R22 longitudinal bungee spring and lateral force trim knob.
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R22 longitudinal bungee spring and lateral force trim knob.
I've searched the Rotorheads archive on this but haven't found a totally satisfactory answer:
http://www.pprune.org/forums/search....archid=2102757
http://www.pprune.org/forums/search....archid=2102863
Robinson are unusual in using delta 3 to correct for coning and inflow roll, as opposed to the mixer box often employed. But why is there a trim knob to apply starboard lateral cyclic? What does the longitudinal bungee spring do?
My confusion here is that it is always described as a force, and not a displacement (unless there is some mild cyclic centreing force).
My thoughts on lateral force trim knob:
This could simply be due to static trim changes at speed - not sure why.
Alternately due to the 72 instead of 90 degree lead angle, with swashplate tilted forewards in forward flight the delta three means the blade at foreward azimuth is pitched a little low and at rear azimuth a little high. This produces a mild port roll in forward flight which needs to be trimmed out.
My thoughts on longitudinal bungee spring:
Absolutely no idea what is does. Anyone help?
http://www.pprune.org/forums/search....archid=2102757
http://www.pprune.org/forums/search....archid=2102863
Robinson are unusual in using delta 3 to correct for coning and inflow roll, as opposed to the mixer box often employed. But why is there a trim knob to apply starboard lateral cyclic? What does the longitudinal bungee spring do?
My confusion here is that it is always described as a force, and not a displacement (unless there is some mild cyclic centreing force).
My thoughts on lateral force trim knob:
This could simply be due to static trim changes at speed - not sure why.
Alternately due to the 72 instead of 90 degree lead angle, with swashplate tilted forewards in forward flight the delta three means the blade at foreward azimuth is pitched a little low and at rear azimuth a little high. This produces a mild port roll in forward flight which needs to be trimmed out.
My thoughts on longitudinal bungee spring:
Absolutely no idea what is does. Anyone help?
Last edited by Graviman; 7th Nov 2007 at 20:35.
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Hi Mart, Enstrom also have a trim system which is a bit more sophisticated than the Robinson one - but the effect is the same. There are 2 springs which control the cyclic centre position which are adjustable in terms of the centre position. The spring that pushes the stick left is much stronger than the right one.
The rotor system in flight imparts a force on the cyclic pushing it strongly to the right. The trim system counters this for the sake of pilot comfort.
Similarly the rotor system strongly tends to push the stick aft and the longitudinal trim opposes it.
I assume the Robinson system works the same way.
Why does the rotor system impart these forces?
No idea. Too much alcohol and too many painkillers tonight to contemplate such weighty matters.
The rotor system in flight imparts a force on the cyclic pushing it strongly to the right. The trim system counters this for the sake of pilot comfort.
Similarly the rotor system strongly tends to push the stick aft and the longitudinal trim opposes it.
I assume the Robinson system works the same way.
Why does the rotor system impart these forces?
No idea. Too much alcohol and too many painkillers tonight to contemplate such weighty matters.
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Hehehe, thanks for the info anyway Gaseous.
Alcohol and painkillers? - so that what gets through those long winter evenings south of the border...
Alcohol and painkillers? - so that what gets through those long winter evenings south of the border...
Graviman - inflow roll exists througout the speed range, not just during the transition where it is certainly most noticeable. Without the lateral trim, you would be forever holding off a stick force (generated by aerodynamic forces on the blades) in the cruise which gets tiring after a while.
I suspect that the longitudinal bungee does mostly the same thing to counter flapback and prevent the nose pitching up in the cruise.
Just buy a proper helicopter with a mixing unit and a cyclic trim system
I suspect that the longitudinal bungee does mostly the same thing to counter flapback and prevent the nose pitching up in the cruise.
Just buy a proper helicopter with a mixing unit and a cyclic trim system
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Thanks, Crab. I realised this morning that the R22 has camber in the blades now. It used to be 63012 symmetrical aerofoils, so i guess it's now 63212.
This means pitching moment will change with AOA, hence speed. That explains the lateral force trim. Much more and hydraulics (or electric actuators) would get a look in.
Still not sure about the longitudinal bungee though. This seems to imply there is a constant out of trim fore-aft force...
This means pitching moment will change with AOA, hence speed. That explains the lateral force trim. Much more and hydraulics (or electric actuators) would get a look in.
Still not sure about the longitudinal bungee though. This seems to imply there is a constant out of trim fore-aft force...
