In level flight all forces must be in balance,that mean center of overall pressure(lift) is in line with C.G.

enter image description here

Disturbance comes in such way that shift C.P. back. enter image description here

Is this glider stable?

If I use aircraft stability definition,glider is not stable ,because neutral point is in front of C.G.

(for me glider is stable,because lift in reality act in CP,so lift make counter clock wise torque about C.G., this plane will put nose down.better to consider pitch moments for every AoA with corresponding position of center of pressure..)

I dont understand why proffesor draw lift at A.C., if lift act at C.P. ?

  • $\begingroup$ It would be helpful if you annotated the video with the time of the diagram you don't understand. It is quite long for searching for it. $\endgroup$
    – Jan Hudec
    Commented Jan 19, 2021 at 15:49
  • $\begingroup$ @JanHudec for example at 5:30 he drawn Cl(lift) vector at A.C..... $\endgroup$
    – user53913
    Commented Jan 19, 2021 at 17:26
  • $\begingroup$ I indeed looks like he's drawing the lift in A.C., and the moment to fix it up. $\endgroup$
    – Jan Hudec
    Commented Jan 19, 2021 at 17:33
  • $\begingroup$ @JanHudec But lift dont act in A.C. ,.. $\endgroup$
    – user53913
    Commented Jan 19, 2021 at 17:37
  • $\begingroup$ Lift acts distributed over the wing. You can compose the forces as acting at C.P. with no torque, or as acting at A.C. with torque. While the former seems easier, for some calculation when you need to take bunch of other torques into account, the later makes more sense! $\endgroup$
    – Jan Hudec
    Commented Jan 19, 2021 at 17:38

3 Answers 3


Disturbance comes in such way that shift C.P. back.

But which way is that disturbance? Is it increase or decrease in angle of attack?

Remember the definition of neutral point: the pitching moment around it does not change with angle of attack. So if the C.P. shifts back further from the N.P, the total lift mus decrease so its product with the longer arm remains the same.

Decreasing lift means the disturbance was decrease in angle of attack.

And what moment around C.G. did the disturbance create?

C.G. is now ahead of C.P., so the resulting pitching moment is nose down. And nose down moment decreases angle of attack.

Therefore decrease in angle of attack is causing further decrease in angle of attack and the design is therefore unstable. It will not return to trimmed angle of attack, it will keep pitching down.

  • $\begingroup$ My ides was that disturbance increase AoA,but is seems that is not possible in my case.1)Why proffesor draw lift at AC, if lift act at CP? LIFT DO NOT ACT AT A.C. 2) Why we dont use center of pressure for stability,like rocket,arrows, and every one else use? $\endgroup$
    – user53913
    Commented Jan 19, 2021 at 14:46
  • $\begingroup$ @EBV821 ad 1) you can draw the lift at C.P. with no moment, or you can draw it at A.C. with the (constant) moment. They are equivalent decompositions of the forces. $\endgroup$
    – Jan Hudec
    Commented Jan 19, 2021 at 15:51
  • $\begingroup$ @EBV821 ad 2) the centre of pressure moves with angle of attack and elevator deflection and you need to know how it moves to find the stability, which is what neutral point indicates. Longitudinal stability is more complicated than simple weathervaning of rockets and arrows. $\endgroup$
    – Jan Hudec
    Commented Jan 19, 2021 at 15:52

Typical Designs

You're starting out from a fairly mistaken idea of how things normally work. In particular, you're showing the horizontal stabilizer lifting upwards.

At least in most normal designs, the horizontal stabilizer does not lift upward at all. In fact, it's normally flying at a negative angle of attack, so its "lift" is actually pushing downward on the tail.

We normally start by looking at the center of lift (CP) of the wing by itself. That'll typically be somewhere around 40-45% of mean aerodynamic chord (MAC).

The CG is at least somewhat forward of that (25% of MAC is a pretty common starting point).

Then you trim the downward lift from the horizontal stabilizer to maintain level flight. The CP acts as basically the fulcrum point. We put the horizontal stabilizer at the end of a long tail to give it a lot of "leverage", so it doesn't have to generate much downward pressure to maintain level flight.

This is what gives static stability. If you stall the wing, it will lose lift. As the wing loses lift, the CG being so far forward will cause the aircraft to pitch nose down. That, in turn, will (at least normally) reduce the wing's angle of attack, so it will recover from the stall.

