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Flying Wing/delta Wing Pitch Control  
User currently offlineSpeedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0
Posted (7 years 9 months 6 days 8 hours ago) and read 10271 times:

As a mere enthusiast, I have a hard time getting my head around the concept of controling pitch without a completely separate surface either for or aft of the main wing. On a conventional aircraft, the horizontal stabilizer (and connected elevator) acts as a separate lift-providing surface (whether positive or negative) located far enough away from the main wing to act as a lever, thus controlling pitch.

At the moment, however, I can't understand how a flying wing like the B-2 controls pitch if its pitch-control surfaces are part of the main wing. I'm familiar with some of its unique control surfaces, like ailerons that split to control yaw (i'm sure there's a word for them). But basic pitch control escapes me. How do control surfaces on the trailing edge of a flying wing adequately control pitch?

From my perspective, delta wings present the same problem. Since the wing occupies the majority of the length of such aircraft, how does pitch control work without a separate horizontal stabilizer hung way out the back of the fuselage? Thanks, as always, for any replies.

O


Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
5 replies: All unread, jump to last
 
User currently offlineJetMech From Australia, joined Mar 2006, 2684 posts, RR: 53
Reply 1, posted (7 years 9 months 6 days 7 hours ago) and read 10264 times:

G'day Speedracer1407,

These are some of my thoughts on the subject, which are in no way authoritative.

IIRC, control surfaces change the attitude of an aircraft about the C of G. They do this by applying aerodynamic forces to the airframe at a certain distance from the C of G. From a plan-form shot of the B2, it appears that the control surfaces are located on the trailing edge, which I would suspect is some distance aft of the C of G.



I am not sure which of these surfaces is responsible for pitch control. IIRC, true flying wings such as the B2 are inherently unstable. Again IIRC, the B2 has a quadruple redundant fly by wire (FBW) control system. I presume that this system makes very frequent minor adjustments to the pitch control surface to either maintain or change the pitch of the aircraft.

A good analogy I once heard is sitting on the bonnet of a speeding car holding onto the handle-bars of a bicycle with the back wheel out at the front. Your task is to keep the back wheel heading in a straight line by making adjustments on the handlebars only.

Such a configuration is very unstable and it is most likely that a human could not keep the back wheel of the bike veering out of control. A FBW control system on the other hand can keep the back wheel of the bike heading in a straight line because it makes hundreds of very minor adjustments to the handlebars of the bike before any significant divergence of the back wheel occurs.

The Concorde has control surfaces on the trailing edge of it's delta wing, which I again suspect are someway aft from the C of G. IIRC, these are elevons, and carry out the tasks of elevator and aileron using sophisticated mixing. I am not sure about the pitch stability of Concorde.



You could almost picture a delta wing as an airfoil where the tailplane and main-plane has been merged into a single aerodynamic entity.

Basically, I think that for any sort of attitude control of an aircraft, the most important thing is that you apply aerodynamic forces at some distance from the C of G of the aircraft.

[Edited 2006-10-24 10:05:47]


JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineSpeedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0
Reply 2, posted (7 years 9 months 6 days 6 hours ago) and read 10249 times:

Quoting JetMech (Reply 1):
Basically, I think that for any sort of attitude control of an aircraft, the most important thing is that you apply aerodynamic forces at some distance from the C of G of the aircraft.

Indeed. And thank you for a prompt reply. But I guess where I get hung up is when i consider the percieved fact that control surfaces on the trailing edge of a B2 or Concorde are integral with the primary lift-providing wing. Thus any change to the shape of that wing via pitch control surfaces seems like it should effect the overall lift, and provide little pitch control. Obviously, my perceptions are wrong, as delta wing and flying wing aircraft fly just fine, even if some of them need FBW for stability. I just don't know where my perceptions fail.

O



Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
User currently offlineJetMech From Australia, joined Mar 2006, 2684 posts, RR: 53
Reply 3, posted (7 years 9 months 6 days 5 hours ago) and read 10230 times:

Quoting Speedracer1407 (Reply 2):
But I guess where I get hung up is when i consider the percieved fact that control surfaces on the trailing edge of a B2 or Concorde are integral with the primary lift-providing wing. Thus any change to the shape of that wing via pitch control surfaces seems like it should effect the overall lift, and provide little pitch control.

I think for what you mention to happen, a wing would need to produce a uniform chord-wise distribution of lift. This wing would possibly be of rectangular plan-form and low aspect ratio with a long chord.

IIRC, the total lift generated by a wing is the result of a non-uniform lift distribution. For wings such as a 747 or B2, I seem to remember that the chord wise location of the peak of the lift distribution curve appears around the region of maximum airfoil section thickness  Confused.

