Robinson R22 Blade behaviour in flight

[WrenchMonkey_96]

Hi all,

 

This is my first post here. I'm currently working on a project involving the R22, specifically the blades. But I need to know a bit mroe about how the blades are behaving in flight.

 

I know that the typical rotor rpm is between 480 and 530 rpm, which means that the blades go through a full flapping motion at a frequency of about 8 to 8.5 Hz.

But I'm also interested in lead/lag motion. I know that lead/lag occurs at the same frequency as blade flapping, but I cannot find any documentation regarding the actual deflection of the blade tip.

 

I've also had trouble finding out how far the blade will delfect axially, so under torque from aerodynamic forces. The R22 tends to operate very close to the stall angle for its blades, but the blade twist will be much smaller than that. I'm guessing of the order of maybe 1 or 2 degrees of twist. But I can't find any info on it anywhere.

 

Can any of you help me out? The main thing I'm looking for is lead/lag amplitude, but torque on the blade would be really helpful too.

 

Thanks.

[Curyfury]

If an average pilot knows the answer to these questions, i will be impressed. Sounds very technical. Something that only a robinson engineer will be able to answer. However, i will add that r22 dont have a lead/lag hinge so i assume the amount is zero, unless there is measurable flex from the solid piece of metal that axis is attached to. Or i can just be completely full of crap and really dont understand anything.

[kona4breakfast]

From the R22 POH, p.1-4 (http://www.robinsonheli.com/manuals/r22_poh/r22_poh_full_book.pdf): Free to teeter and cone, rigid inplane ... -8 degrees twist.

So the blades are free to cone, but not hunt (lead/lag). If you've ever seen the videos from the hub of a blade in flight, though, you know it moves. Do you have access to a discrete element analysis program? If not, you'd have to make a lot of approximations (even with DEA, you still would). The honeycomb and skin would add significant rigidity in that axis, which would make it difficult to model.

 

It'd be easier to model a swept-tip blade at a hover under varying gross weights. Swept-tip blades not only delay the onset of compressibility but they also increase twist in the blades by moving the aerodynamic center at the tip, creating a moment about the spar that twists the blades further, increasing, slightly, aerodynamic performance under high power demands.

[WolftalonID]

Well the POH and the maintenance manual both have rotor specs. Both publications are found in pdf format on robinsons website.

[WrenchMonkey_96]

From the R22 POH, p.1-4 (http://www.robinsonheli.com/manuals/r22_poh/r22_poh_full_book.pdf): Free to teeter and cone, rigid inplane ... -8 degrees twist.

 

I read through the handbook, thanks for the link. It helps me out in a lot of other ways too. But I don't understand the blade twist. Is it a range of 8 degrees? As in +4 to -4 degrees, or 8 degrees in one direction?

[heliflyknow]

I'm pretty sure it's the change in blade pitch from the root to the tip- higher at the root so that the lift along the blade is more constant.

[iChris]

 

I read through the handbook, thanks for the link. It helps me out in a lot of other ways too. But I don't understand the blade twist. Is it a range of 8 degrees? As in +4 to -4 degrees, or 8 degrees in one direction?

 

The blade twist is for two reasons: to improve hover performance by making the induced velocity more uniform; and to reduce the angle of attack on the retreating tip to delay blade stall at high speed. Twist that helps on the retreating tip hurts on the advancing tip by reducing the angle of attack and the lift carried on that portion of the disc. In addition, twist that is beneficial in powered flight is detrimental in autorotation.

 

For these reasons, and because high blade twist has resulted in severe vibrations at high speeds, designers use it sparingly. Instead of the -20° or -30° optimum for good performance in hover and minimizing retreating blade stall at high speeds, the designers tend to use -6° to -16° as a compromise.”

 

The helicopter blade is a complex subject because of the number of interacting variables involved. The long thin structure of a practical blade is not rigid and flight loads will cause the blade to flex. In forward flight the loads have a powerful alternating component and this generates harmonics.

 

A further complexity is that the flexing of the blade will affect the aerodynamics; a phenomenon called aeroelasticity. The aerodynamics affects the flexing, which creates an ongoing loop. Aerodynamic forces act on the blade causing blade flexing and excite resonances. Blade flexing changes relative airflow and angle of attack, leading back to changes in the aerodynamic forces that continue the loop.

 

This loop may be stable or unstable. Part of the design process must be to ensure that aeroelastic effects always result in overall blade stability.

 

The shape of the blade is completely defined by the root cutout, the tip shape, the degree of twist and taper and by the blade section used, which may change with radius. The way the blade responds to this excitation is complicated. The blades torsional forces depend on the location of the center of pressure with respect to the center of mass.

 

As an example of variations on the conventional, the UH-60A used a non-linear twist modification at the blade tip to compensate for tip vortex interference.

 

 

 

 

If you’re looking for specific blade deflection measurement data for a full-scale rotor system like the R22 you should go direct to Robinson.

 

For general information on deflection, aeroelasticity, check the link below:

 

Rotary-Wing Aeroelasticity

Edited August 3, 2015 by iChris

[FlyingBuma]

iChris covered blade twist with his usual attention to detail...

......

But I'm also interested in lead/lag motion. I know that lead/lag occurs at the same frequency as blade flapping, but I cannot find any documentation regarding the actual deflection of the blade tip.

.....

 

With a semi rigid 2 bladed rotor system, like the R22 uses, an underhung teetering hinge is used to allow the blades to flap, compensating for dissymitry of lift between the advancing and retreating blades.

As the blades flap to accomodate dissymitry of lift, the underhung hinge moves each blade closer or farther from the rotor mast. This keeps the center of gravity of the blade at reletively the same distance from the mast. The retreating blade flaps down (less coning angle) and is pulled closer to the mast. The advancing blade flaps up (more coning angle) and moves away from the mast.

Because the CG of the blades stays pretty much constant distance from the rotor mast, there is minimal coriolis effect on the blade. Also the semi rigid design does not have lead/lag hinges.

Therefore, semi rigid rotors do no have the typical lead/lag found in fully articulated rotor systems.

[Eric Hunt]

FlyingBuma, you have not quite grasped what it is all about.

 

In the hover, with the disc coning upwards, each blade has its centre of gravity at the same distance from the mast. But when the coned disc is tilted forward by using cyclic feathering, to generate forward thrust and to move in that direction, the front blade appears longer (in the horizontal plane) and the rear blade appears shorter.

 

Conservation of Angular Momentum would require the longer blade to slow down, and the shorter blade to speed up, to keep the angular momentum constant. However, in a 2-bladed system, this would mean that both blades are now on the advancing side, creating a massive out-of-balance situation.

 

The underslung system allows the front "longer" blade to tuck in closer to the mast, and the back "shorter" blade to poke out further, minimising the problems of angular momentum. There are still big stresses in the 2-blade rotor head, but being underslung reduces these stresses.

 

Flapping to equality due to dissymmetry of lift is a whole other totally misunderstood aspect of aerodynamics, done to death in other threads, and is unrelated to moving forward, because it only relates to NOT changing the pitch with cyclic.