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I can see that if the tow is perfectly aligned with the axis of the engine thrust, that it is equivalent to pure parasitic drag and similar to some reduction of thrust. However, in the real world the tow is basically always misaligned to some degree or the other, with occasional significant yaw/pitch moments on the tail of the tow plane.

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None

The effect of the glider is purely drag, which the tow plane must compensate for with more thrust. Both aircraft lift themselves. The tow plane thrust counteracts the induced and parasitic drag of the entire flying unit.

The effect on aerodynamic performance would be comparable to a twin with one engine out, or trying to fly with speed brakes open, with the added weight of the glider also affecting acceleration. The weight issue is offset with the wings of the glider, so now you have a draggy biplane lifting more weight on only the towplanes thrust.

The tow plane, lifting only itself, will have the same stall speed (for its weight) theoretically, but ...

In practice, one may greatly increase safety margin by adding extra airspeed while taking off and climbing to help avoid the potentially disastrous effects of uncommanded changes in pitch, which could pull the tow plane over its stall AoA or nose it into the ground.

Proper location of the tow attachment on both aircraft would be useful in minimizing pitch tendencies, ie, close to the center of gravity, as well as careful piloting of the glider, and trimming of the towplane.

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  • $\begingroup$ Actually, one might expect a glider to do very well lifting its weight with as little drag as possible. Working on a 14,000 mile bipe 747 with a symmetrical variable AoA top wing as an extra fuel tank. $\endgroup$ Oct 10 '21 at 17:26
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    $\begingroup$ The dangerous problem is not so much stalling the tug as the glider doing too well lifting its own weight, getting higher than the tug, and pitching the tug nose down. This risk increases if the glider's "correct" position on a short tow rope is affected by the tug's downwash, and a small upwards excursion quickly turns into a big one if it comes out of the downwash airstream. Turbulence can cause similar hazards. $\endgroup$
    – alephzero
    Oct 10 '21 at 20:41
  • $\begingroup$ @RobertDiGiovanni Assuming it's not in the wash of the plane. $\endgroup$ Oct 11 '21 at 3:53
  • $\begingroup$ On your last sentence, I presume you meant to refer to the centre-line of the aircraft, as attaching to the centre of gravity is (in practice) very bad. $\endgroup$ Oct 11 '21 at 14:54
  • $\begingroup$ @Mark Brockington why is it "bad"? Any additional drag forces (not built into the towplane) could cause dangerous pitching torques. Remember an aircraft has a CG on the vertical axis too. Generally thrust and drag (from the glider too) will be somewhere near it. $\endgroup$ Oct 11 '21 at 16:02
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Well again, a glider would have zero effect on a stall speed of the tow plane, because the tow plane technically does not have a stall speed (remember wings only stall if they exceed the critical AoA - this can happen at any airspeed and/or at any flight attitude).

Now most tow planes use a release hook mounted under the fuselage just aft of the tailwheel. That’s just about as close to the CL thrust axis of the aircraft as structure will allow. If anything, this placement should tend to lower the nose of the tow plane and decrease its AOA.

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Looking at this question from a fundamentals of flight perspective. The airplane towing the glider would have its forces in balance as they usually would, however there is a glider in the back that is causing quite a bit of drag.

Since airplanes climb due to an excess of thrust, the airplane will climb at a much lower rate. The pitch attitude would be lower during the climb to maintain the proper VY speed. The pitch attitude at which the airplane stalls would be lower but under steady state flight (1G), the stall speed would remain the same.

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  • $\begingroup$ Parts of your answer are not technically true. Airplanes climb due to an excess of lift. They can maintain that excess lift (and thus climb) if they can provide thrust to counteract the additional drag that that the lift provides (assuming that the parasitic drag doesn't change much whether climbing or in level flight). $\endgroup$
    – Jon V
    Oct 11 '21 at 16:38
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    $\begingroup$ I'm sorry. You are incorrect. An airplane climbs due to excess thrust. $\endgroup$
    – wbeard52
    Oct 11 '21 at 21:57
  • $\begingroup$ @JonV -- see for example aviation.stackexchange.com/questions/40921/…, and other related answers to the same question. To a first approximation, in a steady climb, lift = weight * cosine (climb angle), and lift is therefore less than weight in a steady climb. $\endgroup$ Oct 12 '21 at 15:31
  • $\begingroup$ Yet at the same time, we can also observe that we need to have lift greater than weight to make the flight path curve from level up into a climbing path -- so if we could never generate more lift than weight, we could never climb -- and so the endless discussion goes on and on . Anyway, for modest climb angles, the excess of thrust compared to drag must be much greater than the discrepancy between lift and weight. (At least, if we are speaking in terms of ratios.) $\endgroup$ Oct 12 '21 at 15:31
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If the glider is being towed in perfect alignment with the longitudinal axis of the tow plane and the thrust, then no - the stall speed will be the same.

In reality the tow would never be perfectly aligned like that and there would be some effect on the stall speed.

For instance, if the glider is low i.e. below the plane axis, there would be a downward component to the force acting on the tail of the tow plane, at the attachment point of the tow cable. This force is equivalent to an increase in weight (i.e increasing the stall speed) and a rearward shift of the C.G.

Nevertheless, during typical, stable tows, even a large 25m glider generates no more than 40N or so of drag. Near the stall speed of a typical tow plane, that might drop to well under 15N. Even at a relatively very low tow, say 30 degrees below the plane axis, that means an equivalent of a mere 1.5lbs "weight" attached to the tail of the tow plane as far as the effect on the stall speed is concerned.

The opposite would apply for a high tow, i.e. a reduction of the stall speed.

Probably no effect by lateral shifts of the glider, assuming the tow plane remains coordinated.

Furthermore, as soon as the stall starts developing the tow plane's speed would start dropping resulting in a sharp decrease of the tension in the tow cable thereby practically eliminating any effect the tow might have had on the stall.

In short, the stall speed may increase or decrease depending on the glider's direction of deviation from perfect alignment with the plane axis, but the change is likely to be negligible in most real world conditions.

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  • $\begingroup$ I'm not convinced a pull-up at the tail will lower the stall speed. This needs to be counteracted by extra negative lift of the horizontal stabilizer, and then to the main wing the situation looks the same. If it's not counteracted by the stabilizer, the tug will quickly pitch more and more steeply down – which certainly means it won't stall, but not that it can get any slower. $\endgroup$ Oct 11 '21 at 12:45

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