What direction are forces applied to control a helicopter? 2

[dspDad]

I was at oskosh this summer and took a demo flight in an (unnamed) gyrocopter. Thoroughly entertaining, but somwhat discomforting for a fixed wing pilot.

After seeing how squirrely gyrocopter handling is, I started researching gyrocopter flight characteristics, which led to asking questions about helicopter flight characteristics.

Because of helicopter wing precession, are the forces generated through the swash plate orthogonal to the desired motion, e.g. apply a force up or down on the front of the blade to turn right/left, etc.

Or will applying an upward force at the front of the blade (admittedly by increasing the angle of attack as it passes in front of the helicopter) cause the helicopter to
pitch up like in an airplane?

I asked a gyrocopter guru about this, but instead of an answer got a comment that the gyrocopter I had tested is one of the more stable gyrocopters out there. (With the implied and probably correct assessment about my cowardice)

The second question is the same, only about gyrocopters. Does swinging the body of the gyro forward and back cause a change in pitch or roll, or some combination? Because the linkage was through a floor mounted stick, I don't know what motion I was really imparting to the blade.

 

[GraviMan]

I'm a fixed wing boy myself - something about beating the air into submission! ;-)

Gyroscopic precession in a rotor blade is taken care of by the rotor aerodynamics. The direction the rotor "disk" is tilted is exactly as mechanical intuition would suggest. If the rotor wing rolls left, say, there will be a gyroscopic torque trying to tilt the rotor disk rearward (i.e. front up, rear down). But, this actually means the rear blades see increased AOA, while the front sees reduced (or even negative) AOA. The practical upshot is that the blades stay in the same plane. I found this amazing when I first saw it!

I am not involved in helicopter design, so can't quantify the forces/torques, but gyrocopters function in exactly the same way as helicopters, only with no rotor hub torque. Again the "stick" is just controlling the rotor disk orientation as mech intuition would suggest (i.e. forward for forward flight etc). The cyclic was really just a way of "trimming" the blades as they rotate, so the pilot doesn't have to put in large control forces.

As for controlability, that's easy. Rotor craft blades handle the lift forces by centrifugal force (watch the rotor disk go conical as a helicopter pulls into the air if you don't believe me). In level flight the disk is so shallow as to be almost flat. Imagine flying a fixed wing with no wing dihedral, and the elevator the same AOA as the main wing (longitudinal dihedral) - it would be almost unflyable!

It would be interesting to see a rotor craft with designed in dihedral - maybe wing tips on the rotor blades would do the trick without affecting centrifugal forces. Mostly choppers just use inertia to reduce the oscillation frequencies, and the pilot damps the movement (sort of an active system ) - that's what Bell bars are all about.

Intermesher what do you reckon - winglets on the unicopter blade tips?

Mart

 

[Sparweb]

On the Bell helicopters, the angle between the pitch control and the cyclic control is not 90^o and neither is at 90^o to the direction of flight.

Go figure.


STF

 

[dspDad]

Thanks for the input. This is very interesting and helps put things in perspective. The increased AOA and partially offset controls is starting to explain some questions. Here is my analysis so far.

When an airplane enters a turn there needs to be a moment (aileron input) accelerating it in the direction of the turn, then a second moment in the opposite direction to stop the increasing bank, then third and fourth moments (opposite aileron input) to bring the plane back to level.

But in a helicopter, the precession generates another force in the pitch axis proportional to the roll rate. (aileron input is zero during maximum roll rate in planes) This precession force does not occur in airplanes so we fixed wing pilots..well we're fixed wing pilots.

It sounds like roll forces predominate over precession forces, at least during gentle flight. Are the manufacturers compensating for part of the required correction by mixing a little pitch control in with roll control? Sort of like mixing aileron and rudder together.

 

[PEW]

Remember that a gyroscope is a completely rigid entity whereas a helicopter rotor is articulated. Precession of a gyro is caused when a force is applied to the rotational axis of the gyro, movement occuring 90 degrees later with respect to the applied force and the direction of rotation.

Helicopters are NOT controlled by the pilot's inputs directly overcoming gyroscopic stability to cause precession. They are controlled by flying the rotor disc to the desired attitude. This is achieved by altering the cyclic pitch blade angle. This can be seen in Kaman's helicopters where the blades are controlled by flap-like control surfaces on the trailing edges of the main rotor blades. There is no way for the pilot to cause precession in the sense of a gyroscope.