Graviman - the cambered blades would produce a pitching moment on the blades with change of AoA but I don't think this would manifest itself as a roll to port. IIRC the early R22s had the same trim arrangement to cope with the symmetrical blades.
Since the lead angle is 72 instead of 90, surely that just means putting the jacks and the pitch change horn in an appropriate place so that forward cyclic still gives forward disc tilt. There may be some cross-coupling acceleration during manoeuvring but not, I would have thought, in the cruise.
Maybe the D3 can't completely counter the inflow roll and, to give a comfortable control load, Robinson have just used springs and bungees in an effort to provide a sensible cyclic position in the cruise.
Since the lead angle is 72 instead of 90, surely that just means putting the jacks and the pitch change horn in an appropriate place so that forward cyclic still gives forward disc tilt. There may be some cross-coupling acceleration during manoeuvring but not, I would have thought, in the cruise.
Maybe the D3 can't completely counter the inflow roll and, to give a comfortable control load, Robinson have just used springs and bungees in an effort to provide a sensible cyclic position in the cruise.
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Interesting so it is not likely a function of camber pitching moment change, Crab.
One of the reasons you can't increase rotor inertia much on an R22 is that the "wee-wa" angle (not sure what a more technical term is) is dependant on Lock number. At the moment it is within 2 degrees the same as the coning correction. The more i think about the design constraints, the more i start to see the benefit of FBW - assuming it can be made cheap and reliable.
It may just be as simple as not quite getting the D3 quite right for inflow roll. The longitudinal bungee spring is likely just for CG trim. I'll have a look pre/post flight this afternoon.
One of the reasons you can't increase rotor inertia much on an R22 is that the "wee-wa" angle (not sure what a more technical term is) is dependant on Lock number. At the moment it is within 2 degrees the same as the coning correction. The more i think about the design constraints, the more i start to see the benefit of FBW - assuming it can be made cheap and reliable.
It may just be as simple as not quite getting the D3 quite right for inflow roll. The longitudinal bungee spring is likely just for CG trim. I'll have a look pre/post flight this afternoon.
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Without going into the fairly simple schematics of it, you will notice that both the H269 and Hiller 12E's have electric beep trim switches which are activated to take the weight off cyclic, laterally, as cruise speed is reached. This is despite the apparant "crooked rigging" of the H269, especially when viewed from behind in the vertical plane, to try to counter act this phenomina of forward flight.
IMHO that more sophisticated aircraft, such as the Bell 47, simply installed hydraulic controls and the pilot simply directs the course of action without even having to think about it.
I note also that later variants of nearly all helicopter designs have these "sophisticated hydraulic controls".
This means of course that modern pilots have less to worry about in the dull routine of translating to cruise airspeed.
There's lots to be said for the old '47.
IMHO that more sophisticated aircraft, such as the Bell 47, simply installed hydraulic controls and the pilot simply directs the course of action without even having to think about it.
I note also that later variants of nearly all helicopter designs have these "sophisticated hydraulic controls".
This means of course that modern pilots have less to worry about in the dull routine of translating to cruise airspeed.
There's lots to be said for the old '47.
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Interesting TET, i didn't realise the Bell 47 had hydraulics now. I was considering the possibility of an electric actuator in an R22 size helicopter. These are becoming cost effective now, and don't suffer contamination and wear problems of hydraulics. I suspect electric actuators will start to make an appearance soon. Anyone know what R66 uses for SAS?
Crab, not much too see - all hidden away behind panels. Cyclic longitudinal bungee is probably simply there, as you say, to reduce flapback trim force. Still there is something to be said for the minimal number of parts count in the R22 design.
Crab, not much too see - all hidden away behind panels. Cyclic longitudinal bungee is probably simply there, as you say, to reduce flapback trim force. Still there is something to be said for the minimal number of parts count in the R22 design.
Graviman - I believe Lock number is a measure of rotor inertia and changes with density altitude as aerodynamic damping reduces.
I tried to get my head round wee-waa when Lu Zuckerman argued about it but it made my brain hurt
You might just have to ask Frank Robinson, I'm sure he's got an email address somewhere.
I tried to get my head round wee-waa when Lu Zuckerman argued about it but it made my brain hurt
You might just have to ask Frank Robinson, I'm sure he's got an email address somewhere.
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Crab,
Lock Number represents the ratio between blade's aerodynamics forces and inertia forces. As density altitude increases, then aero forces will reduce. I'll post the formula later if you're interested - i don't have Prouty to hand.
