Our Good Friend Cyclic Right Trim

[blave!]

So we all love the cyclic right trim on the R-22. Kind of makes your arm say thank you on longer flights by relieving the tendency of the cyclic to pull left in high speed flight, all with a simple, little, elastic cord. The question is what are the aerodynamic forces at work precisely that cause the left stick forces we negate by pulling the trim?

[FlyingDodo]

The question is what are the aerodynamic forces at work precisely that cause the left stick forces we negate by pulling the trim?

 

A short answer from a rookie:

Reverse Flow. Higher airspeeds create a larger area of reverse flow on the retreating blade.

Less surface area to create efficient lift on the left side. Left roll ensues. Right cyclic input.

Sore shoulder. Pull trim. Happy shoulder.

[PhotoFlyer]

Think simpler. The rolling tendency is compensated for by blade flapping, and it is not caused by the stall or reverse flow region. If there was a left rolling tendency it would be caused by Dissymmetry of Lift, and would only occur if the rotor system was unable to flap.

 

The R-22 is rigged so there is little lateral control pressure in a hover. Hover = high translating tendency.

 

Move into forward flight and translating tendency is reduced (less left pedal), but the cyclic is still trimmed for a hover (trimmed to the left), hence the pull. Make sense?

[apiaguy]

photoflyer is correct... that dang tail rotor is pushing you to the right (in US helicopter) and robinson has set up the helicopter to have low forces in the hover

Boy that cyclic trim is nice in the 300.

[spw1177]

photoflyer is correct... that dang tail rotor is pushing you to the right (in US helicopter) and robinson has set up the helicopter to have low forces in the hover

Boy that cyclic trim is nice in the 300.

 

 

Actually, Photoflyer is wrong. I took my CFI checkride with Bob Golden who was one of the test pilots for the R22. If you have been to the safety course and heard the story, he was the guy who had to ditch one of the prototypes off the coast. He told me that most of the left roll that is produced by dissemtry of lift is actually taken up by the pilot correcting with the flight controls (at Vne it is something like 80%). Only a small portion of the force is cancelled out by flapping. And he got his info from the guy who designed the helicopter.

[PhotoFlyer]

Can't really argue with that, but if 80% is taken up by the pilot then why bother with flapping hinges?

[apiaguy]

yeah, I don't know where my brain was last night.... When I read the thread I put my response thinking the original question was asking about left cyclic needed to counteract translating tendancy in the hover.

[FlyingDodo]

Guess I should know better than to be the first to respond to a technical/aerodynamics question.

Rookie indeed.

 

Yes, flapping is supposed to solve dissymmetry of lift (apparently inefficiently in the R22, according to spw1177) but reverse flow is still always encountered at higher airspeeds right? And while that "reversed" blade section still produces some lift, it's almost negligible in the overall equation, so why wouldn't this create at least some left roll? I'm thinking of the Lift formula here: it's not different blade velocity causing the problem, it's surface area. Almost like dissymmetry of lift with a different cause.

That's how my thinking goes, anyway. Maybe someone can set me straight before I teach it wrong.

[spw1177]

Can't really argue with that, but if 80% is taken up by the pilot then why bother with flapping hinges?

 

 

The flapping hinge also relieves some of the forces involved with dissemtry of lift that would otherwise be applied to the rotorblades. If you observe a helicopter flying straight at you at high speed you will notice the rotor disc is not level with the helicopter or the ground for that matter. So without a flap hinge the blades would have to bend up and then down for every revolution to keep the disc at that tilt, that is what happens on a helicopter with a rigid head, which places a lot of stress on rotorblades. Rigid rotorheads have composite blades that can handle the repeatitive bending, but metal blades would fatigue and eventually fail.

Edited November 13, 2007 by spw1177

[PhotoFlyer]

The flapping hinge also relieves some of the forces involved with dissemtry of lift that would otherwise be applied to the rotorblades. If you observe a helicopter flying straight at you at high speed you will notice the rotor disc is not level with the helicopter or the ground for that matter. So without a flap hinge the blades would have to bend up and then down for every revolution to keep the disc at that tilt, that is what happens on a helicopter with a rigid head, which places a lot of stress on rotorblades. Rigid rotorheads have composite blades that can handle the repeatitive bending, but metal blades would fatigue and eventually fail.

 

The disc not being level with the helicopter is a matter of design and CG. As for the disc not being level with the ground, why would it be? If the helicopter is in forward flight it wouldn't keep going forward very long if you leveled the disc. It is displaced laterally because of translating tendency and dissymmetry of lift, which is really what you said. I guess I really don't understand why you mentioned disc level. I may be missing something, and if so, please explain. And again, as for 80% being taken up by the pilot in a Robinson, I could argue the point, but if thats what Robinson says then I don't see the point in discussing it. No reason not to believe it, unless someone else has information that says otherwise.

[FUSE]

I thought that the reason we have trim was because of translating tendency. Because of translating tendency the rotor mast is tilted slightly to over come this during hovering manuvers, but once in flight you don't have translating tendency so you put the trim on to negate the fact that the rotor mast is tilted.