The pilot's inputs cannot act instantly. When an input is made, the blade does begin to change track immediately but the maximum deflection (high point of the blade disk) occurs further round the blade orbit. The controls must be arranged to have an advance angle to allow for this such that they act in a logical sense for the pilot. If you look at films of Igor Sikorsky's flights in his very early machines, you can see that he did not realise this to begin with. His control inputs are made 90 degrees out to the response of the aircraft, such that a an aircraft pitch alteration is achieved by what would be a roll input in a fixed wing.

 

[dspDad]

The gyroscopic forces act when the axis of rotation changes and are present by virtue of the physics (conservation of angular momentum). Applying the forces at the root of the blade (gyrocopter) or aerodynamically on the blade itself(helicopter) do not change the necessity to apply sufficient torque to re-orient the blade.

After reading SparWeb, I examined an R-22. They have the controls offset about 15 degrees. So in a turn, 4 parts of the control input go into precessing the blade and 1 part into rolling the helicopter. Doing some estimates on an ultralight gyrocopter, it looks like the ratio is about 19 to 1 precession to roll.

Unless my calculations are totally whacked (has happened once or twice) then the flight dynamics of gryos are related to
1) Center of Gravity shift steering mechanism and
2) Large precessional forces that predominate the control input in normal flight.

In more extreme flight (tighter banks, faster turns) it seems the problems would be exagerated.

Any confirmation or rebuttal of this analysis so far?
Thanks,
DspDad

 

[PEW]

Forgive me if I'm wrong but your first paragraph seems to indicate that you are thinking along the lines that the helicopter pilot directly provides a force which will overcome gyroscopic stability possessed by the rotor blades / disc. Not true. What he does provide is a force which cyclically changes the angle of attack of the blades, hence the use of the term cyclic stick as the primary aircraft control. The cyclic input is arranged to occur 90 degrees in advance of the desired high point of the blade disc. The 90 degrees advance is achieved by a combination of advance mechanical tilting of the swashplate and the angle subtended by the pitch change arm mechanism on the blade root. The altered aerodynamic forces produced by the blade overcome the "gyroscopic" force tending to keep the disc in the original plane. The disc effectively flies to the new tip path plane. The forces fed back into the swash plate become very large in heavier helicopters and have to be taken care of by hydraulic servos. The helicopter I fly for a living has a dual hydraulic system because the aircraft is unflyable without servo assistance.

 

[dspDad]

Pew,
Thanks for the response. Swash plate forces have minimal (but not necessarily zero) effect on the overall control of a helicopter, but are the only control available in a gyrocopter because the blades have a fixed pitch. But the force required to tilt the blade is the same both in direction and magnitude regardless of how it is applied.

If my assumptions and calculations are correct, then helicopters should be 'squirrelier' as you begin to horse them around in tight turns. Airplanes, by contrast, don't care much how fast you roll into a turn or how steep it is except that the g's get uncomfortable and you lose lift.

If the helicopter has a fly-by-wire or some other compensation built in, then it could handle fairly well even in extreme maneuvers.

From your experience, do helicopters get a lot harder to fly in fast turns and rapid maneuvers than in normal rate turns, and second, do military helicopters (or commercial) have a fly-by-wire that compensates for precession to reduce pilot workload when flown aggressively?

I'm designing a low cost fly-by-wire positive control system for a gyrocopter (strictly a hobby). Most of what I have learned is counter-intuitive, so I appreciate the input from you who fly the real thing.

 

[PEW]

Glad we are singing off the same hymn sheet!

The requirement for an advance angle certainly still applies to an Autogyro. The blades cannot instantly fly up or down to adopt the required disc attitude.

Regarding the Robinson R-22, I have read a lot of discussion about cross-coupling effects caused by a design peculiarity of its flying controls, but as I only flew one once I can't really comment on that aircraft.

Helicopters can't sustain "G" in a turn to any great degree, generally because of the large increase in main rotor torque required to maintain the airspeed. A 45 degree AoB sustained turn is about the most you will ever see on most full sized ones. Any more than that and the speed rapidly bleeds off or you run out of torque, or would exceed the torque limits.

Larger helicopters don't have "fly by wire" as such but they have a Stability Augmentation System (SAS) which takes out the "squirreliness". This involves the use of gyros of some sort, either rate or tied or both, depending on the complexity of the requirement. The ones I fly have laser ring gyros, a bit expensive for a model. As I'm sure you are aware, helicopters are dynamically unstable beasts due to aerodynamic effects on the rotor disc and the "follow through" of the fuselage. You haven't seen "Dutch Roll" until you have tried a large helicopter, SAS out, at speed. however, they can be "tamed" enough to fly coupled approaches, auto hover etc.