If the CG were arranged where you've shown it, behind the CP, you wouldn't get that automatic stall recovery--if the wing or (especially) the horizontal stabilizer stalled, the CG would be far enough back that the aircraft would tend to pitch nose up, increasing the angle of attack, stalling the airfoil even more deeply (a "deep stall", which is usually unrecoverable).

Alternative Designs

Although it's fairly unusual, you could do a design where you kept the CG somewhat behind the CP, but loaded the wing more heavily than the horizontal stabilizer. In this case, both the wing and the stabilizer would be lifting upwards, but since the wing is more heavily loaded, at higher angles of attack, the wing would stall before the stabilizer, and when that happened, the nose would drop.

To achieve the front wing being more heavily loaded than the rear, you typically end up having to shrink the front wing, and (to maintain overall lift) expand the rear wing. And that gives a canard design, such as the Rutan Vari-EZ and Long-EZ, and the Quickie Q2. Thanks to their use of canards, the Rutan designs are extremely stall resistant. The Quickie claimed to be completely stall proof (and as far as I know, that was accurate).

  • $\begingroup$ In gliders the horizontal tail is near neutral and produces a bit of lift near the speed of the best L/D, so your alternative designs paragraph captures gliders better. Details depend on c.g. location, of course. And when the canard wing stalls, wouldn't that qualify as a stall as well? $\endgroup$ Commented Jan 19, 2021 at 7:21
  • $\begingroup$ @PeterKämpf: Some competition gliders do. Almost no other does (and not all competition gliders either). As far as the canard stalling goes: not in the conventional sense. Basically, all that happens is that if you keep pulling back, the nose simply quits rising. Part of the canard is undoubtedly stalled, but that's about it, so the plane slows down, but otherwise it just keeps flying almost normally. $\endgroup$ Commented Jan 19, 2021 at 7:45
  • $\begingroup$ @JerryCoffin Isnt (CP)lift act at approx 25% of chord line? Where do you find 40-45% ? $\endgroup$
    – user53913
    Commented Jan 19, 2021 at 14:50
  • $\begingroup$ @EBV821: it sounds like you're thinking of the aerodynamic center (which is normally at 25%, at least for an uncambered airfoil). The Center of pressure can/does move around quite a bit, depending on alpha. $\endgroup$ Commented Jan 19, 2021 at 16:55
  • $\begingroup$ @EBV821: It could be too much--I was going from memory, so the exact number that came out of my <strike>sieve</strike> brain could easily be wrong (or something that stuck in my memory because it was unusual). $\endgroup$ Commented Jan 19, 2021 at 20:59

EBV821, you seem to be very determined to learn a lot quickly, so I hope this helps:

Aircraft are unique in that they are really doing 2 things at once: holding themselves up and moving through the airmass.

The focus here is on directional stability. Matching center of pressure with center of gravity takes care of holding any object up, now we must focus on the forces around the center of gravity due to movement.

The $nuetral$ $point$ is simply the dividing line between directional stability and directional instability.

So, alas, you gliders are directionally unstable, therefor they cannot be staticly stable.

Notice if you increase the size of the tail, you can move the net aerodynamic center back. Or, you can simply move the weight forward.

Center of lift is balanced with tail trim, Directional stability is dependent on placing CG ahead of the Net (wing AND tail) AC (neutral point).

In your case (second diagram), CP moves back, glider pitches down, and continues to pitch down even if disturbance is removed.

  • $\begingroup$ @RobertoDigiovanni In your case (second diagram), CP moves back, glider pitches down, and continues to pitch down even if disturbance is removed. When glider pitch down, CP shift to original position at first picture,so pitching stop..that was my original idea.. $\endgroup$
    – user53913
    Commented Jan 19, 2021 at 14:57
  • $\begingroup$ @EBV821 thats why I mentioned planes doing 2 things. Moving CP back is what (more) down elevator will do. The reason the rotation continues is the plane is directionally unstable. If it were directionally stable the rotation stops, and reverses when disturbance is removed. As you go on, you will find directionally stable can be staticly unstable with change in speed due to changes in CP, depending on airfoil type and positioning of CG. $\endgroup$ Commented Jan 19, 2021 at 15:59

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