why does the nose pitch forward when you lower collective?

[Thomas coupling]

Because of the coupling between the controls and tail stabilator.

[cl12pv2s]

The instantaneous reaction to lowering collective is for the lift on the advancing side to reduce MORE than the lift on the advancing blade. [The amount of blade angle reduction on adv and ret sides is reduced by the same but the ratio of lift is different].

Thomas,

This doesn't seem to be the case when I run the numbers through a simple 'lift formula' spreadsheet. A change of pitch doesn't appear to change the ratio of lift between the adv and rtg sides, seeing as the only parameter in the lift formula being changed is the pitch and that is being changed the same all over. (That's why its called a 'collective'!) In fact the the opposite of what you said happens... the amount of lift reduction is different but the ratio is the same. That's because of the V-sqaured in the equation.

The only thing that changes the ratio of lift is the forward airspeed of the aircraft. Of course this would make sense as the whole point about dissymetry of lift is the difference in forward speed. .i.e. in a hover no dissymety of lift! If you are in a hover and pull much collective you still don't get dis-of-lift! Forward speed is the key.

Here's how I see it!

In forward level flight for a 'given' speed, there is a 'given' amount of blowback / flapback (whatever you call it)...thus the pilot will need a 'given' amount of foward cyclic to balance that dis-of-lift which gives rise to the 'given' attitude of the aircraft. It's all in balance; a system in equilibrium. (It could be argued that actually there is no dis-of-lift in steady forward flight, but that's for another thread!)

Lowering the collective simply un-balances the equation. Now there is less lift. To bring the system back into equilibrium, there must be a change in the other parameters. If the pilot does nothing, the speed of the aircraft will initially begin to bleed away, thus reducing the dis-of-lift. The cyclic position is now too far foward and the nose of the aircraft drops, sacrificing the level flight in order to maintain airspeed.

What happens next is up to the pilot. If he wants to maintain level flight with the new collective setting, then speed will be sacrificed and aft cyclic must be used in order to balance the dis-of-lift once again.

You don't notice much airspeed change while all this happens, due to the 'law of momentum'. The reduction of pitch does not instantly cause a reduction of airspeed, but it does cause an inequilibrium between the cyclic setting and the degree of flapback (dis-of-lift). The momentum fights against the 'in-equilibrium'. The momentum is lost at the same rate as the equilibrium is restored.

If one adds in the effect of drag (which would be different at different attitudes), then there is going to be some change in airspeed. Let's say the aircraft was originally in level flight at the most aerodynamically efficient attitude and speed. Then lowering the collective here will cause all the same stuff to happen as above. However, as the aircraft reaches a less efficient attitude, then the speed will be lost, causing a greater pitch down if not corrected by the pilot.

Conversely a pilot who is flying above the 'efficient' attitude / speed, will find the opposite. Lowering the collective will produce the inequilibrium state, but this might not be so marked as the aircraft adjusts to a more efficient attitude. In fact initially the aircraft may not seem to pitch down at all.

This is noticeable in aircraft which have definite speeds (VToss / VBroc). The amount of collective needed to initiate a descent can be very different depending on what speed you are at and what speed you are going to!

I am wary of the explanation regarding the downwash on the horizontal stab, as I think this doesn't obey the premis of 'action and reaction'. The downwash action must be complimented by a reaction at the rotor. Sit on a child's swing with your legs out in front of you. Then reach forward and try to push your legs down...it doesn't work! Just like if you were to get a huge fan and try to blow your legs down. That's why you can't put a huge fan on the back of a sailing boat and hope to go any faster!

The only part the horizontal stab might play in the whole affair is that when you drop the forward speed the stabilizer (depending on its shape and AoA) will work differently, and I don't discount that this may have some affect.

Well that's food for thought anyway,

cl12pv2s

[Colonal Mustard]

It could be that magical power ... G R A V I T Y.. only joking,summit to do with the vertical stabiliser and airspeed

[SAR Bloke]

in nil wind the nose will not drop.

Not true. Try it on a heli with no control mixing (switching off SAS or Stab may make it easier to see). The effect is still there in nil wind.

[Bertie Thruster]

Try it while flying straight and level backwards.

[212man]

I'm assuming that either chuteless' pc is being used by someone else to post in his name, or that he has been on the p***s! His last three or so posts bear no resemblance at all to his earlier ones, which were well written and provided useful contributions to their threads.

Compare this:

""...king hell"
the anwser is as you lower the lever, your body moves forward thus moving the cylic forward
the accelerative attituide of this manouvereey(okay this is fun but but where the f''''k is the spell check , i apoligise for acceelerationm)
caused by this the helicopter want to move away from you because it really does'nt like YOU
after this the aircraft type(weather big or small R-22 or s 76) suddenly remembers that it cant fly on it's own it then WANTS TO HUG YOU AT A RATE OF KNOTS THAT IS NOT HEALThY TO H8MANS

you only swing when you're winning
lots of love and kisses chuteless
(PS you dont want all the answers from here do you ?)"

With this:

"I'm currently writing a single pilot helicopters CRM course
and trying to stay away from the usual 2 pilot plank case studies,
I've got the NSTB reports for the 3 a stars that crashed on the same day in Juneau, Alaska, Sept 10th 1999.
I'm wondering if any of you guys can help with background info
on flight paths, crash site, photos etc.

Also any case studies other helicopter accidents you would like to see in CRM

cheers in advance for any help.

Chuteless"

[Bertie Thruster]

..................you could also see what happens to the nose when the collective is lowered while flying level sideways. (suggest above 500ft)..............

[Thomas coupling]

CL12..:

L = (1/2) d v2 s CL

d, s, are the same on both sides of the disc (adv and retreat).

