Stability

Dear Fellow Aero Engineers/Aviation Enthusiasts/Students

What is THE most stable aero-structure?

glider? flying wing? swept-forward wings? delta? aeronvironment?

I'm trying to design a platform which provides maximum (positive)stability with minimum manoeuvrability.

*I mean positive stability by a platform which returns to its original trim condition when it's disturbed by wind-gust; especially wind blowing from under the aircraft heading upwards (i.e. z-axis).

Thank you.

The best planform for this is the Marske Flying Wing... it readily adjusts nose down to an updraft... as in a thermal.

See our web site for more details.

What is your web site address?

I have heard and/or read about this on other posts, also that the B2 flying wing has a ride similar to a dime store pony in that it rocks back and forth in turbulance. What is this and does it require a stronger air frame? I seem to recall an article about how the Pioneer could fly faster in turbulance with a lower wing loading because the pilot wasn't bashing his head against the canopy like he would in a regular glider. Is there an explanation of this somewhere?

It's probably true. It certainly is for the homebuilt canards such as the Long-EZ.

The thing is that the elevator, whether its at the back or the front, is operating at a higher wing loading than the main wing, and thus has less response to vertical gusts. When the wing in the front hits an upwards gust it will go upwards, and when it hits a downards gust it will go downwards. Not much you can do about that. But what about a fraction of a second later when the entire aircraft is in the new air mass, but hasn't had time to accelerate up or down much yet?

With a conventional aircraft, the main wing accelerates up (in an upwards gust) faster than the tailplane, thus causing a nose-up pitching movement which increases the lift and G load generated even more.

With a canard aircraft, the (rear) main wing accelerates up (in an upwards gust) faster than the canard, thus causing a nose-*down* pitching movement which *decreases* the lift and lowers the G load.

And the reverse in a downwards gust.

How large this effect is is going to depend on a lot of factors. I wouldn't be surprised if you could optimize a design so that at high speed (where you don't need much change in angle of attack to cause a big change in lift) you could cancel out a large part of the gust loading.

I don't know how much you'd *want* do to it because, like any stability, it's going to cut down on control response then you *want* the aircraft to do something. For a long-distance cruiser it's probably a good idea, especially if it doesn't happen so mcuh at pattern/thermalling speeds.

Flying wing aircraft such as the Pioneer or Genesis have positive pitching moment airfoils.(reflex wing) As the wing moves through the air there is a downward pressure on the trailing edge and it wants to pitch up slightly or continue flying depending on the amount of reflex built into the airfoil. This means that at all speeds, but especially at higher speeds, the center of lift is at or near the leading edge of the wing. One way to visualize this is to imagine a yaw string placed at the side of the canopy and used as an angle of attack indicator. The center of lift of the yaw string is at the tape junction (leading edge) and the string flows backward indicating the relative wind. Any change in pitch causes a change in the relative wind and the string instantly moves to indicate this change. A flying wing moving through the air behaves in a similar way. The center of lift near the leading edge anchors the wing in the air with the leading edge as the pivot point. The trim setting determines the speed at which air will flow over the wing. If the wing is trimmed for 70 Knots and the glider enters a strong updraft the wing will instantly react by changing pitch so that the air moving across the wing is STILL TRAVELLING AT 70 Knots. (give or take phugoid oscillations, friction etc...) Since the air is now moving vertically, the glider gains altitude but the nose drops slightly causing a forward rocking motion. When the glider encounters strong sink, the relative wind is now coming from slightly above the wing and the wing reacts by pitching up slightly while the glider sinks but it still maintains the trim speed of 70 Knots IAS. The effect is to add a forward and backward rocking motion to the lift and sinking motion of the air. This is a very different sensation from a regular glider but in fact it increases the sensation of lift when encountering very weak thermals. Now imagine a Genesis and a Discus flying at 110Kt. The Discus(and all other non-flying-wing gliders) has a negative pitching moment airfoil. As the wing moves faster through the air, the center of lift moves aft. At 110Kt. the center of lift is behind the trailing edge of the wing causing the negative pitching moment. The horizontal stabilizer counteracts the pitching moment but there is a strong vertical force on the stab. When the Discus encounters strong lift the wing pitches up because the pivot point is somewhere behind the wing and the tail still has vertical pressure on it. The G force is vertical. A strong down draft causes the wing to pitch down and the force is still vertical so the pilots head slams into the canopy. On the Genesis and probably more so on the Pioneer, the G forces are spread between the vertical climb and sink and the forward and backward rocking motions from the change in pitch. Therefore, the ride is more gentle although different. Now imagine you are the main spar, ailerons, elevator and tail boom of the Discus (or Nimbus) and a flying wing. On the Discus at higher speeds there is a great deal of lift generated by the wing but there is also a great deal of forward or downward torque on the wings. The main spar is flexing upwards and is being twisted forward. As you go into a right turn the left aileron goes down increasing the torque on the left wing and the right aileron goes up decreasing the torque on the right wing. Since the Genesis and Pioneer maintain their center of lift near the leading edge of the wing at all speeds the torque is absent (or very minimal). The main spar is flexing upwards but is not being twisted forward. A turn generates torque on the spar but the change is not nearly as great as on the Discus. Ditto for the tailplane. The elevators on the Discus (or Grob 102) have a lot of vertical pressure on them at higher speed flight. As they change their angle of attack the changes in pressure are very high. The Pioneer and Genesis elevators maintain the pitch axis and trim but they don't have to counteract the torque caused by an airfoil which wants to dive into the ground. The aerodynamic stresses on them are much less. What causes flutter? Which aircraft are more likely to shred their control surfaces when they encounter turbulance during higher speed flight? The 'ride' of a Pioneer, Genesis or any other flying wing is certainly different due to the forward and aft rocking motion in turbulance but the overall stress on the airframe and pilot are much less. The stress on the airframe is less on almost all aspects of a flying wing. Tail booms, air brakes, ailerons, elevators, rudder, wing spars, tailbones, headbones and bladders.

