why does the nose pitch forward when you lower collective?

[helmet fire]

Sorry TC for my delayed response.
My point re the Huey and Black Hawk was that as slicks they behave one way, then with gunship fit and ESSS respectively, they behave another. Neither of these fitments in any way affects the control mixing nor control to stab mixing. In the case of the Huey, there is no control mixing at all, nor any coupling between collective and stabiliser. So do you have another solution to my "dilema" other than the one I proposed?

[Droopystop]

TC,

I am missing something here:

Lift = 1/2 x rho x v squ'd x S x CL

I agree the only thing that changes is CL by virtue of the change in collective pitch. At each point of azimuth, the rho, V and S remain the same before and after collective input. Since only collective pitch is changed, the pitch angle changes by the same amount all around the disc. Assuming that the angle of attack is directly proportional to the pitch angle and that the CL is directly proportional to angle of attack, then the CL reduces by the same amount all the way around the disk. Is that not what you are saying?

So if the CL is halved (say), then the lift is halved, all the way around the disk since all the other parameters are constant. So how can there be a new dissymetry of lift to cause flapping?

As for the horizontal component of thrust vs parasite drag.....

Point taken. Got an arrow going the wrong way.

[cl12pv2s]

TC,
No one is disagreeing that the change in magnitude is differerent.

Just wondering if you agree with this statement?

"With respect to the following formula [Lift = 1/2 x rho x v squ'd x S x CL] (and notwithstanding parastic drag, horizontal stab angle, CofG), a collective change in AoA will result in an unequal change in the 'magnitude' of lift on both sides, but the ratio of lift left side to right side will remain constant.

This is what I and Droopy are saying. If the ratio remains the same, than the equation is balanced. Surely the correct 'cyclic' trim is dependant on the ratio of the lift on both sides.

break...break...

Nick,
You said that it is all to do with a whole number of factors that are complexly integrated to produce this phenomenon we talk of. I agree with you here. However, you say that the horizontal stab 'angle change is the greatest' factor.

My question is, isn't the 'horizontal tail angle change' a result, rather than a cause? I don't discount that the horizontal tail is the main factor in determining aircraft attitude in flight. That's why they are utilised in certain aircraft as the main adjustment of attitude (ESSS) and almost every medium and heavy jet*.

However in aircraft without the ability to change the AoA of the horizonatal tail, the phenomenom still exists. Any AoA change must be a result of the nose drop and not a causal factor.

*Last point, has anyone put this question to a bunch of plank-divers? Same question to plank drivers!

cl12pv2s

[[email protected]]

cl12pv2s - your assertion seems correct but only if you assume the CL line is constant and linear - if a change from 4 -2 degrees pitch on the advancing side produces the same change in CL as a change from 12-10 degrees pitch on the retreating side does, then your ratio does stay the same. Does this actually happen - without having the CL curve for the aerofoil section it is difficult to say but I expect Nick has some general principles to apply.

However, I am with you completely on the tail angle being a symptom and not a cause. Nick is implying that ETPS and many other good textbooks are wrong about the raising and lowering of the lever causing pitch up and down.

[NickLappos]

cl12pv2,
Remember, angle of attack is what the air sees, not what your eye sees. A rate of descent causes the air to come from below the horizontal tail, so the AOA increases, as does the upward lift. This makes the nose go down, and is the biggest contributer to the nose down effect of collective down, IMHO.

[IntheTin]

Now that answer makes sense, thanks. Straight forward with no complications!!!

[FanPilot]

Originally Posted by NickLappos

cl12pv2,
Remember, angle of attack is what the air sees, not what your eye sees. A rate of descent causes the air to come from below the horizontal tail, so the AOA increases, as does the upward lift. This makes the nose go down, and is the biggest contributer to the nose down effect of collective down, IMHO.

There you go, and hands up who thought my response was totally wrong. (Thanks Nick, I am quite chuffed you more or less agree with my response)

Something you might try, which was demonstrated to me when I was in california (in an R22) is flying backwards at a reasonable rate of knots. What happens when you move the stick forward? (If you do try it, be careful, ease the stick)

This is the main reason I thought about the horizontal stabiliser. You don't realise how much force it does have until you do a manouver that catches you out.

