why does the nose pitch forward when you lower collective?

[lmlanphere]

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).

------------------
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 :ok:

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...:hmm:

[SAR Bloke]

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 :ugh:

[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]

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.:bored:

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.

[34']

i've had so many possibilities of what may/may not/should/could/might do for this it's getting a little confusing :confused:

i've drawn every option as guys have mentioned it and all have some form of merit...suppose it's what you feel best trying to explain and if someone else comes up with another concept that you prefer you'd probably go to that one... i'd just like to know EXACTLY what causes it :8 , just incase the examiner asks me...just so i can be ready for the slippery old goat :ok:

34'

[TiPwEiGhT]

I would have to agree with RotorFossil's explaination, that's what I teach at least.

TiP:ouch:

[stacey_s]

Ask a 'good Engineer' and he'll tell you why!!

[Helipolarbear]

Your kidding, of course??!!:}

[Dis-Mystery of Lift]

Best answer I have is,
In forward flight the advancing blade is producing more lift than the retreating.So when you lower the lever the percentage of lift lost is more on the advancing side.Due to this being felt 90 degree's later the nose pitshes forward.:D
Hope that helps

[Hilico]

Ah - this was the thread I was thinking of.

http://www.pprune.org/forums/showthread.php?t=214775

It talks about the disk tilting forward, rather than the aircraft. Hmmm.

[Helipolarbear]

KISS PRINCIPAL:)

[crispy69]

The way I had it explained to me was it happens because of Flap back.

It is easier to see with a picture but if you can imagine a two bladed machine in forward flight. This machine is producing lift, both blades are producing the same lift but have different AoA's and airspeeds.
Simple so far right!

Now put some values on lift for simplicity I will use small numbers
We currently have a lift value of 20 on each side.

Advancing side has a higher airspeed and lower angle of attack
Airspeed = 10 AoA = 2
10 x 2 = 20

Retreating side has lower speed and higher AoA
Airspeed = 5 AoA = 4
5 x 4 = 20

Now we lower collective by 1 degree

Now advancing side is
10 x 1 = 10

Retreating side
5 x 3 = 15

You have more/less lift on the retreating/advancing side this precesses 90deg to make nose pitch down.

So while flying we automatically correct for flap back with cyclic, but this setting only works at a given speed/collective setting. if you change either of them you have to reset the cyclic.

I am sure there are probably many factors to this question but this is my main understanding of how this works.

Crispy

[Doc Cameron]

Stacy,

Just wondering if you know any good engineers to ask? (I guess that is the baited question you were posing?).

Cheers,
Doc

[34']

Thanks crispy

I'll get the whiteboard into action and see if i can get the brain around it :ok:

This examiner had better ask me this question or i'm just gonna give him the info anyway ;)

[imabell]

in nil wind the nose will not drop.

most of the other answers must not be from pilots. if they are then it is very embarrassing.

this thread demonstrates the main arguement against low time pilots becoming instructors, a complete lack of knowledge of basic aerodynamics.

so much for the jar system.

[34']

we all have to learn the information somewhere... if you have many hours of experience that could be of help to us starting out then that would be great...we don't make the rules...i'm just trying to learn as much as i can...

[Dave_Jackson]

34'

If the examiner does not give the helicopter's configuration, you can quite honestly state that 'pitch reduction due to torque reduction, is because of the pitch-torque coupling'. :eek:


After he has given you a mark of zero, you can then say that you were talking about the Intermeshing configuration and ask for your full mark. :ok:
___________________________


www.unicopter.com/0842.html#Torque_Pitch (http://www.unicopter.com/0842.html#Torque_Pitch)

[34']

...and if he asks me to draw it... i know i don't have enough colour pens for that :rolleyes:

[helmet fire]

I was taught that it was primarily the effect of reduced downwash over the horizontal stabiliser, but that like verything in a helicopter there are multiple forces at play. Those included the moment arm on the CofG (see Shawn Coyle's post) and the possibility of an uneven redistribution of lift across the disk (not flapback) as per fingwing207.