Wee-wa works since the rotor system can be considered a gyroscope, independant of the airframe. When you say pitch forewards, you create a moment 90 degrees azimuth ahead (i have no doubt you already know all this). The moment causes the rotor to have a constant pitch velocity, dependant on cyclic position. The fuselage follows as the TPP lift vector moves away from shaft axis - in fact apart from flapback and fast manouvres, TPP follows axis plane closely.
However as the rotor is nutating in say forewards pitch, the front portion is seeing a higher AOA than rear portion. So just like inflow roll there is an associated starboard roll for american rotation. Robinsinson take 18 degrees out from the pitch link to dial in a bit of left cyclic. This conveniently is close to coning roll correction, but (as we've discussed) they didn't quite account for inflow roll.
As a professional design/analysis engineer i'm always uncomfortable explaining
various aspects of aircraft design to professional pilots, but always do it with the best intention. Please feel free to correct me.
Lock Number represents the ratio between blade's aerodynamics forces and inertia forces. As density altitude increases, then aero forces will reduce. I'll post the formula later if you're interested - i don't have Prouty to hand.
Wee-wa works since the rotor system can be considered a gyroscope, independant of the airframe. When you say pitch forewards, you create a moment 90 degrees azimuth ahead (i have no doubt you already know all this). The moment causes the rotor to have a constant pitch velocity, dependant on cyclic position. The fuselage follows as the TPP lift vector moves away from shaft axis - in fact apart from flapback and fast manouvres, TPP follows axis plane closely.
However as the rotor is nutating in say forewards pitch, the front portion is seeing a higher AOA than rear portion. So just like inflow roll there is an associated starboard roll for american rotation. Robinsinson take 18 degrees out from the pitch link to dial in a bit of left cyclic. This conveniently is close to coning roll correction, but (as we've discussed) they didn't quite account for inflow roll.
As a professional design/analysis engineer i'm always uncomfortable explaining
various aspects of aircraft design to professional pilots, but always do it with the best intention. Please feel free to correct me.
Last edited by Graviman; 10th Nov 2007 at 17:58.
I am afraid we might start the old 'is the rotor a gyroscope or not' argument again if we are not careful. I don't think it precesses or nutates - I believe the phase lag is due to aerodynamic forces. I know that many across the pond firmly believe it is gyroscopic force - it just depends where you were taught and when.
I think your explanation of Lock number is the same as mine - the rotor inertia doesn't change but the aerdynamic damping does and therefore the ratio and thus the Lock number changes with density altitude. I know this can cause changes in rotor response because the phase lag angle varies with Lock number and this results in pitch/roll cross couples.
Surely, as you apply forward cyclic, the forward moving blade experiences a reduced AoA, which is why it moves down.
I think your explanation of Lock number is the same as mine - the rotor inertia doesn't change but the aerdynamic damping does and therefore the ratio and thus the Lock number changes with density altitude. I know this can cause changes in rotor response because the phase lag angle varies with Lock number and this results in pitch/roll cross couples.
Surely, as you apply forward cyclic, the forward moving blade experiences a reduced AoA, which is why it moves down.
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Crab, you are quite right about gyros and Lock no. Aerodynamic forces do swamp inertia forces.
Prouty explains it best when he explains blade as being in resonance as it goes around hub, due to pendulum forces. Basically from a blades perspective it is pitching sinusoidally as it increases azimuth. It doesn't matter whether you consider inertia or aeroforces, max pitch will cause max upwards velocity min pitch will cause max downwards velocity.
The only real factor on pitch lead angle (outside of coning, inflow, weewa etc) is blade natural resonant frequency modification to flapping frequency. Indeed changing the Lock number changes the damping of the resonance, so alters the azimuth lead angle slightly.
Prouty explains it best when he explains blade as being in resonance as it goes around hub, due to pendulum forces. Basically from a blades perspective it is pitching sinusoidally as it increases azimuth. It doesn't matter whether you consider inertia or aeroforces, max pitch will cause max upwards velocity min pitch will cause max downwards velocity.
The only real factor on pitch lead angle (outside of coning, inflow, weewa etc) is blade natural resonant frequency modification to flapping frequency. Indeed changing the Lock number changes the damping of the resonance, so alters the azimuth lead angle slightly.
Last edited by Graviman; 10th Nov 2007 at 18:02.