This is a very complex subject which I admit goes beyond my level of knowledge, even though I have been flying them for a living for twenty years or so; and despite the fact that I do like to read a bit more basic theory than many pilots seem to. I therefore suggest you read some good books!

I can highly recommend Ray Prouty's books, conveniently named "Helicopter Aerodynamics", "More Helicopter Aerodynamics" and "Even more Helicopter Aerodynamics".
Ray has a way of keeping the mathematics out of his explanations, probably because he knows that maths turns most pilots off after a couple of pages.

Have you heard of the UK based website "PPRUNE"? If you log into that, you can spark off a whole plethora of discussion about this subject on the "rotorheads" page. I'll bet a certain Lu Zuckerman is just waiting for someone like yourself to come along and ask the right question! I would think it would only be a matter of a few hours before he gets involved. Just don't tell them I started you off!

Best of luck!

PW

 

[Sparweb]

PEW: "The helicopter I fly for a living has a dual hydraulic system because the aircraft is unflyable without servo assistance.

There are heavy heli's that can be flown without the hydraulic servos: The Bell 204B and 205A-1 are both flyable when the hydraulics fail - it was a requirement of their civilian certification (don't know if it was a military requirement, too). This is not the case with the Bell 212.

"Dual hydraulics" is a term that confused me until I got somebody to carefully explain it to me. People can mean two things when talking about "dual hydraulics".
a) all hydraulic systems have a backup system,
b) the cyclic and collective controls are boosted by separate hydraulic systems.

In the 204B, you have only b).
In the 205A-1, you have only b) on the collective, and both a)&b) on the cyclic control.
In the 212, you have both a)&b) on cyclic and collective controls.
In each successive model, the gross weight and maximum forward airspeed of the heli goes up significantly, so you can see that Bell was dealing with progressively increasing weights, disc loadings, etc. for each model change, even though the basic airframes are virtually indistinguishable.

I once did a project looking at reducing the control forces in the collective controls of a 212 rotor head. By doing so, it became suitable for installation on a 205A-1 airframe.



STF

 

[PEW]

Sparweb,

Yes, I've flown the 204 and the 212. They both have two rotor blades / teeter rotor systems, which as I understand it, require lower pilot input forces when hydraulics out. I have flown the S-55 Whirlwind hydraulics out too, even that was a bit of a handful.

Much of my time has been on AS330Js and S-76s, both having 4 bladed articulated heads, which are heavier and not at all as docile. I've also flown the S-70 and the CH-53. All of these aircraft use two parallel hydraulic systems, each one controlling both the main rotor and tail rotor servos. A single hydraulic failure on either type means you put the aircraft down asap because there is no hope of controlling the show if the other system fails. (I think the analogy of a very small person trying to control a wild bull elephant on a lead would be appropriate if the unthinkable were to happen).

 

[buni44]

hi dspDAD,
I wouldn't worry too much about precession as the rotor rpm is relatively low around 300 for most helicopters.
the angle mentioned here(I think it was 15 degreees) is a natural angle of the cone due to the aerodynamic forces in forward flight. the angle varies(increases) with speed.
the pilot has control in pitch and roll through the cyclic that in turn controls the swash plate; as the swash plate modifies its angle the control rods are changing the pitch angle of the blade depending on the azimuth. of course the blade is tapered, there is flapping movement (self dampened) and yawing.the equationsin bideminsional for infinite lenght are not very scary and may give a pretty good approx of global performances including some variables suc as the cone angles. for blade loads more complex approach is necessry using the vortex filament theory.

the hydraulics on a medium tonnage helicopter has two functions : supplying the AP hydraulic block and the servounits that controls the swash plate. flying with AP hydraulic off on such a helicopter might be an embarrassing experience for many pilots. some simply don't have what it takes..the servos are bi-chamber type each being supplied by a different system. while with one system operational you may still land safely if not performing a quick turn with complete losss of hydraulic power I doubt that very much.

 

[Intermesher]

Just some comments on phase angle (phase lag).

A basic teetering rotor has a phase angle of 90-degrees. This is the result of gyroscopic precession; or to be more accurate, aerodynamic precession. (I.e. the blade flies to position.)

The Robinson has a phase angle of 72-degrees. This is because its rotor has delta-3.

An absolutely rigid rotor, if it was possible to make one, would have a phase angle of 0-degrees.

 

[dspDad]

Thanks for the comments. I picked up a couple of Ray Prouty's books and another one mentioned as a good reference that hasn't arrived yet.

Intermesher and Buni44 use the term phase-angle. How is phase angle defined? What is delta-3, and why would a rigid rotor have a zero phase angle? It would still have angular momentum.