I did find that flying Balck Hawks reinforced the lesson that it is primarily due to downwash reduction over the horizontal stabiliser. The Stabilator (big wing at the back) is driven by a computer that changes it's angle dependant upon a number of inputs, one of which is collective bias. This senses the rate and magnitude of collective movement and angles the stab to keep the nose level. When you lower the collective in the Black Hawk, the nose stays level, but the stab drives up to achieve it.

This explanation would suit all arguements, however the one that made lean toward the downwash theory was what happened when we lowered collective whilst fitted with the external support wings (ESSS). Those wings are situated forward of the rotor mast, and the stab computer doesn't know they are on. When we lower the collective, we get a pitch up. That would make sense with the downwash reduction over the ESSS not being countered by the stab computer, but it also backs up the CofG moment arm arguement. It tends to discount the effect of any redistribution of lift across the disk because that theory would dictate a nose down regardless of the ESSS.

I found the same thing on Hueys without a computerised (but moving) stabiliser. I did a maintenance test flight with an improperly rigged stabiliser, and during the max power check I contacted the forward cyclic stop trying to keep the rising nose under control. The only way out was to reduce collective and that restored forward cyclic authority. This suits the downwash arguement well. Lastly, when fitted with gunship "wings", the aircraft behaves like the Black Hawk with ESSS, ie it pitches nose up rather than down.

[bladepitch]

without getting to technical
i would have thought it was as simple as a loss of lift on the advancing blade and the postion of the cyclic in foward flight when the collective is lowed

[munchkins]

Ask a 'good Engineer' and he'll tell you why!!
Hey Spacey, if you're such a "good Engineer" why don't you chime in with your version?
Munch Munch

[wiisp]

Lot of funny and complicated answers on a straight forward question.. :hmm:
Lowering collective reduces AoA, and in forward flight this reduces flapping and make the rotordisc tilt forward, this will cause the liftvector to tilt forward and anybody who been awake on the lesson where the helicopters stability has been the subject, will understand why the fuselage tilts forward.. Stabilizers and precession and those factors are not the main contributor in this matter.
Fly safe:)

[Thomas coupling]

Dis Mystery has it right.

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]. Move this round 90 (ish) degrees due to flapping to equality and the front of the blade dips - helo noses down.
At the same time, on SOME helos, this goes one step further with a change in C of G dynamics.
Once the helo starts to descend.....then the tail stabiliser comes into effect -exacerbating this dip with the nose.

[helmet fire]

As I said above, I was taught the redistribution of lift too. But why does the Black Hawk with ESSS and Huey gunship pitch up when lowering 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]

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]

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. :eek:

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.

[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! (http://www.pprune.org/forums/showthread.php?t=224917)

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]

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]

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. http://www.unicopter.com/Chairshot.gif

[34']

Well I never thought that I'd opened such a big can of worms as this :8 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. :ok:

[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:bored:

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!

[Dave_Jackson]

cl12pv2sWouldn't the reduction at the changes at the 180/000 azimuths cause a left right roll and not a fore/aft pitch/


You are correct.

I was thinking in terms of 'Absolutly' Rigid Rotors (zero phase lag). The previous reply in this posting has been removed.

Dave

[Deemar]

cl12pv2s

You are correct.

I was thinking in terms of 'Absolutly' Rigid Rotors (zero phase lag). The previous reply in this posting has been removed.

Dave

Surely an absolutely rigid rotor is an theoretical construct only? Any real world (physical) rotor system is going to have some flex to it. Even if you don't have flapping hinges, the blades themselves will flex and act like flapping hinges.

Dan

[Dave_Jackson]

Deemar

You are correct ~ as well.

There is no such thing as an 'Absolutely' Rigid Rotor but the phrase 'Rigid Rotor' was already taken. http://www.unicopter.com/Cry.gif

The expression "'Absolutely' Rigid Rotor" is used in the context that 'absolute' is the optimal, albeit impossible, objective.

For more; http://www.UniCopter.com/0815.html

Dave

[Matthew Parsons]

crab said, "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."

I probably did, but that's not exactly what I meant to say. I think most that are in favour of the flapping/coning argument are correct if you limit the reaction to a very short time period. If you're a designer or a tester, limiting it this much is needed, but if you're just trying to understand the helicopter I think you should go beyond the instantaneous reaction and look at a larger picture.

