Pitching moment due to drag, thrust and weight longitudinal component.

[b_kimoun 0]

Hi everybody! I wish to know if drag, thrust and weight longitudinal component (weight x sin(gamma) at climb and descent) contribute to pitching moment and how. Thank you very much.

[ronmarks 13]

Well, yes. Any force not acting through the center of mass of the a/c will create a couple that will tend to cause rotation about an axis normal to both the arm and the force itself. A high thrust line will produce a nose down couple, for example. By the way, many sim a/c I have flown have an unrealistic down pitch when you pop the gear. The designer reasons that the gear is below the wing, so this couple should cause a very noticeable down pitch when the gear is extended. However, in real aircraft, the extra drag of the gear under the wing increases wing CIRCULATION. This increases wing lift and increases downwash on the tail. So the net result of extending the gear is some deceleration with very little trim change. Often there is a tiny bit of ballooning - the opposite of what would be expected from the drag effect alone.

[mgh 89]

How does the extra drag on lowering gear the increase the circulation on the wing?

[ronmarks 13]

How does the extra drag on lowering gear the increase the circulation on the wing?

Background. Circulation is a very fundamental thing with any lifting system:If one subtracts the forward velocity from the flow around a lifting wing, the result is a circular flow or vortex viewed from the wingtip. This circulation is up in front of the wing , down behind, forward beneath and aft above. The intensity of this vortex is proportional to the lift per unit of span. Of course, when the forward velocity is added back, there is no actual circle of flow except where this vortex "falls off" the wing at the wingtip. So, anything which increases velocity above the wing or retards or reduces velocity below will increase wing lift with no change in angle of attack, etc. A split flap works this way. The lift increment is about like 5 degrees of extra angle of attack. The flap can be anywhere under the wing but works best near the back. I might add that an extreme effect of this (which I personally experienced as a guest pilot) is that of an airtanker dropping a load of fire retardant. When toggling the drop it required a very aggressive application of nearly full down elevator for 2 or 3 seconds to maintain pitch attitude due to the aerodynamic effect of the released load under the aircraft. Even though I understood and anticipated well, the effect is dramatic and tests a pilot's instincts for attitude control. So, this circulation thing is as real as it gets and like, say ground effect, needs to be applied to get the most accurate simulation. r

[mgh 89]

I don't think that's the answer. If such an effect existed it would affect only a very small part of the wing adjacent to the gear. Also, circulation is a concept derived from potential flow theory which does not account for drag. Do you have any reference to support your view. In the example you give, the need for nose down elevator is to counteract the rapid loss in aircraft weight as the retardant is dropped. Also, as it' generally dropped from the fuselage the effects, if any, on the wings would be minimal

[b_kimoun 0]

Hi, but how would you explain the fact that the aircraft pitches up as one accelerates( increases thrust)? That is, how does the necessary up pitching moment build up?Thanks.

[WarpD 824]

Ok, I just gotta ask... taking Aerodynamics in college or something?

[ronmarks 13]

I don't think that's the answer. If such an effect existed it would affect only a very small part of the wing adjacent to the gear. Also, circulation is a concept derived from potential flow theory which does not account for drag. Do you have any reference to support your view. In the example you give, the need for nose down elevator is to counteract the rapid loss in aircraft weight as the retardant is dropped. Also, as it' generally dropped from the fuselage the effects, if any, on the wings would be minimal

Well, I don't think a lifting wing's horseshoe vortex system was derived from potential flow theory. It was in fact one of the first things that was discovered about wing lift long ago. The vortex system seemed reminiscent of magnetic induction to scientists of the day, so the resulting forces where called induced lift/drag as I remember. Prandtl and others refined the details.While there is a strong effect close to an object in a flow of course, there is also a more general effect. The skinny inner cord of a sailplane substantially effects the general flow 30 feet away. This is why wings are powerful and their gift of flight seems almost magical to many including me. Regarding the extreme pitching tendency induced by a retardant drop, the jettisoning of weight is a very small part of it - as well as obvious. Four or five seconds after the drop, with the load well clear, the plane is back to "normal" and only a small trim change remains. So, the big surprise is produced by the load damming up the flow under the wing increasing lift and downwash on the tail.Another thought relates to an airtanker tragedy that occurred not long ago. A converted C-130 had a wing spar failure just as it was dropping its load. Relieving the a/c of weight would not break a wing, but a lift pulse produced by a dispensed load would. It was recorded on video by some one and may be on youtube somewhere. Re a comment that as an a/c accelerates flies faster?, the nose goes up. This is produced by the angle of the wing and tail. The forward surface must have a greater effective positive angle than the rear plane to have positive static stability. All a/c I know of are like that ( and have to be to be certified FAA part 23 , 25). And finally, If a previous commenter was referring to me about about some one taking a college course, Aero 101 maybe. No, all this stuff was installed in my head years ago. Much of my background is engineering study, collaboration with experts, and even long conversations with John Thorp (Sky Scooter, Piper Cherokee, t-18) as well as contributions to the development of the Stevens Acro, conversion of the Navy S2F to airtanker service and a few other things. I looked a the calender recently and realized I am now definitely a geezer not a college guy, but I'm still learning.

