Tertiary effect of pedal

[GipsyMagpie]

Here's a puzzle for you:

In forward level flight in a US helicopter with AP and SAS off you put in a little pedal (let's say left). The aircraft yaws left, then it rolls left. Then it pitches...but which way?

Then you try it with right pedal...what happens?

Better still, why?

[helimutt]

So come on then, tell us and save us the hassle of going out tomorrow and trying it. Not that i'd suggest taking AP and SAS out and leaving go of the controls to find out.

[Ascend Charlie]

Nose down both times. Don't fall for the myth that it will pitch up if you yaw right, it just doesn't happen.

[army_av8r]

May I ask a few questions? Are we assuming that the controls are being held in the position required for level flight prior to the intentional upset? And, is the aircraft allowed by the handling pilot to depart from the entry altitude?

[G0ULI]

I suspect this is one of those trick questions intended to demonstrate why gyroscopes turn 90° to applied force etc.

The gyroscopic force exerted by the rotor may impart some initial pitch up or down, but the aerodynamic forces acting on the whole aircraft are what counts.

Stick some rudder in on a fixed wing aircraft and it yaws, then rolls, then pitches down as lift is lost. Basic 101 flying lesson. Why should rotary craft be any different?

[army_av8r]

assuming all things remain equal, a left yaw due to large pedal input will cause the obvious yaw, this will put the relative wind off the right side of the aircraft as the nose swings left. The right crosswind will cause the rotor to blow back to the left rear. This will cause that left roll, and then as the aircraft rolls, it will begin to descend. This vertical airflow across the tail will cause a pitch down due to the building descent and upward airflow. Right pedal will be the same except that all the roll/ yaw goes right. There is more to say but I'll leave it out for now and see where this goes.

[Ascend Charlie]

Oh dear, Gouli still believes in gyroscope theory.

[GipsyMagpie]

May I ask a few questions? Are we assuming that the controls are being held in the position required for level flight prior to the intentional upset? And, is the aircraft allowed by the handling pilot to depart from the entry altitude?
pedal held with initial input, collective and cyclic fixed. the pitch occurs within 3-5 seconds so altitude hasn't changed much.

[GS-Alpha]

I'm a pretty inexperienced helicopter pilot, but I'll have a go as a Physicist and say the reasons are all about the new 'direction of travel' air passing across the disc and therefore incorrect positioning of cyclic to counteract the flapback. You're initially in equilibrium with cyclic in the correct position to counteract all flapback. Applying pedal in either direction causes that counteraction of flapback to now be in the wrong direction because you'd need it to be more in the direction of the slip. This now means you haven't got enough roll input in the direction of the 'out of balance condition' airflow, so you flapback from that new direction which gives roll in the same direction as the pedal input. Similarly, you've also still got too much forward pitch counteracting flapback that is now no longer coming from quite the same direction as it was in the balanced condition, so the nose of the aircraft pitches down. This will happen with either pedal input and indeed with either direction rotor travel.

[GipsyMagpie]

Stick some rudder in on a fixed wing aircraft and it yaws, then rolls, then pitches down as lift is lost. Basic 101 flying lesson. Why should rotary craft be any different?
Think about your basic lift theory of the main rotor in forward flight. The main difference is that the advancing and retreating blades are in completely different environments. Hopefully if you do fly helicopters you understand flapping to equality, dissymmetry of lift, flapback, inflow roll, inherent sideslip...so yes, rotorcraft are different.

[GipsyMagpie]

I'm a pretty inexperienced helicopter pilot, but I'll have a go as a Physicist and say the reasons are all about the new 'direction of travel' air passing across the disc and therefore incorrect positioning of cyclic to counteract the flapback. You're initially in equilibrium with cyclic in the correct position to counteract all flapback. Applying pedal in either direction causes that counteraction of flapback to now be in the wrong direction because you'd need it to be more in the direction of the slip. This now means you haven't got enough roll input in the direction of the 'out of balance condition' airflow, so you flapback from that new direction which gives roll in the same direction as the pedal input. Similarly, you've also still got too much forward pitch counteracting flapback that is now no longer coming from quite the same direction as it was in the balanced condition, so the nose of the aircraft pitches down. This will happen with either pedal input and indeed with either direction rotor travel.
Good logic except one point. The aircraft might pitch down eventually because of loss of lift/airspeed, but not initially in one direction.

Just consider I am talking about a small lateral airspeed here (sideslip). First I get (lateral) flapback. Then what? Think about what comes after flapback when we have a small forward speed....

And if you nay sayers say the aircraft always pitches down I bet you it doesn't - and I'd wager the effect is worse the more power you pull.

And before anyone says don't try this at home, I'll say just that.

[jellycopter]

I've demo'd this effect hundreds of times in AS350. A left yaw produces pitch down, a right yaw produces momentary pitch up followed by pitch down. The opposite is true for CCW rotors.