Thanks,
DspDad

 

[Sparweb]

I don't see how flying the blade to its "new" angle of attack requires exactly 90 degrees. I thought it was dependent on blade mass, torsional stiffness, etc., not an aerodynamic requirement per se.


STF

 

[Intermesher]

dspDad

Short answers to you questions about phase-angle and delta3 can be found on

http://www.UniCopter.com/B329.html.

Both words are linked to more detailed information, if desired.

To be more precise, an absolutely rigid rotor would not have a zero phase angle, but it would be very close. The aerodynamic precession will strive for 0-degree phase-angle while the much weaker gyroscopic precession will strive for 90-degree phase-angle.

Assume that two rotors have their maximum pitch angle at 270-degree azimuth.
On a basic teetering rotor, the blade will climb to its highest point at 360-degree azimuth. The disk will now pull the craft's nose down.
On a absolutly rigid rotor, the blade will also have its maximum lift (pitch) at 270-degree azimuth, but because it cannot teeter or flap it will 'pry' the helicopter to the side.

Phase lag (phase angle) must be adjusted so that the rotor will cause the craft to go in the same direction as the pilot moves the cyclic.

 

[Intermesher]

The above web site address should read;

http://www.UniCopter.com/B329.html

The first word following the address is 'Both'

Is it possible to edit Eng-Tip posts??

 

[PEW]

Quote: "I don't see how flying the blade to its "new" angle of attack requires exactly 90 degrees. I thought it was dependent on blade mass, torsional stiffness, etc., not an aerodynamic requirement per se".

Sparweb,

What you refer to is phase lag - it's a law of physics!!

Rather than an aerodynamic requirement, it occurs much like the way in which a displacing force applied to a gyroscope results in a displacement apparently 90 degrees after it looks like it might. No-one's invented this, it's just the way things happen.

The term "New angle of attack" is a possibly erroneous thing to say.

(We are about to get more complicated... Just one definition here to avoid confusion: Rotor disc = the rotational path taken by the whole rotor system, rather like a spinning disc).

A rotor blade experiences 2 airflow velocities; one due to it's rotation ("rotational flow&quot and the other due to airflow induced to flow through the rotor disc, usually downwards through the disc if the rotor system is in powered rotation. This second flow is known as "induced flow".

These two airflows give a resultant flow, usually called the "relative airflow". The effective angle of attack can only be measured with regard to this relative airflow. Angle of attack actually varies along the length of a straight, untwisted blade due to the difference in rotational velocity proportional to the radius from the hub and localised differences in induced flow across the disc.

It is therefore actually difficult to refer to "THE" angle of attack unless you talk of a very thin slice / element of an individual blade.

As a blade flies up, it experiences an increase in induced velocity (it's upward velocity is added to the downward induced flow velocity) and this has the result of reducing the angle of attack to it's original value. The overall lift produced by the disc is actually the same but the disc now has a different attitude. This is known as equalisation of lift or flapping to equality. It is the secret of controlling the disc and therefore the whole helicopter.

Disc attitude is definitely NOT the same thing as "angle of attack".

One thing worth grasping is that the induced flow is a direct result of the production of lift and the bigger it is, the lower the angle of attack of the blade for the same rotational airflow velocity.

 

[Sparweb]

PEW,

I grok the term "disk attitude". I do tend to think primarily in fixed-wing terms, and it's been a while since I went through a blade-element analysis of a rotor - I've obviously forgotten a few things. Here I am with a small turbine rotor sitting on my desk beside me! What can I say?

I remember now that a teetering rotor tilts to one side due to this principle.

Intermesher,

In the past when I've posted something erroneously, I've red-flagged my own post (with a suitable explanation). Dave Murray is a busy guy, so please don't take up too much of his time on little corrections, but he won't mind at all if you want to remove a bad post - even if it's your own.

STF

 

[GraviMan]

Hmmm, just reread this thread. The more I think about it the more I think there is a fundamental design flaw in the Sikorsky derived swash plate mechanism. This definately leads to a helicopter with tricky flight characteristics. I've done a bit more digging on various rotor hub mechs.

Check out the thread below for my non-mathematucal treatise on the subject: ;-)

http://www.eng-tips.com/viewthread.cfm?SQID=70338&SPID=6&page=1

I take it hydraulic systems in big S-61 type of machines directly control rotor hub orientation?

BTW figured my Ray Prouty book may have gone to an earlier address. Cain't wait to get my brains up to speed with yawl helicopper dudes! Just studyin' fluid dynamics (2nd degree) to warm up. The only trouble is that my "career" led me to design 6x6 trucks...

Mart