I don't know about the B2, but it's true of the Pioneer. The reason is the very low moment of inertia about the pitch axis that is typical of Marske flying wings. This allows the static stability of glider to cause a pitch down into a vertical gust fast enough to reduce the angle of attack and relieve the wing spar loading. Theoretically, this should translate into higher permissible turbulence penetration speeds.

Put another way, a flying wing glider stays much nearer it's trimmed angle of attack in turbulence than a conventional glider with a long tail and large moment of inertia about the pitch axis.

Since the pilot is sitting very near the CG, little of this motion is felt. You mainly notice it on the 'G' meter which shows much lower forces than would be expected.

Bill Daniels

[Flying wing]

I strongly doubt that. If it were that way then the center of gravity was *behind* the center of lift, creating a pitch-up momentum that would have to be counter-acted by a downward deflection of the elevator. What is obviously not the case...

Of course: If the aircraft has got a wing with negative sweep (like the Genesis) the center of lift is at the leading edge of the wing root (where the pilot's center of weight is) - but seen over the whole wing, it is more aft, which is about 25-35 percent of the mean aerodynamic chord.

Hmm... I also doubt that. A center of lift that far aft could not encountered by the horizontal stab.

The negative pitching moment is simply created once the center of lift moves behind the center of gravity - and this happens at any speed over the speed for best l/d. All conventional gliders are designed in a way that the horizontal stab will not create any lift at the speed of best l/d (about 90-100 kph).

Not as strong as many people think - Cl of the horizontal stab is usually below 0.2 at high speeds.

Hmmm.... I do not think this is correct. Given that the elevator is in a fixed position, the aircraft is trimmed for a certain angle of attack. Flying into rising air will increase the angle of attack, therefore the aircraft will take its nose up in order to get back to its trimmed AoA. Slowly. There is *not* a sudden change of pitch.

The G-Force that can be felt is mainly due to the vertical acceleration that is caused by the rising air (higher AoA - higher lift, therefore acceleration), not (!) by the pitch-up.

The same goes for negative g's - they are not caused by rapid pitch-downs, but simply by the negative vertical acceleration when the aircraft is passing sinking air.

Well... in fact the spar is not twisted at all. The torsion nose is what stops the wing from twisting - the spar's only function concerning torsion is that it forms one side of the torsion nose (not even that on modern designs where the whole wing shell acts as a torsion box).

Hmmm.... as soon as you deflect a control, you will get a torsion momentum. The strength of this torsion momentum is only a function of control size and deflection angle (with minor influence of the airfoil).

With asymmetrically deflected ailerons the asymmetric torque is the same as in a conventional design - but of course the overall torque is lower. With neutral ailerons there is indeed only very little torque on a flying wing (Horten designs and SB-13 are completely different here).

Don't confuse stick forces with aerodynamics The cause for the high stick forces that make it impossible for most non-flapped gliders to be trimmed for level flight over 200 kph is simply that the spring providing the artificial trim force is not strong enough to overcome aerodynamic forces at high speeds that want to centralize the elevator according to the airflow. The stick force does not represent the aerodynamics at the horizontal stab at all.

Hmm.... As long as you stay within the allowed speed range, no aircraft is going to shred its part in turbulence. Today wing flutter caused by sructural weakness of the wing is not a problem anymore. Usually wing flutter is caused by aileron flutter - and THIS can happen on any aircraft, be it conventional or flying wing.

Tail boom: Flying wings don't have one. Clear advantage for them. On the other hand - I've never heard of a tail boom that broke inflight.

Air brakes: Please enlighten me why extending airbrakes of identical design and size at identical speeds should show an advantage on a flying wing.

Ailerons: See above.

Elevators: Never heard of an elevator disintegrate inflight.

Rudder: The rudder does not know about the airfoil on the wing. Therfore there's little cause to assume that stress on it should be different at all. On the contrary - rudders ore usually smaller on conventional aircraft than on flying wing designs.

Wing spar: It only carries vertical g-loads. Not torsion. Not much difference if I pull 5 g's in a discus or in a Genesis - the forces on the spar will be the same.

Structurally there is not much of an advantage of a flying wing to a conventional design. This is underlined by the fact that the Genesis (as the only flying wing in the same performance and speed range as conventional designs) is not lighter.

Bye

It has already been done - take a look at the B-1B bomber and its small canard wings on the nose. Their only purpose it to stabilize pitch in vertical gusts. They are doing this by active movement, but the effect is exactly what you describe.

Bye

There are 2 ways of describing the aerodyamic forces on a wing, both are equivalent, but you can't mix both. One way is to reduce them to an unique force applied to a fixed point, usually 25% of the mean chord, plus a (positive or negative) pitching moment. The other way, which works only when the unique force above is not zero, is to combine this force and the moment into a unique force applied at a suitable point, sometimes called center of lift. It is clear that in this second description, for a conventionnal cambered airfoil, the center of lift can go far aft of the wing. This is because in the first description, these airfoils have a pitching moment even at zero lift, so in order that in the second description the unique force provides the same pitching moment, as the force goes near to zero, the lever arm has to go near to infinity.

Actually, with a conventional (unstable) wing airfoil, both should be in opposite direction. If as you said the position of the CG is so that no lift is generated by the tailplane at best L/D and so no trimming force is needed (assuming a symmetric airfoil for the tailplane), when flying at higher speed, i.e. lower angle of attack, the unstability of the wing tends to further lower this angle of attack and must be countered by a downward force on the whole tailplane, although the moving part itself is subject to an upward force. Isn't that which makes all moving tails give strange