In my opinion, the cause is flapback as a consequence of RRPM changes. Bear with me here while I offer up some ball park figures.

If the helicopter is yawed, let's say at 30 degs per second, that will give an increase or decrease in RRPM of 5rpm. If yawing with the direction of rotation, the RRPM rises, if yawing against the direction of rotation, RRPM falls. This RRPM change is relative to your position in space, and the surrounding air. It is not sensed, or corrected by governor as the RRPM sensed by the governor is in relation to the aircraft and this hasn't changed.

So when yawing with the direction of rotation, the RRPM rises leading to increased lift on the advancing blade and reduced lift on the retreating blade. This effect is noticed at approx 90 degs due to phase lag and voila, the nose pitches up momentarily. The effect is short-lived as the yawing is short-lived; as soon as the yawing motion stops, the RRPM (in space) return to normal.

The same effect on RRPM can be observed in the hover when yawing left or right resulting in a slight climb or descent.

[GS-Alpha]

Ok. Lateral flapback occurs which is not counteracted. This causes the disc to be more oblique to the wind. The angle of attack of all blades has just increased due to the same collective being maintained along with the same cyclic but relative airflow coming from more under the disk, causing more flapback so continued roll. This slows the airspeed, reducing the amount of flapping to equality due to the disymetry of lift reducing. Therefore the rearward (left relative to direction of travel) blade will climb and the right descend. So if left pedal, you get pitch up and if right pedal, pitch down? I guess as airspeed reduces, the yaw will also increase.

[[email protected]]

I think it will differ from aircraft to aircraft and is a function of the airflow across your horizontal stabilisers on the tail. Some aircraft have two on the tail boom, some have one big one on the tail.

You would think that accelerating the horizontal stab (which is what you are doing when you yaw it) would increase its Vsquared and then produce a nose pitch up (since it is designed to minimise fuselage attitude changes as speed increases) and on some aircraft I think this is exactly what happens, even if the effect is temporary as Jellycopter indicates.

The downwash on each side of the disc in forward flight is not the same because of the AoA distribution according to Prouty.

Additionally, on aircraft with two horizontal stabs there may be different trim tabs, gurney flaps etc.

So a combination of stab design and downwash variations, I believe, means that the nose will pitch up (perhaps only briefly) yawing one way and pitch down going the other.

Many aircraft require subtly different attitudes to maintain speed in turns from one way to the other which may well be due to the same effects.

[JohnDixson]

And it gets more complex when considering a canted tail rotor. Stabilator sensor inputs on the S-70 for example, use, in addition to the obvious airspeed and collective position, pitch rate and lateral acceleration. The result is effective: if everything else in the SAS/AFCS system is shut off and the stabilator is left active, pushing on either pedal results in a pretty flat pitch attitude response.

[G0ULI]

I thought I had discounted gyroscopic action in my post. Obviously I wasn't clear enough.

I am at a bit of a loss to understand how this materially affects the handling of a helicopter from a pilot's point of view if the effect would seem to be automatically compensated for by the normal pilot reactions when a deviation from altitude and course is detected.

I can see how this effect might have very meaningful consequences during inadvertant entry into IMC without an instrument rating and suitable panel.

It would be nice to hear how this seemingly counter intuitive nose down attitude in response to yaw/rudder input affects flight operations in a real life scenario. Are all rotary wing craft purposely designed to react this way so the pilot response is identical no matter which way the aircraft yaws?

[Ascend Charlie]

The "Effects of Controls" exercise is demonstrated by holding all controls in a fixed position, while the one that is to be demonstrated gets moved.

The instructor is not correcting for any primary or secondary effects, just holding everything else fixed while the poor little unstable beast gyrates itself into an ever-worsening situation. Hence the description of "stick-fixed instability".

In real life, Bloggs feeds in some pedal to achieve some aim, and he has to feed in all the other controls to some extent to keep things the way he wants them.

And as far as gyroscopes go, helicopters don't. It is merely a simple and understandable way of describing some things that happen to a helicopter. But gyroscopic, it ain't. Gurgle up some of Nick Lappos' posts from years ago, including one called "Helicopter urban myths."

[GipsyMagpie]

It would be nice to hear how this seemingly counter intuitive nose down attitude in response to yaw/rudder input affects flight operations in a real life scenario. Are all rotary wing craft purposely designed to react this way so the pilot response is identical no matter which way the aircraft yaws?

It doesn't with stab out, that's my point - it's assymetric. When the SAS is in it doesn't happen because it's damped out. For a pilot who inadvertently dumps the SAS in cloud or loses it during an electrical snag, this tertiary effect could be confusing particularly as the effect is assymetric. With good references you automatically compensate. Not so easy IMC.

[G0ULI]

Thank you, that makes sense to me now.