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

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    $\begingroup$ An aircraft does not need a down force on the tail for stability. All that is required is less lift per area. $\endgroup$ – Peter Kämpf Jan 1 '18 at 13:41
  • $\begingroup$ 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. $\endgroup$ – Charles Bretana Jan 5 '18 at 1:22
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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

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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.

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  • $\begingroup$ "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. $\endgroup$ – Koyovis Jan 1 '18 at 17:23
  • $\begingroup$ @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? $\endgroup$ – Charles Bretana Jan 2 '18 at 2:47
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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.

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    $\begingroup$ The original Mirage iii was a tailless delta without a canard. The dual cockpit trainer version did have one. $\endgroup$ – Koyovis Jan 1 '18 at 13:01
  • $\begingroup$ 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. $\endgroup$ – Charles Bretana Jan 10 '18 at 22:02

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