WW2 aircraft tail load, up or down ?

Discussion in 'Aviation' started by Timppa, Jun 9, 2013.

  1. Timppa

    Timppa Active Member

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    This is a technical question. I know that the horizontal stabilator load does not have to be down for the aircraft to be stable. (I hope nobody disagree on this). I believe that most aircraft had basically upward tail load for most flight conditions, but are there any numbers ?
     
  2. drgondog

    drgondog Well-Known Member

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    Timppa - conventional tail behind a cambered wing must have at least a slight down lift to overcome CMac at zero lift. CM changes (decreases) with AoA. When AoA changes the contribution of the elevator must offset the Delta CM due to the new airspeed (as f(AoA)).

    The question is about stability at all speed ranges - the elevator trim required is both positive and negative depending on where you are in the CL range with aft cg being the most scrutinized condition at low speed.

    Near CLmax , after the aircraft is trimmed for equilibrium for a specific velocity, the elevator trim is 'up' (negative angle gamma by conventions- which decreases the Lift on the tail) as the velocity decreases below equilibrium.

    Conversely - for a speed above the trimmed velocity, the elevator deflection angle is down (deflection 'down' by convention is 'positive') thereby increasing the lift on the tail

    Just about any aero book will devote a chapter on Stability and the derivatives of Tail Lift Coefficient as function of AoA and angle of elevator deflection.. the more complicated game is solving for Static margins and associated trim drag contributions..
     
  3. swampyankee

    swampyankee Active Member

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    Insofar as I remember (I don't feel like digging out my books on aircraft stability control), there must be a net down load on the horizontal stabilizer of an aircraft with conventional configuration to have positive stick-free stability. Some aircraft may have operated with sufficiently aft c/g to have net upward lift on the horizontal tail, but the technical term for these when maneuvered was "death trap," as they were liable to pitch up and go into a flat spin.
     
  4. gumbyk

    gumbyk Well-Known Member

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    With a down-force on the tailplane, if the aircraft slows (engine failure, throttling back, etc), the aircraft slows, the down-force reduces, and the aircraft nose pitches down, which increases airspeed again (albeit with a descent).
     
  5. Ivan1GFP

    Ivan1GFP Member

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    In general, you can count on the horizontal stabilizer providing down force for normal flight conditions. There are exceptions however: The P-51D Mustang with its 85 gallon (IIRC) fuselage tank even half filled had the stabilizers providing significant lift. This made the aircraft very unstable and also "unsafe" to fly. With significant lift provided by the stabilizers, if the aircraft was stalled, the stabilizer would stall first which would increase the angle of attack of the wing and likely cause a structural failure because of the increased load. That is why the fuselage tank had to be burned off before the drop tanks and also why even though the tank was 85 gallons, it was never filled to capacity.

    The Aircraft's Aft Center of Gravity limit is established to prevent this stall and structural failure condition from happening.
    Aircraft are usually tested (by the manufacturer) with the center of gravity as far aft as allowable to get the best performance numbers. It helps to not have the stabilizer providing downforce to counteract the lift by the wings. The problem is that as the aircraft's center of gravity is moved rearward, the longitudinal stability is reduced. The aircraft gets very wobbly on the vertical axis. It often flies what is called a "Phugoid".

    So, the short answer is: Yes, the stabilizers can provide lift and it will give the best straight line performance, but it is generally avoided because the controllability and stability are not so good.

    Hope this helps.
    - Ivan.
     
  6. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    I don't think the aircraft was "unsafe to fly' in normal operating conditions, unsafe while performing any kind of upset maneuvers or areobatics.
     
  7. tyrodtom

    tyrodtom Well-Known Member

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    My understanding ( which is little ) is you want the tail at less of a angle of attack so it will stall last, the wing will stall first. The nose drops, restores lift over the wing.
    If the tail is providing lift, and it stalls first, the tail drops and you've got no way to restore the aircraft to flight attitude. You fall out of the sky in a flat spin from which the aircraft cannot be recovered no matter how much altitude you have.
    Aircraft with canards, are set up with lift on the canards, that way if the canard surface stalls, the nose drops, restoring flight.
     
