View Full Version : Flapback/forward
Helimutt's thread turned out real well, so did Lu's on the French rotation.
A lot of good stuff is still coming out of both (I don't mean mine !) So I thought I'd put a newie on. Maybe the focus will continue to shift away from the ugly stuff if we do that? And look, no Robbies !
OK, so - Why does the disc NOT flap forward
by a similar amount (when speed is decreased to zero) as it flaps back when speed is increased FROM zero?
I have many things to say on the matter but I'd like to see what comes up first.
SPS, Outside the square on a shrinking oblate spheroid in an expanding universe.
SPS,
It does. Try a level deceleration from 80 to 40 kts - select a 5 degree nose up attitude and lower the lever to maintain height. Note the exact position of the cyclic to achieve the initial nose up attitude. If you keep the cyclic in this position as the speed reduces the nose will gradually drop and to maintain 5 degrees nose up you will have to bring the cyclic further back to overcome flap forward.
rotorque
19th Jan 2001, 12:54
Crab,
Could the elevator have anything to do with this. IE: less lift (albeit downward)with less airspeed. (Or as someone once told me less downwash on the tail plane - ha ha)
Cheers
Horizontal stabilisers whether they be on the tail boom or tail plane and whether they be fixed or movable are there to reduce the fuselage attitude changes as a result of speed changes. Without the down force of a horizontal stabiliser the crew would end up hanging forward in their straps at speeds much above 100 kts due to the nose down attitude required to tilt the disc enough. With a horizontal stabiliser you still have to tilt the disc forward and keep moving the cyclic forward to overcome flapback as speed increases, but the increasing down force on the stabiliser due to speed pushes the nose up and keeps the fuselage much more level. Movable stabilisers such as the Apache has are even more efficient - watch an AH -64 transition and the fuselage attitude hardly changes at all - just what you want from a weapons platform.
I was thinking on the disc only, but seeing as this is mentioned Crab is correct.
The Heli's C of G moves forward when the disc is tilted forward as the fuselage will follow it. If that were allowed to go unchecked the Heli would tumble over forwards.
The Horizontal stabilser (in some Heli types)
applies a downforce on the tail proportional to the speed of airflow over it, resulting in the nose being lifted. The downforce supplied by the H stabiliser is proportional to forward speed.This effect extends the allowable range of forward movement of the Heli's C of G. (So it won't tumble over!)
A moving H stabiliser can extend the range even further, or be smaller and do the same job.
Whilst it is said that flap forward DOES happen (and I agree, most prevalent at around 80% of best rate of climb speed)
we do not meet a similar amount of flap forward as speed is reduced from (say)
35 kt to zero as we find with flap back when increasing speed from zero to 35kt. There is much more flap back than forward in this range(it would appear).
Why does this happen?
SPS, sitting at computer oriented S N in a world oriented N S.
Lu Zuckerman
21st Jan 2001, 07:15
SPS
Read my post on page 3 of the Coriolis force thread.
------------------
The Cat
SPS, it doesn't happen.
The flapping forces generated by changes in airspeed are exactly the same whether you increase or decrease speed and therefore the control movements are the same.
Your description of why a horizontal stabiliser is required is fatally flawed; when the disc is tilted forwards the total rotor thrust is tilted with it and produces a nose down pitching moment about the C of G, as the speed increases a couple exists between the horizontal component of rotor thrust and parasite drag of the fuselage about the C of G which encourages the nose down attitude. BUT, the helicopter does not tumble over forwards because there is another couple between the vertical component of rotor thrust and the aircraft weight which acts in the opposite direction about the C of G.
The fuselage will only pitch nose down until the two couples are in balance (think of the balance of couples in tail rotor roll).
The horizontal stabiliser, as I mentioned before, augments the restoring couple and permits high speeds without excessive nose down attitudes. It also allows for a greater range of safe C of G positions fore and aft of the datum so that everyone doesn't have to sit under the rotor mast.
OK, I accept all of that, it is what I was taught too.
BUT- Balance your pencil on a level surface.
