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Since Delta winged aircraft don't have horizontal stabilizers to produce down force, how is that problem solved ?

Federico
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Arth Patel
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    An aircraft does not need a down force on the tail for stability.

    All that is required is less lift per area.

     – Peter Kämpf Jan 01 '18 at 13:41
  • All aircraft, to maintain flight in any static, stable condition, must be in rotational equilibrium abut their center of gravity. i.e, the sum of all pitch moments about the CG must sum to zero. Any condition where the sum of pitch moments is not zero will cause a pitch or Angle of Attack change, which will continue, either (if stable) until the aircraft reaches pitch moment equilibrium, or (if unstable) indefinitely. Lift per unit area has absolutely nothing to do with this. – Charles Bretana Jan 05 '18 at 01:22
  • The most numerous jet ever produced, the MiG-21, was a tailed delta. You appear to be asking about tailless aircraft, which is different thing altogether. Are you even thinking there is a difference between swept and delta wings in this respect? Please edit your question to clarify which you mean. – Guy Inchbald Jul 06 '21 at 16:24
  • @CharlesBretana While what you say is absolutely correct, the last sentence is wrong. In order to have that desired pitch moment change over angle of attack, the relative lift per area between forward and rear surfaces / parts of the wing is what counts. – Peter Kämpf Jul 06 '21 at 17:33
  • @Peter, Perhaps, this discussion, being detailed, should be elsewhere, but are you calling it wrong because of my use of the word "Absolutely"? Clearly, Only including relative values of "Lift/area" is not sufficient (how far from the CG are these lift forces applied?!), but potentially misleading, as it is the the moments that are created, and how those moments change as AOA changes, that determines stability or lack thereof. Would you not agree? – Charles Bretana Jul 08 '21 at 14:16
  • To answer the OPs question requires that you analyze the lift *distribution" over the surface of the wing from leading to trailing edge. We use vectoral summations of the the actual, real, lift forces so often, that we forget that they are engineering conveniences used to create clarity or make calculations easier. The actual forces are from molecules hitting the surface of the entire airframe. On a tailless delta wing aircraft, the lift forces on the front of the wing pitch the nose down, and the back of the wing creates down lift (and a nose up moment) that compensates for this. – Charles Bretana Jul 08 '21 at 14:22

3 Answers3

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The effect of a horizontal tail can be built into a tailless aeroplane in two ways:

  • By integrating into the wing profile: a horizontal s-shape with the trailing edge turned up.
  • By combining positive sweep with negative twist.

From a decades old course book, paper copy only

Koyovis
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Because, although we often conceptualize Lift as all acting through the "center of Pressure" of the wing surface, this is a fiction, done only to aid in visualizing the total lift, and it's effects, and to aid in doing simple calculations that rely on this approximation.

In actuality, Lift itself is an artificial abstraction, as it is just a portion of the aerodynamic force acting on every square inch of the aircraft body. And this force, at each point on the aircraft surface, creates two physical effects.

  1. The force accelerates the aircraft, in the direction of the force is applied, according to the formula F=ma. The total aircraft acceleration is the vector sum of all the individual accelerations at each point on the surface.

  2. Secondly, at each point of the surface of the aircraft the force applied creates a rotational force (a torque) about the aircraft CG. dependent on the magnitude if the force and the distance between the line of the force and the CG (the moment Arm)

With a delta wing aircraft the wing is designed so that the trailing end of the wing will be generating a small downwards force even when the bulk of the wing forward of the trailing edge is still generating an upwards force. Although the sum of the all the forces is still upwards, and creates positive lift, the small downwards force from the trailing edge is much further from the CG, so it has a longer moment arm, and therefore creates sufficient nose up torque to balance the nose down pitching moment from the rest of the force on the wing, which is closer to the CG with a shorter moment arm.

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In order to achieve dynamic stability with any aircraft where pitch control is implemented manually through direct pilot inputs, the overall pitching moment from increasing positive lift must be nose down, (i.e., - the sum of all Aerodynamic forces must be behind the CG). This is so that minor deviations from equilibrium will create a correcting pitch moment which will bring the aircraft back into pitch equilibrium. This requires that the control surface, in order to balance this nose pitch moment, must create a nose up pitch moment. Aircraft with their pitch control surface at the tail, therefore, incur a drag penalty (tail lift down reduces total positive (up) lift, so to achieve level flight the sum of all positive lift must be greater, increasing drag).

In modern aircraft, the introduction of computer driven flight control systems has mitigated the need for this. In the F-16 (subsonic) for example, the Center of pressure is actually slightly ahead of the CG, and creates a nose up pitching moment. The tail surface, therefore, which is substantially behind the CG, must create a nose down pitching moment to achieve equilibrium. This means the tail is actually creating up lift as well as the wings, augmenting rather than decreasing, trim drag. Stability is artificially created by the flight control software. In flight, you can see the stabilator constantly moving up and down as it responds to small deviations in pitch, keeping the aircraft stable.

Charles Bretana
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  • "In modern aircraft, the introduction of computer driven flight control systems has mitigated the need for this". Very few modern aircraft are aerodynamically unstable like the F-16 is. Very useful in a very manoevrable fighter jet, not so in an airliner that needs to bring people home safely. – Koyovis Jan 01 '18 at 17:23
  • @Koyovis, I confess I do not know which aircraft, (other than the F16), take advantage of this capability. In addition to the increase in maneuverability, It also provides a significant reduction in trim drag and fuel efficiency. But it seems that once you have made the commitment to let a computer be responsible for flight control, with triply redundant systems, it would be foolish to not also take advantage of the drag reduction and increased fuel efficiency that this approach provides. Do you know which aircraft (if any) use this approach? – Charles Bretana Jan 02 '18 at 02:47
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    You start right with “overall pitching moment from increasing positive lift must be nose down”, but then the part about tail creating pitch up moment does not follow—well, it does around suitably chosen point, but it can create positive lift as long as it creates less of it per unit area than the wing—nor does the part about drag—when the tail flies in the downwash, creating downward lift may still create thrust that, offsetting some of the induced drag increase on the main wing. – Jan Hudec Jul 06 '21 at 16:57
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Most of Delta-Wing aircraft are equipped with a canard in different ways: fixed (Mirage III), movable (Typhoon) or retractable (Tupolev Tu-144). This canard is lifting surface similar to tail, which is ahead of wing and creates lift, which will produces pitch-up moment. It should be mentioned that tail's negative lift also produces pitch-up moment, which is required for trimming the aircraft.

JetSonic
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    The original Mirage iii was a tailless delta without a canard. The dual cockpit trainer version did have one. – Koyovis Jan 01 '18 at 13:01
  • What do you mean by the "tail's negative lift" ? do you mean the negative lift from the aerodynamic forces on the aft portion of the main (delta) wing? If so, then you are correct, and the presence of a forward canard on many delta wing aircraft reduces the need for this negative lift on the aft portion of the delta and reduces the trim drag and range/endurance performance hit from that trim drag. – Charles Bretana Jan 10 '18 at 22:02
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    There were many tailless deltas without canards, including Concorde. And fixed or retractable canards don't help with control anyway, so they are irrelevant for the discussion. – Jan Hudec Jul 06 '21 at 17:32