The formula is balanced due to flapping to equality in fwd flight (or in the hover with a wind).

When you lower the lever, the CL changes on both sides by the SAME amount because it is the collective (thanks for reminding me!).
Part of the coeff of lift is the A of A. It is this which is reduced (collectively) on the adv and ret side.

The instantaneous effect of "destabilising" the lift formula is a reduction of CL by the same amount, but when this is factored into the above formula, you will notice that the side with the greater "V" inherits the greatest reduction in overall lift (L).

The blade then starts to dip down on the advancing side due to the relative reduction in lift on the adv side. Phase lag causes this effect to be experienced approx 90 degrees later at the front of the rotor plane. The rotor dips down and aft cyclic is required to return the a/c to stability.

If my memory serves me right

[Bertie Thruster]

On a R/C model helicopter, in straight and level inverted flight, would the effect be the same?

[Flingwing207]

Er... ...like a lot of us are saying:

Put a helicopter on ground with 60KT headwind. RPM top of green, collective all the way down - assume this is zero lift. So there is zero dissymmetry. Now raise the collective a little, the disk will begin to tilt back (flap back) due to a little dissymmetry. Raise collective more, more dissymmetry due to more lift. If you want to keep the disk level as you increase lift, you will be pushing the cyclic forward to adjust AoA - the more you raise the collective, the more forward cyclic you'll need to hold the disk level against the otherwise increasing dissymmetry.

This won't happen in a no-wind condition. For "dissymmetry of lift", you need the dissymmetry (of relative airspeed) and the lift!

[Bertie Thruster]

If I did everything Flingwing said in that 60kt wind and ended up in the air but stationary over a spot on the ground, would I be hovering or flying at 60kts?

[KikoLobo]

At least in a jet ranger.

You have a mixing of the controls in the controll column... Meaning that when you lower the collective you have some mixing going on on the forward cyclic control rod. In the case of this helicopter its actually moving the control rods. At least that;s what the factory instructor said to me

In spanish the mixing lever is called "LA BAILARINA"

[Flingwing207]

Originally Posted by Bertie Thruster

If I did everything Flingwing said in that 60kt wind and ended up in the air but stationary over a spot on the ground, would I be hovering or flying at 60kts?

Yes you would!

[[email protected]]

Guess what? in forward flight when you raise the lever the nose pitches up - you add pitch to all the blades and the one on the advancing side sees a larger increase in lift because it has the higher speed - the result, due to rotor flapping (not precession) is blades high at the front giving pitch nose up. When you lower the lever, the opposite happens. Lots of helicopters have mixing in the control runs to oppose this so that a nose down cyclic input is given as the lever is raised and vice versa.
The argument in the hover could well be due to downwash effects but a helicopter is a different animal in the hover than in forward flight and collective/cyclic cross couplings are different in aircraft types.

[Bertie Thruster]

But what about hovering and downwash when your groundspeed is zero but your IAS is 60kt? (as per Flingwing)

[[email protected]]

Silly me, I should have pedantically stated that I was talking about a hover in still air - slow day at work Bertie?

[Droopystop]

TC,

Before you lower the lever, both sides are in equilbrium, with the total rotor thrust acting through the C of G in all planes. So the transverse distribution of thrust is balanced. If you lower the lever, the only thing that changes in your lift formula is CL as you say. If the CL changes by the same amount around the disk, then wouldn't the distribution of lift remain the same? (Yes it would). Since you are not changing V, if you halve the CL on one side, you halve it on the other. So if CL changes by the same amount around the disk, there would be no flap back/forward. Remember that the relationship between AoA and CL is not linear and the CL around the disk will be distributed differently after the lever is lowered. So there will be some form of flapping, but the extent and direction of flapping would depend on the airflow regime and the blade design.

When one lowers the lever, the total rotor thrust is reduced, thereby reducing the horizontal component. There is an imbalance between this and parasite drag causing a pitch forward couple. Also, as the aircraft starts to descend, the AoA of the horizontal stabiliser decreases, reducing the downward component of the force acting on the tail, causing a tail up couple. I am sure there are other more subtle effects as well.

[Impress to inflate]

On an AS 332L when you drop a small amount of collective the nose pitches up a small amount. Can anyone answer that odd one ?? Or is it because it's French ? Answers on a post card

[Thomas coupling]

Droopystop: do the maths. Start with the formula in equilibrium (on both sides obviously) and then enter a reduced AofA into the formula on both sides. Everything else in the formula is same.
The CHANGE in "L" on the advancing side is LESS than that on the retreating side, swing round 90 degrees and you get nose down.

Also:
If one utilises the CofG thrust diagram, which you talk about..

If we looked at the horizontal thrust vector emanating from the rotors - then lowering the collective would reduce this vector which is now LESS than it was before, which is proportionately less than the (parasite) drag horizontal vector (in the couple), consequently the nose will tilt UP.

Try again.....................

[NickLappos]

For exact answers, think not of helicopters!

The basic causesfor helicopter cross couplings are complex, and the sum of several factors, some of which cancel each other to some extent.

Dumping collective makes the nose go down, mostly because of the horizontal tail angle change, somewhat because the CG might be aft enough to contribute, and somewhat because at the new collective position, the cyclic is now not at the trimmed position.

To slice out each, set up a small experiment for each. The tail has no effect in a hover, so set a high hover and lower the lever. If the nose dumps down, it was not horizontal tail angle change, so look to the other two factors.

Then do the hover experiment, but with a vastly different CG. Apply logic to the results.

My experience says horizontal tail is the biggest factor, but all are contributary.