However, I did ask my flight school what it is that causes it, as there were quite a few replies here. They said that it was due to reduced downdraft on the tail boom. I commented that I thought it was the horizontal fin. They said that there is likely to be a combination of reasons, but primarily the downdraft.

FP.

[[email protected]]

But FP, as you reduce the downwash on the tail boom, you also reduce the downwash on the fuselage that is forward of the rotormast and on many aircraft that is of a bigger area than the tail boom.

Nick, I am sure what you say is right, but only once the RoD is established - the nose down pitch when lowering the lever is instantaneous - the effect of the tail stabiliser surely is most apparent after the nose has dropped, the speed has increased and then the change in AoA on the stabiliser acts to bring the nose back up, giving speed stability.

Especially since most horizontal stabilisers are upside down wings and an increase in AoA will give a pitch nose up.

[Matthew Parsons]

crab,

The ETPS book focuses on the coning and flapping response, but does mention fuselage and tailplane as subsequent effects.

I think most here are talking about the control derivative, or the pitching due to collective cross couple. I believe that is primarily a coning and flapping effect as discussed.

Others are talking about the pitching due to a vertical disturbance (following the collective input), which you could argue is a secondary effect, but unless you're trained to seperate the effects both appear to happen together.

Therefore I think it is important to consider the whole picture. Don't think of the helicopter as many isolated structures, but as a whole, complex aerodynamic problem.

[cl12pv2s]

Nick,

While FanPilot seems to be very happy to slap his own back, I would go so far to say that I can see your point about the AoA on the horizontal stab (I admit I didn't see it from that angle originally). In fact I'd say that it is the most plausable explanation so far, and I am struggling to find a problem with it! However, I still have a couple of niggles. I haven't had time to think them through properly. So at this point, I'm not disagreeing with you!

First of all, just to clear this up...

Especially since most horizontal stabilisers are upside down wings and an increase in AoA will give a pitch nose up.

Crab, for a wing which is 'inverted' a RoD would cause a decrease in AoA. This would mean less 'upside-down' lift and would have the same result as Nick suggests.

However, I am wrestling with the notion that the pitching down happens before a rate of descent is sufficiently established to act on the horizontal stab as you suggest. I'm sure the horizontal stab is contributory (and probably even the determining factor to the aircraft's attitude), but I'm not sure it is the cause of the initial pitching. I'll have to put some more thought to that...

Anyway, something on another forum got me thinking about conservation of momentum being the cause of the pitching down rotation around a center of mass on the pitchwise axis. In the same way that a motorbike will squeeze onto the front forks or a car will load up the outside tyres when cornering. Now that would be an immediate reaction to the loss of the horizontal (forward) component of thrust. I'll have to put more thought to that too!

What I am unconvinced by is the idea that 'downwash' plays a part. I explained that before.

Dissymmetry of lift is one which I dislike unless it is considered in terms of the forward speed and not a collective change in the AoA of the blades. I stall don't think a collective change in AoA causes a significant change in dissymmetry of lift. (BTW Crab, you're right, I did make an assumption on the Coefficient of lift. Not that there is a linear relationship though, but that any non-linearity is negligable when applied to the lift formula.)

Matthew Parsons says that a number of factors all work at the same time. I think he is probably right...The challenge is working out which ones do and which ones don't.

Well, it's all good for the brains anyway!

cl12pv2s

[Dave_Jackson]

I second Nick's motions.

Some have mentioned that the fuselage must be descending for the HS to be able to pitch the craft nose down. Others have suggested that the craft pitches nose down, then it starts to descend. The pitch of the fuselage and the descent of the fuselage are both motions of the fuselage, The only difference between the two is that one is linear and the other is rotational.

If it can be agreed that descent will result in HS pitching the nose down, then does it not make sense that without descent the HS could actually be resisting the nose from pitching down (Depending upon its location in respect to the rotor's downwash).

Perhaps it might be of value to think of the rotor and the fuselage as two separate entities. Then consider that teetering rotors are very loosely coupled to the fuselage, whereas extremely rigid rotors are more tightly coupled. The speed with which different rotor activities affect the fuselage are dependent upon the firmness of this coupling.


