor blade to try to us it's own CG during flight, but unlike a free body flying wing with 6 full axis of freedom a semi ridged rotor blade is restrained at the root. The CG of the rotor blade is just one force on the free body diagram. If that force happens to a be on the aerodynamic axis than there will be no aerodynamic twisting force introduced. If the CG of the blade is not aligned with the mechanical torsion axis of the blade then there will be a twisting force introduced. Only when the cg of the blade, the aerodynamic neutral axis and the mechanical torsion axis align will there be no torsional loads on the rotor blade.......other than aero torsion loads due to the variation of AOA along the blade.

Typed this kind of quick. Brother in law's water heater just sprung a leak and he has 11 thumbs.
 

Jean Claude

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Maybe my bad translation, Leon.
What I mean is that if you introduce the angle of one degree as you mention, it shifts the profile forward at the same time as the center of gravity of the blade, as I drew.
So I do not see why you suppose that the torsion axis would stay aligned with the other blade, as you drew

In add, the unbalance vibrations produced by this misalignment will of course be corrected by the user with the same misalignment on the other blade. So the reference axis to consider for the twisting moment is the one that joins the points that I indicated. Not the line of hub bar
. Sans titre.png

The torsional torque is determined by lift and the opposite component of the centrifugal force. The lift is assumed applied to the Aerodynamic Center and thus of course we must add the torsion due to the Cm0 profile.
Sans titre.png
 

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Hot Wings

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Jean Claude;n1143746 said:
Maybe my bad translation, Leon.
And it could be my lack of rotor experience. I was kind of hoping to get your response on this thread. I'm a bit busier today than I expected and I need some time to put my words together properly. Thanks for taking the time to help me learn!

Sorry to hear about the fire today. It was/is a loss to all of us, not just the French.
 

Jean Claude

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Yes, the destruction of this witness of architectural technics and manners of almost a millennium ago is a great loss for heritage
 
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