[PhotoFlyer]

I thought that the reason we have trim was because of translating tendency. Because of translating tendency the rotor mast is tilted slightly to over come this during hovering manuvers, but once in flight you don't have translating tendency so you put the trim on to negate the fact that the rotor mast is tilted.

 

Of course there is still translating tendency in forward flight. You are using less left (or right if thats your flavor) pedal because the tail rotor is more efficient in forward flight (ETL), but the tail rotor is still helping to control yaw. Otherwise how would you explain the left skid low tendency of the helicopter in forward flight. It couldn't be attributed to dissymetry of lift because then the bank would cause a turn (unbalanced forces). Of course if your helicopter has an offset mast it wouldn't be left skid low in forward flight.

[FUSE]

Of course there is still translating tendency in forward flight. You are using less left (or right if thats your flavor) pedal because the tail rotor is more efficient in forward flight (ETL), but the tail rotor is still helping to control yaw. Otherwise how would you explain the left skid low tendency of the helicopter in forward flight. It couldn't be attributed to dissymetry of lift because then the bank would cause a turn (unbalanced forces). Of course if your helicopter has an offset mast it wouldn't be left skid low in forward flight.

 

I don't know, I disagree a little. I don't think that the tail rotor is more efficient beyond ETL. You don't need as much pedal because of the weather vaining affect that happens. Same reason that if you lose your tail rotor you can continue forward flight. Dissymety of lift is not enough to cause an actual turn, it's not that extreme. I mean, when you have retreating blade stall the ship doesn't all of the sudden make a turn to the left. The left low skid tendency is because the pilot doesn't put enough right lateral cyclic.

 

I don't think you have translating tendency in forward flight above ETL, that sort of goes against the definition. I also thought that most helicopters tilted their masts a couple of degrees to help with translating tendency. And all the tilting of the mast does is make it so during a hover you are able to have your cyclic centered. But once you go above ETL and the translating tendency is no longer there then now the cyclic is off center and your always pulling it to the center. So the trim puts that pressure on the cyclic for you so it stays centered.

[betr_thn_Icarus]

I was told that the R22 has this right trim adjustment because the rotor mast is designed slightly off horizontal CG to compensate for pilot weight. ...maybe when solo can't remember

 

-SOK

[PhotoFlyer]

Same reason that if you lose your tail rotor you can continue forward flight.

 

How do you know if your tail rotor has failed? Assuming a complete failure (not just stuck pedals, or broken control system), the helicopter would yaw to the right. If the tail rotor wasn't doing anything, why would it yaw? And if the tail IS doing something, why would translating tendency go away?

[FUSE]

If the tail rotor wasn't doing anything, why would it yaw? And if the tail IS doing something, why would translating tendency go away?

 

Well, the first question, I believe that the tail rotor is doing something... Now since I believe that the tail rotor does assist in keeping you in trim I can't explain why translating tendency goes away. I just believe that it does because of the definition of Translating Tendency.

 

Translating tendency causes a hovering helicopter to move laterally in the direction of tail rotor thrust.

 

I have never read anything that says there is translating tendency during flight.

[IFLY]

Where are your pedals in an auto? That should be neutral thrust because no more torque to overcome. When your flying straight and level you have close to even pedals showing the tailrotor is still creating thrust laterally. You still have translating tendency during flight.

[spw1177]

Photoflyer

 

The disc not being level with the helicopter is caused by dissemtry of lift, the advancing side will fly higher (more lift) than the retreating side (less lift). The reason I mentioned it and it may not be clear from my previous post, is that the flapping hinge prevents the rolling force caused by dissemtry of lift from being directly applied to the mast. The primary function of a flapping hinge is not to equal out the lift produced on the advancing and retreating sides (it helps but only just), but to keep the helicopter from flying in a continous circle towards the retreating side in forward flight. You can't transfer a moment through a hinge. This doesn't, of couse, cancel out that rolling force, but keeps it from directly acting upon the helicopter. This in turn takes a lot of stress of of the helicopters components.

 

Fuse:

 

No mast tilt in an R22. It was a matter of debate when I was getting my CFI, so one of the other students called the factory and confirmed no tilt.

[southernweyr]

Yeah, no lateral tilt in the R22 or the Schweizer, but if your assistant chief pilot thinks otherwise don't say anything because you might get fired.

[FUSE]

Where are your pedals in an auto? That should be neutral thrust because no more torque to overcome. When your flying straight and level you have close to even pedals showing the tailrotor is still creating thrust laterally. You still have translating tendency during flight.

 

I think your totally off here.

 

When flying straight and level you don't have the pedals close to even because the tail rotor is creating thrust laterally. You have them close to even because the main rotor is creating torque.

 

So I'm curious why does the definition specifically state hovering? And I learned to correct for translating tendency it was cyclic input needed, not pedal inputs. The pedals are not used to correct for translating tendency from what I know. Below is how the Rotorcraft Flying Handbook explains translating tendency.