[WarpD 824]

Ok.. no, I didn't ask you specifically if you were taking a college course. As for your claims regarding flow disruption, etc... I think this has more to do with basic laws of motion.

[b_kimoun 0]

Hi you all ! WarpD asking :"Ok, I just gotta ask... taking Aerodynamics in college or something? ". No, I am just trying to understnad flight mechanics and how an iron bird stands in equilibrium in the air and moves around its axes. I am using FS2004 for a long time and thought that it is time to go further to understand how an aircraft flies.Now concerning ronmarks's reply : "Re a comment that as an a/c accelerates flies faster?, the nose goes up. This is produced by the angle of the wing and tail." , I don't think you are right, since speed ( and thus q the dynamic pressure) varies equally for both surfaces so that the total pitching moment remains constant and does not change with change in speed.

[mgh 89]

A wing's horseshoe vortex system is derived from 2-dimensional inviscid irrotational potential flow theory because provides the theoretical relationship between circulation and lift. A vortex’s potential function is ϕ = Γϴ/2π where Γ is the circulation and ϴ is the angle from the origin. It’s a very general theory and fairly somple. It doesn’t determine the lift on a wing section and gives a lift curve slope of 2π regardless of the shape of the wing section. The

of the 2002 C130 accident shows the retardant being released from under the fuselage well away from the wings and seems not to be damming up the flow under the wing. Also, the aircraft was pulling up at the time the starboard wing detached. The NTSB determined that the cause was a fatigue failure as a result of undetected cracks in the wing structure. A fatigue failure can arise when a wing is being loaded or unloaded - it's not dependent on the direction of the load. Can you give a reference in the FARs to the section that states the forward surface must have a greater effective positive angle than the rear plane? The fundamental requirement for static longitudinal stability is that there must be a negative change in overall pitching moment about the centre of gravity following a positive change in angle of attack - symbolically, dM/dα< 0. This is illustrated in the attached figure.

[WarpD 824]

Bottom line... basic physics... in level flight lift=weight. Drop 100,000lbs of weight... serious excess of lift all of a sudden, aircraft baloons up. Really, really basic.

[mgh 89]

Many things can be simply explained by considering the basics.

[ronmarks 13]

mgh's view that there are only local effects by an object in the flow field of a wing system is very common. But it is incorrect.If you wiki lifting line theory, which I am sure you have, you will notice that to derive the circulation and thus lift at any one point on the span, you have to consider all other points on the span. In other words, there is interaction from tip to tip in a wing system. This interaction does not occur just on the lift line or lift plane, but a considerable distance above and below the wing, roughly 1/2 a span. In fact, the basic equation for induced drag is based on the energy required, and thus the drag produced, to propel a cylindrical mass of air with a diameter of one wingspan, downward, the reaction of that being lift. That diameter is properly thought of as the domain of the wing. So, any object which causes a velocity change (thrust/drag) within that diameter will change the circulation and change the wing lift to some extent. The proof of the pudding is that doubling the drag of most a/c by lowering the gear produces surprisingly little trim change because the gear drag couple is compensated for by the circulation effects I have mentioned. The airtanker drop effect is well known. I have personally experienced it and dealt with it as a pilot. Since I was well acquainted with wing theory then, I recognized it for what it was. It is quite different from a load separation, say dropping a streamlined object such as a bomb of equal weight. Dispensing retardant produces an ADDITIONAL large, but transient lift pulse because it radically increases wing circulation until well clear of the a/c. Since it is a liquid or slurry, it violently exchanges momentum with the surrounding air and becomes a larger aerodynamic object than what is apparent.Nothing other than circulation effects explain these observations. BTW, interaction distance with a wing is related to span, and little else, so 15 feet away from a 130 foot wing is close enough to have a strong effect on circulation. The onset of ground effect is span only related and is a circulation effect as well. On the foreplane/aftplane loading biz:Every practical example of a statically stable system will yield a more highly loaded fore plane than aft plane. The rule applies to conventional, canard, tandem, or whatever layouts. At normal aspect ratios, if loading differences are large enough, this will yield dL's to overcome wing section moments and other destabilizing factors and produce static longitudinal stability. One might be able to violate this rule by using a very lightly loaded low aspect ratio foreplane, say AR<.5 and still have stability because the low aspect ratio of the foreplane will produce a very low dL/dAlpha at modest angles. At high angles, however, the system would be expected to become unstable in the worst way - divergent. Evidence of static stability can be demonstrated by increasing positive stick forces to maintain steady level flight throughout the a/c's speed range. This is more elaborately stated somewhere in FAA part 23, 25 as a requirement for certification. Anyway, thanks for the thoughtful responses. This all seem to be good therapy for some recent clusters. :) r

[WarpD 824]

I'm impressed. I can't recall ever seeing a CFI completely and absolutely ignore the most basic physics of flight.