It will be statically stable because its C of G is directly over its base. Now try and balance the pencil with its C of G NOT over its base,(not vertically)Wieght is not uniformly exterted on the pencil's base. Of course, it will fall over every time you let it go. It is statically unstable.This is because the C of G has been moved forward.
Because there is no other force opposing the acceleration of gravity acting on the mass of the pencil. With the transition to forward flight there are 4 forces providing 2 opposing couples which end up in balance.
I have read some stuff on Helicopter dynamics and stability and it is mostly Greek Flute Music that puts you to sleep after the first paragraph.
Ok, I'll answer fully later (healthy debate only!) V busy right now...!
Not much time and I hope I do this justice.
Crab - No eggs to suck here, many read and follow.
The pencil is statically (and dynamically, for that matter) unstable if tilted. What could prevent this? A stabiliser. Your finger,anything you like. If the stabliser exerts sufficient force in the right direction (opposing gravity in this case)the pencil will not fall over because the forces are again balanced (as they were when it stood up straight).
The Helicopter is statically stable in forward flight by virtue of the H stabilser
(only in this sense, of course) As you say, the pilot is using more and more forward cyclic as speed increases (and when there is no more cyclic movement available this shows one of the limits of forward speed). The stabiliser provides a balancing force, preventing the Heli from 'nosing over'. It is just a lever/ arm principle, small force further away from fulcrum can balance a bigger force that is closer to the fulcrum.
To prove, what would happen if (hope it never happens to anyone) the H stabiliser fell off close to VNE? Only tilting the disc back could save the situation (if that is possible in the time available). If nothing were done the Heli could only topple over
as there is no force to balance those pushing the nose down.
Im not saying that the Heli could not fly forward from zero speed to some speed without a H stabiliser because it could, but the speed it may reach would be reduced severely, it could not cope with the forward movement of C of G as well as it could with a stabiliser. I said the H stabiliser extends the allowable range of movement of the C of G and I'm still happy with that.
As for the movement of C of G, this is readily accepted to be the main factor in static rollover, ie when the C of G moves outside the skid, usually portrayed as happening sideways (although it can happen in all directions). It must also be that the C of G moves forward as the fuselage tilts forward and this must be balanced (in the same way anything about to topple could be held) by a stabiliser.
Intersting side issue - If we give the heli a weight (say 2000) and move into forward flight up to VNE the aerodynamic force exerted on the stabiliser will have a quantity too, one acting in the same direction as weight. It might be 50 (for the sake of illustration). The 'total weight' of the Heli has increased with speed,(2050)one of the reasons we have to use more and more power to travel faster and faster.
You don't get 'owt for nowt, as they say.
SPS, wishing I was better at fiddling with files so I could use my book graphics to illustrate things. Anyone interested in giving me a quick rundown (not using a car please!)
SPS, as I said earlier the drag from the fuselage stops the C of G moving forward - the fuselage hangs under the disc in the hover and is dragged along in forward flight. It will never be statically unstable in this condition.
The pencil is only statically stable - dynamic stability requires positive static stability as the object must try to return to its starting position and overshoot. If the oscillations increase in amplitude it has negative dynamic stability (not good) if the oscillations decrease in amplitude it has positive dynamic stability (a good thing for aircraft).
The pencil falls to the ground without oscillating and has therefore only negative static stability when the C of G is placed outside of it's "footprint".
If the H stabiliser fell of at VNE the aircraft would pitch nose down (aft cyclic could chop off the tail)and accelerate - if it did not destroy itself then flapback would pitch the nose up again which highlights dynamic stability with speed increase.
I was painfully aware that I had not closed off the rotor tilt to rear/high tail at speed problem as I drove to work. That's what you get for rushing eh?
Agree on the dynamic instability but only with the stabiliser on because it is also reponsible for the tail going down (as flapback is) as the second half of the wave cycle of instability begins. Nothing to push the tail down although the disc will flap back, granted. I now wonder if the tail would be cut off anyway, ie. before things got as far as we have projected them.
High tail and lots of flap back = Not good!
But any chance of getting back to the original thread?!! :)