Last but not least, it might be of value to think that I don't know what the hell I'm talking about.

[34']

Well I never thought that I'd opened such a big can of worms as this but thanks to everyone that's joined in.

I shall watch with interest as you guys sift through the technicalities and sort out a solution.

[Aser]

However, I am wrestling with the notion that the pitching down happens before a rate of descent is sufficiently established to act on the horizontal stab as you suggest.

I agree with you, I think that we need to start thinking about why do we need a horizontal stab in an airplane.The instant reaction to less collective must related to the movement of (Center of pressure or Aerodynamic center) from/to the c.g. and the moment generated.

Just my 2 cents.

[FanPilot]

During the PPL course, you are shown the reasoning for the tail rotor. It is demonstrated on the board for theories sake that to counteract the torque would be two anti-torque rotors, one at the front, one at the back.

With the impracticallity of two anti-torque, it is argued that double the output from one cancels the need for the other, however, we get a secondary effect of rotor drift and one skid low.

Now, my point, if we could theoretically put another horizontal stabiliser at the front at the same distance as from the rear, then, would we experience a pitch down?

FP.

[Dave_Jackson]

FanPilot,

If you eliminate the primary cause, i.e. the HS, then 'flap-back ~ blow-back' [http://www.unicopter.com/B267.html#Flapback] of the rotor will probably have the next greatest effect on changing the craft's pitch. However, flap back is related to the craft's relative airspeed and lowering the collective does not, initially, reduce the airspeed.

Reducing the collective will reduce the coning angle and, IMHO, I suspect that this will result in a small instantaneous reduction in the flap-back. The reason for this reduction has to do with the fact that the reduction in the angle of attack of a blade at 180-degrees azimuth will be different from that of a blade at 0-degrees azimuth ~ if the cyclic stick is not given a slight lateral shift.
It should be noted however, that two knowledgeable people have argued against this position in the past.

[[email protected]]

cl12pv2s - It was Nick who said the increase in AOA on the stabiliser would make the nose go down. My point is more that as the nose pitches down, the tail must move up as the aircraft will rotate about its C of G thus seeing an increase in AoA for an inverted wing and initially giving a nose up moment.

Matthew, I think you are saying that you agree that flapping/coning is likely to be the initial reaction to a lowering of the lever. I think that is the answer to the original thread question and all the stabiliser stuff is a secondary effect once the RoD has been established.

[cl12pv2s]

It was Nick who said the increase in AOA on the stabiliser would make the nose go down.

Crab,

Yes, It was indeed...but I think he was just being sloppy and really meant 'an increased conponent from below'!

While I'm here, I may as well look into what Dave said...


Dave you said...

Reducing the collective will reduce the coning angle and, IMHO, I suspect that this will result in a small instantaneous reduction in the flap-back. The reason for this reduction has to do with the fact that the reduction in the angle of attack of a blade at 180-degrees azimuth will be different from that of a blade at 0-degrees azimuth ~ if the cyclic stick is not given a slight lateral shift.

1. Wouldn't the reduction at the changes at the 180/000 azimuths cause a left right roll and not a fore/aft pitch/

cl12pv2s

[Droopystop]

Can someone explain to me how lowering the collective results in flap forward/reduced flapback, that then results in a pitch nose down.

Concering the AoA of the horizontal stabiliser: the moment the lever is lowered, the aircraft descends, causing the change in AoA. This is pretty instantaneous (or at least as instantaneous as a change in AoA on main rotor blades that causes flapping). In other words, you are going to get a nose down before the VSI decides to waver off the horzontal.

[Thomas coupling]

Droopy: When you lower the collective:

FIRST, the Aof A reduces, and THEN the a/c descends. Not the other way round.
If the Aof A didnt reduce, the a/c wouldnt lose lift and descend?

I'm still looking for my aerodynamics guide book re - the CL bit

Whatever the outcome - each 'input' (collective/AofA/stabiliser/CofG/cross coupling) I'm sure the only man who could list it as it happens is Prouty!

[Droopystop]

Caught again typing before thinking!

I think I might have to rename myself dopeystop!