why does the nose pitch forward when you lower collective?

[lmlanphere]

collective v. pitch attitude

can someone explain why the nose pitches up with an increase in collective, and down with
a decrease in collective? If I were to guess, I'd say it had to do with blow-back, but my books don't explain this. thanks.

[Skycop]

The horizontal stabiliser gets affected by the changing downwash, giving the effect you describe.

[Skycop]

OOPS Sorry, post repeated.

Also, the changes in airflow caused by the subsequent climb / descent also act on the stabiliser.

[This message has been edited by Skycop (edited 07 November 2000).]

[Thomas coupling]

The instant you pull on the collective, both lift and drag increase. The greatestlift increase takes place where the max angle of attack changes take place [which is when the advancing blade is at 90 degrees to the airflow] (3 o clock for anti clockwise rotation and 9 o clock for clockwise rotation). Drag also increases but this is compensated for by the rotor governor. Because the effect of this lift is not felt for another 90 degrees, approx (dependant on rotor head design); then the outcome is a tilting up of the main rotor at the front of the disc. This phenomenon is instantaneous whereas the influence of the tail stabilisers is secondary (after the a/c starts its climb).

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TC

[34']

I'm searching a clear answer from anoyone that knows???

When lowering the collective in straight and level flight, the nose pitches down.

Answers have ranged from flapforward (but there is no change in IAS so no change in amounts of disymmetry and resultant flapping?), precession from decsent flow on the dic (there would be descent flow equally over the entire disc?), descent flow on the horizontal stabalizer forcing nose down attitude or reposition of A/C CofG due to loss if lift.

Come now all you technical types

Cheers all for the help...

[FanPilot]

Could it be due to that as you lower the collective, you are now descending and that horizontal fin at the back (as well as the drag from the boom) is now levering the cockpit downwards?

FP.

[Stringfellow Dork]

Less downwash onto the tail boom?

[Hilico]

This was discussed a couple of months ago. However, a small contributor is the reduction in the downwash on the horizontal tail surface(s); but the big one is the reduction in flapback. The stick position is forward in the cruise to overcome it; it is proportional to collective pitch; hence when you reduce collective, you don't need so much forward cyclic. If you leave the stick where it is, the nose will go down, so you have to pull it back.

Think I've got that right.

[34']

..but isn't 'flapback' a result of flapping which is a result of airflow onto the disc from the front? ( namely airspeed ) If the IAS is the same why would you get a change in 'flapback'? You don't get 'flapback' in the hover ( unless you get a gust/wind onto the disc from the side/front).

So with a constant IAS the amount of 'flapback' should stay the same or does the reduction in Induced Flow due to down collective have an effect on Inflow Angles and AoA and hence Total Rotor Thrust orientation?

Mmmm...

[PPRuNeUser134364]

the big one is the reduction in flapback. The stick position is forward in the cruise to overcome it; it is proportional to collective pitch; hence when you reduce collective, you don't need so much forward cyclic. If you leave the stick where it is, the nose will go down, so you have to pull it back.

So why does it also happen in the hover on a still wind day?

[34']

SAR Bloke... i think your onto the point i'm trying to make. I don't think it has much to do with flapback.

If you down collective on a still wind day (no flapback happening ) the nose still picthes down. Forget forward hover as you would then have 'flapback' and the 'bubble' to throw into the pot

[Matthew Parsons]

In a hover the effect of the downwash on the horizontal stab will still be there.

There is drag from the fuselage being in the downwash that can change this balance.

In both conditions, if your center of gravity is forward of your center of lift, a reduction in lift will result in a pitch down.

With a canted tail rotor, you add other effects.

AFCS and mechanical mixing will add to these effects.

What was already said about flapback is plausible.

I've probably missed a few.

Remember that the helicopter is not a simple aerodynamic model. It is not a good idea to try to pin one cause on an observed effect.

Matthew.

[mongoose237]

Another one to throw into the pan...

An explanation of another contributory factor that I heard some time ago:
When you lower the collective lever, you reduce the lift coefficient on both blades by the same amount (presuming a two-bladed helicopter for the sake of simplicity). However, the increased V on the advancing blade compared to the retreating blade means the reduction in lift is not uniform across the disc, which results in the nose drop.

[Shawn Coyle]

How about it being due to a change in the balance of forces?
In most helicopters, the rotor is aft and (well) above the CG - this is the main source of the lift and thrust forces. Reducing the lift force but not the thrust (forward tilt) force will upset the balance of forces, and in an attempt to re-establish equilibrium, the nose will drop.

[rotorfossil]

I thnk the major reason in most helicopters is the change from a downwash over the horizontal stabiliser in cruise flight which keeps the cabin level, to an upflow in descents, particularly in autorotation. In some types this effect would be excessive (Bell 206 for instance), and the stabiliser has to have some feature such as a spoiler to reduce the up thrust in high rate descents.

[Flingwing207]

It's not a reduction in 'flapback' per se, but flapping and precession has everything to do with it. The faster you go, the more forward cyclic you hold. Forward cyclic is decreasing pitch on the advancing side of the disk, and increasing pitch on the retreating side of the disk. This is helping equalize the large difference in relative airspeed across the two sides of the disk (which we know as dissymmetry of lift). Essentially, the pilot is applying cyclic correction against the blade's desire to flap.

Now if there is no lift, then there can be no dissymmetry. Lots of lift, lots of dissymmetry. No airspeed, no dissymmetry, lots of airspeed, more dissymmetry. So if you are going fast with lots of power pulled in, you have lots of lift and airspeed creating lots of dissymmetry. You are also applying lots of correction (forward cyclic). Take out some power - lift - and you reduce the 'desired' flappping amplitude. If you don't also reduce the 'correction' applied with the cyclic, the nose will pitch down due to precession.

[Camp Freddie]

having read all the replys, the one I like the most is that as you start to descend the upflow of air onto the horizontal stabilizer creates vertical drag and levers the cockpit downwards.

I think I am going to start telling people that from now on, as for whether it is true or not who cares ! it sounds good to me, and thats all you need

regards

CF

[34']

unless it's a question for an instructors exam...hence i'd like to know the full details..

but alas...once that's done then maybe i'll agree with your theory K.I.S.S.

34'

[FanPilot]

I am an instructor, and from what I remember, this is what I was told during my PPL. However, I didn't get asked this, or even learn this from my instructor course.

However, I am a "restricted" instructor, i.e. new, I just got to do so many solo sign offs and 100 hours of instructing to become unrestricted.

[Johe02]

I believe (and teach) it is the couple formed between the centre of lift and the centre of gravity. . diagrams needed here but you might be able to figure it out.