  8. KiwiBiggles

    KiwiBiggles Member

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    Given that the tailplane is a less efficient lifting body than the mainplane (symmetrical section, lower aspect ratio, lower Reynold's number), I would expect that ideal trimming would have no load on the tailplane at all, giving the lowest tailplane drag. This trim would also give the highest stability for a given size of tailplane.
     
  9. gumbyk

    gumbyk Well-Known Member

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    That's also the most dangerous. That is where there is zero stability longitudinally.
     
  10. Timppa

    Timppa Active Member

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    See especially chapter 4 in the following document :
     

    Attached Files:

  11. Ivan1GFP

    Ivan1GFP Member

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    What you are describing is the "Neutral Point". It isn't the most dangerous CoG location. It is possible to be stable even with the CoG behind that point. At some point further aft, there will be enough load on the aft "lifting" surface so that it will stall before the front. THAT is the dangerous point, but most folks avoid that because by that time, the aeroplane will tend to wander all over the place.

    We keep thinking conventional aeroplane with a big wing in front and little bitty stabilizer in back. The general rules apply regardless of configuration. It could be a canard or something like Langley's Aerodrome and the same rules apply: The aft surface cannot stall first.

    For stability, what needs to happen is that if the aircraft is perturbed a bit, the aerodynamic force will straighten it out and restore the original direction of flight. If that restoring force is high, the aircraft is VERY stable. If the force is low, the aircraft has LOW stability. If there isn't any restoring force, the aircraft is NEUTRALLY stabile. If the force tends to move the aircraft away from its original direction of motion, it is UNstable.

    - Ivan.
     
  12. swampyankee

    swampyankee Active Member

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    I recall that one of the airline manufacturers (in the US, they're all Boeing now; this was about 30 years ago) concluded that the optimum for minimizing operating costs was a slightly stable conventional configuration.

    As an aside, one of the issues preventing airlines from using aircraft with the canard configuration is that they tend to have more demanding runway requirements and with the wings largely behind the c/g, it's tough to find someplace to put the fuel.
     
  13. nincomp

    nincomp Member

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    The Beech Starship solved the runway and poor max lift problems with a variable-sweep canard. The canard would sweep foreword to allow flaps to be deployed on the main wing. The wings would move rearward under cruising conditions and to compensate for changes in balance as fuel burned off. ...And it was beautiful! I saw it fly once.
    starship.jpg
     
  14. Timppa

    Timppa Active Member

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    There was a discussion in PPrune:

    J.C. Gibson (British Aerospace, Warton), wrote an reply to Air International of this misconception:
    (See also post 10, from the above author)
     
  15. Ivan1GFP

    Ivan1GFP Member

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    I was told that a 747 will sit on its tail if the wing tip tanks are filled first.

    - Ivan.
     
  16. bobbysocks

    bobbysocks Well-Known Member

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    i am not sure if i believe that just from fuel. i do know that on certain ac you had to be careful boarding passengers ( distributing the weight ) when the cargo holds were full or they would tilt back. when i worked the ramp loading the smaller commuter planes..some ( saab a340?) even had a brace you had to attach to the tail prior to loading pax or bags.
     
  17. nincomp

    nincomp Member

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    How about fuel and a couple of mothers-in-law sitting in the back seats?:rolleyes:
     
  18. bobbysocks

    bobbysocks Well-Known Member

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    THAT.. i can believe....
     
  19. gumbyk

    gumbyk Well-Known Member

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    We loaded a helicopter into a 747 freighter, and they had a stand under the tail to prevent it from sitting down while loading. IIRC it was only really a problem if all the freight was at the rear, such as when loading the first few items.
     
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