 

TRANSLATING TENDENCY OR DRIFT

 

During hovering flight, a single main rotor helicopter

tends to drift in the same direction as antitorque rotor

thrust. This drifting tendency is called translating tendency.

[Figure 3-2]

To counteract this drift, one or more of the following

features may be used:

 

• The main transmission is mounted so that the

rotor mast is rigged for the tip-path plane to

have a built-in tilt opposite tail thrust, thus producing

a small sideward thrust.

 

• Flight control rigging is designed so that the

rotor disc is tilted slightly opposite tail rotor

thrust when the cyclic is centered.

 

• The cyclic pitch control system is designed so

that the rotor disc tilts slightly opposite tail

rotor thrust when in a hover.

 

Counteracting translating tendency, in a helicopter

with a counterclockwise main rotor system, causes

the left skid to hang lower while hovering. The opposite

is true for rotor systems turning clockwise when

viewed from above.

[FUSE]

Fuse:

 

No mast tilt in an R22. It was a matter of debate when I was getting my CFI, so one of the other students called the factory and confirmed no tilt.

 

That's good to know, I never knew if for sure the R22 had the tilt, where I learned that was from the Rotorcraft Flying Handbook. It sounds like to me from that book that maybe the R22s flight control rigging is designed so that the rotor disc is tilted slightly opposite tail rotor thrust when the cyclic is centered.

[PhotoFlyer]

Well, the first question, I believe that the tail rotor is doing something... Now since I believe that the tail rotor does assist in keeping you in trim I can't explain why translating tendency goes away. I just believe that it does because of the definition of Translating Tendency.

 

Translating tendency causes a hovering helicopter to move laterally in the direction of tail rotor thrust.

 

I have never read anything that says there is translating tendency during flight.

 

Using that same logic: I have never read anything that says there is no tranlating tendency in forward flight.

 

A force is a force, and newtons 3rd law still applies. If the tail rotor is counteracting torque, or just helping to keep the helicopter in trim, it is creating thrust. A hovering helicopter will drift in the direction of that thrust, a helicopter in forward flight would slip in the direction of that thrust. Just because you never read it, doesn't mean it isn't true.

 

spw177:

 

Your right, a moment can't be transfered through the flaping hinge...Unless there is centrifugal force resisting movement about that hinge.

 

Lets look at another area of helicopter aerodynamics to examine this.

 

Transverse Flow Effect is caused by a difference in lift on the front and rear portions of the rotor. This difference causes the blades to flap. However, left uncorrected the helicopter will pitch up and roll to the right because of transverse flow. The pitch is a result of the upward flap of the forward blades. So why was that moment transmitted?

 

I am not disputing that in the R-22 the pilot counteracts dissymetry of lift. If Robinson said it I don't need to dispute it, they designed the helicopter.

 

I'd also like to point out that if you couldn't transmit moments through the flapping hinge to the helicopter than it wouldn't be able to maneuver. After all, don't you adjust the disc position with the cyclic to cause the helicopter to turn? Or is this a case where only the moments we want to be transmitted will be?

[spw1177]

I'd also like to point out that if you couldn't transmit moments through the flapping hinge to the helicopter than it wouldn't be able to maneuver. After all, don't you adjust the disc position with the cyclic to cause the helicopter to turn? Or is this a case where only the moments we want to be transmitted will be?

 

 

In a helicopter with a semi-rigid head no rolling or pitching moment is transmitted from rotor head to the mast. The rotor system pulls and the fuselage follows the lift vector. If you tilt the disc right the lift vector pulls right and the fuselage is pulled along with the rotor disc. This is why teeting heads have problems in low-G conditions, because when the disc is unloaded there is no tendancy for the fuselage follow the lift vector. No moment across the hinge and still able to maneuver.

 

I would also like to add to the translating tendancy discussion. You both seem to be discounting the anti-torque component generated by the vertical stabilizer. The force it generates conteracts much of the engine torque when the helicopter is in forward flight (dependant on airspeed). This is a large part of why that at higher forward airspeeds the less power pedal is needed (yes the tail rotor also goes through ETL and becomes more efficent which is also a component of this). The tail rotor is producing some thrust, but much less than it does in a hover. It not called translating tendancy in forward flight, because translating tendancy only happens in a hover by definition, but sideward thrust produced by the tail rotor still has some affect on how level the helicopter flys in forward flight. But the CG has a much greater affect on which skid hangs lower in forward flight.

 

And to the pedal position in an auto discussion. Again remember the vertical stabilizer. The engine is not longer appling torque to the main rotor, but the vertical stabilizer is still trying to yaw the aircraft to the left. So right pedal is required to fly the aircraft in trim. The vertical stabilizer is the reason that the helicopter remains controllable at a high enough forward airspeed. But, it will still yaw to the right a little because it has lost the anti-torque component supplied by the tail rotor.

Edited November 17, 2007 by spw1177

[southernweyr]

The "flapping hinges" on the Robinson helicopters are there so that Robinson could patent the rotor system. The true flapping hinge is the teeter hinge in the middle.