The long root chord line of the delta wing would "spread" out this lift distribution in a chord-wise sense, but I seem to remember that much of the lift of a delta wing comes from massive vortices shed from the Leading Edge. I think these leading edge vortices are particularly prevalent at high AOA's.


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JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6385 posts, RR: 54
Reply 4, posted (7 years 9 months 5 days 18 hours ago) and read 10179 times:

Quoting Speedracer1407 (Reply 2):
...I guess where I get hung up is when i consider the percieved fact that control surfaces on the trailing edge of a B2 or Concorde are integral with the primary lift-providing wing. Thus any change to the shape of that wing via pitch control surfaces seems like it should effect the overall lift, and provide little pitch control.

Dear Speedracer, I think that I can explain it.

When on a delta plane the pilot pulls elevator up, then the total lift of the wing will indeed DEcrease.

But the lift decrease is entirely at the trailing edge of the wing. At the leading edge the lift will not change (yet).

Since the trailing edge is behind the center of gravity (CG) then the tail will drop slightly. When the tail drops, then it increases the overall angle of attack (AOA) of the wing. This AOA increase increases the total lift all over the wing far more than the local lift decrease at the traling edge. The plane will pitch up and climb.

Same the other way around. Pushing down elevator will INcrease the total lift, but only on the trailing edge. The tail will lift, the AOA will decrease (maybe become negative), and the plane will dive.

It's all a question about balance.

A tailless plane must be balanced a lot more precisely than an ordinary plane with a tail. That's the reason why they have never been popular as civil transports. In fact most transport planes have a generously sized horizontal tail so the flight crews don't have to worry so much about fat pax in one end of the plane and empty seats in the other end. Anyway, all planes have certified forward and rearward balance limits, a generously sized tail only makes the limit range wider.

Look at old bomber planes which are similarly configured to modern transport planes, such as B-47, B-52, Tu-16, Tu-20/95 etc. They all have a relatively smaller horizontal tail. They were all designed to carry one nuke bomb at their CG, and balance calculation was piece of cake. Very unlike a Ryanair B737 with no booked seating and no known weight of each individual passenger.

Then how could the Concorde be operated without a tail?

I'm sure that exactly balance calculation was a big issue before each Concorde flight. And then the pax payload was relatively small, only 4.5% of TOW. On a 737 it can easily be well over 20% of TOW.

The Concorde also had the capability to adjust it's CG during flight by shifting fuel between forward and aft tanks. That was also needed to adjust for changed aerodynamics and wing lift distribution when shifting between sub- and supersonic speed, but that's a different story.

Many modern transport planes can also adjust CG inflight, but that's only for efficiency reasons. When the tailplane can fly with the smallest possible negative AOA, then the total drag - and fuel burn - is minimized.

Yes, a tailplane on a transport plane always flies with a negative AOA, although a small one. That produces what is called stability on the pitch axis.

When a plane is disturbed in flight by a turbulence, then let's assume that it creates a one degree pitch up. It may mean that the wing AOA will increase from, say, 8 to 9 degrees, while the tailplane has it's AOA reduced from, say, minus 2 to minus 1 degree. That will increase the wing lift only slightly, while the negative lift of the tail will be cut in half. A proportionally much greater change of force on the tail. The result will be that the tail will lift more than the rest of the plane and stabilize the original AOA values. Same (only opposite) when a turbulence forces a pitch down.

That stability is much harder to produce on a tailless plane, but it is done in much the same way, only by shaping the wing profile in a special way. (That does not count for the B-2 which rilies on artificial - computer controlled - pitch- (and yaw-) stability).

There has been a lot of talk about BWB transports - Blended Wing-Body planes, which are essential tailless planes. They could potentially be more fuel efficient than ordinary tailed planes. I don't believe in them, not because they could not be enonomically efficient and safe, but because they would be too complicated to operate because of too narrow CG range. They could be made with a quite wide CG range, but that would call for a very stangely shaped wing profile, and then all the efficiency gains would be gone due to increased drag.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offline787atPAE From United States of America, joined Oct 2006, 143 posts, RR: 4
Reply 5, posted (7 years 9 months 5 days 15 hours ago) and read 10171 times:

Ooh, this is a good one. The best way I can answer this question is to think of a plane in flight trimmed so there are no moments about the plane's cg. To cause the plane to rotate around its cg, does it matter where the control surface is? No. As long as there is enough strength in the surface's actuation to deflect the surface from the trim position, this will cause a moment around the plane's cg. And, to sum up that Newton guy, when you create a new moment on a plane (or any object in general), the plane will rotate. Gotta love physics.  Smile

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