The larger picture is that when you lower collective, you reduce the blade pitch, create a rate of descent, and throw the helicopter out of equilibrium. From those you get reduced coning which changes the disc angle, changes in lifting and dragging surfaces due to the rate of descent, and moments about the center of gravity due to being out of equilibrium.

By answering the question with all the above effects (and even more that could be added) you make it very difficult for a designer to do anything about it, but you help the helicopter pilot understand what is happening to his machine.

Matthew.

[imabell]

crab is right,

the pitching motion is instantaneous, the horizontal bits may add to it but it is secondary,

the pitching of the disc, up or down, occurs whenever you raise or lower the collective or whenever you accellerate or decellerate the aircraft. a simple aerodynamic fact.

if you lower the collective and hold the cyclic position the nose will pitch down and the aircraft will accellerate, the nose will then pitch back up, flapback, and pass through its original position in a descelleration, this is static stability the nose will then pitch down and accellerate again, flapback, these up and down oscillations will increase to destruction if not controlled, dynamic instability.

flapback, the tendency of the disc to go back to where it was, occurs because of simple aerodynamics whenever you raise or lower the collective or accellerate or descellerate and don't compensate with cyclic.

[CyclicRick]

Simple: It's because when you lower the collective the weight of it is nearer the front, you see, simple C of G thing :ok:

[handbag]

thread revival - I've had my nose in this traditional pprune thread for the last hour or so and was interested to see if anybody had anything to complement it with?

[rotorfossil]

In most helicopters, the horizontal tail surface is operating to a greater or lesser degree in the downwash from the main rotor in powered flight. Surely this must mean that as soon as the collective is lowered, the airflow over the surface is modified and causes a nose down pitch before the descent is established. Unfortunately, in practice it is impossible to disentangle the various effects. However, it is noticeable that larger helicopters still exhibit the nose down pitch even though their inertia means that they don't alter the flight path into a descent quite so quickly as for instance an R22.

[delta3]

It is kind of amusing in retro spect to see how speculative and complicated answers tend to be.

I would use the following example: what happens to a plane if you remove the forward wings....

You can of course start a whole thread on how the tip vortices of the forward wings react with the tail, but the fore most important thing is that you remove upward lift at the front of the air craft, so all other things equal, the nose must come down, that in fact is exactly the purpose of the tail.

Perhaps a more serious result of that: high speed low level flight in a heli is more dangerous than planes: if you lose your engine your nose may point faster to the ground than you are able to recover by putting cyclic backward while flaring...

m2c, d3

[EGOSEC135Juno]

thread revival - I've had my nose in this traditional pprune thread for the last hour or so and was interested to see if anybody had anything to complement it with?

This long running thread is good. But only Thomas coupling seems to have the answer. It’s all to do with differential flap back. While there is no relative V change made by raising and lowering the collective, there is a difference in percentage change of alpha. On the advancing side, a lever raise changes alpha by far more in percentage terms than on the retreating side, because the advancing side has a lower alpha to begin with. Across the fore/aft axis, when phase lag is applied, the disk climbs more on the advancing side than the retreating side, causing a ‘flapback’ effect leaving the disk higher at the front. The opposite is true for a lowering of the lever - lever down, ‘flap forward’, disk lower at the front.

[Bravo73]

Woah. 14 years. :D

[[email protected]]

This long running thread is good. But only Thomas coupling seems to have the answer. It’s all to do with differential flap back. While there is no relative V change made by raising and lowering the collective, there is a difference in percentage change of alpha. On the advancing side, a lever raise changes alpha by far more in percentage terms than on the retreating side, because the advancing side has a lower alpha to begin with. Across the fore/aft axis, when phase lag is applied, the disk climbs more on the advancing side than the retreating side, causing a ‘flapback’ effect leaving the disk higher at the front. The opposite is true for a lowering of the lever - lever down, ‘flap forward’, disk lower at the front.
I think I said that in post #54

[MightyGem]

Woah. 14 years. :D
Must be a new record.

[212man]

24 years since it started - almost quarter of a century!

[ascj]

I figured it was none of the above but due to gyroscopic precession of the tail rotor.

[Lala Steady]

I figured it was none of the above but due to gyroscopic precession of the tail rotor.
And apparently M. Vuichard has a much better technique for stopping it:E