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Slow delta wings?

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deskpilot

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Well, Heellooo Lynne. Welcome to my thread. Yours is the first in 3 years and caused me to re-read the whole thing. I still like my design but have moved on to more conventional designs. That being said, I'm also working with Hugh Lorimer with his Sgian Dubh tailless design. Not sure if it should be referred to as a flying Wing or flying Plank.

photo shop.jpg Hugh ran into legal and health problems so this design hasn't flown yet. It is complete and ready to go though. If all comes to fruition, I think we could have a very easy to build single seater with retracts, so hopefully fast enough to keep pretend fighter pilots happy.
 

plncraze

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The Dean Delt-Air as he subject of a lot of articles in Sport Aviation. If only it had not flipped all the way over....
 

Norman

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That's right. Though, from this image I don't see a huge thrust line vertical offset of the Delt-Air 250:
delta kitten | flight international | skylane | 1962 | 0026 | Flight Archive
The problem is not that the high thrust line produces a huge moment but that the control system does not have enough authority to overcome that moment. A tailless delta relies entirely on airfoil pitching moment for pitch control. The thrust line doesn't have to be very high to produce a significant moment. The weight on the main gear of a tricycle also produces a nose down moment and compression of the nose gear shock absorber by the thrust moment decreases the wing incidence. All of these moments stack up to hold the plane on the ground while the power is on then when the pilot cuts power at least half of that disappears. If the plane was above takeoff speed it may well rotate by itself and then lift the weight off the main gear eliminating the rest of the negative moment. Now the plane wants to rotate even more and will probably stall if the pilot doesn't immediately push the nose down which he probably won't because all this has happened very fast. This phenomenon has been disused here before so I'll just link back instead of try to rewrite it at 3:30 am
http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/1454-facetmobile-lifting-body-kits-not-dyke-delta.html#post8023

Aircraft Data N6379T, 1961 Dean Herbert Franklin Jr DELT-AIR 250 C/N 001

I would be very interested to hear any new input as I'm sure everybody else would. This is a very important problem that has killed several people
 

Mike W

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I experienced exactly the same problem as Norman mentions with one of my plans built MW6's. During the take off run on grass, the aircraft would not rotate and take off. Eventually when the throttle was closed to abort the flight, it rotated and lifted off, but obviously in a more sedate fashion than the Dean Delta. Then on re applying power, the aircraft would grudgingly climb away.

The only difference between this aircraft and all the others was the shape of the nacelle. See picture. The drawings show some guidelines for constructing the nacelle but the shape is left more to the builder, so that he can add a bit of his own character into the aircraft. The builder of this aircraft built a slab sided nacelle with a flat floor and a flat downward sloping top which obviously acting as a canard fore plane adding to the list of other nose down moments, such as the high thrust line and wheel drag in the grass The tailplane couple was just not powerful enough to lift the nose until the couple due to thrust was removed. On lift off the wheel drag couple was eliminated allowing the thrust component to be restored while leaving enough elevator control to manoeuvre the aircraft.

TEDS MW6 001.jpg
 

delta

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I believe I read from before that he had set his ground angle of attack to 8*. It doesn't look like that much to me. This pic represents 8*.
Do I see leading edge slats and did they have a part in this tragedy?
I guess you could say anything less than supersonic is slow.
I'll bet his wing loading was pretty high.
 

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Kingfisher

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The problem is not that the high thrust line produces a huge moment but that the control system does not have enough authority to overcome that moment. A tailless delta relies entirely on airfoil pitching moment for pitch control. The thrust line doesn't have to be very high to produce a significant moment. The weight on the main gear of a tricycle also produces a nose down moment and compression of the nose gear shock absorber by the thrust moment decreases the wing incidence. All of these moments stack up to hold the plane on the ground while the power is on then when the pilot cuts power at least half of that disappears. If the plane was above takeoff speed it may well rotate by itself and then lift the weight off the main gear eliminating the rest of the negative moment. Now the plane wants to rotate even more and will probably stall if the pilot doesn't immediately push the nose down which he probably won't because all this has happened very fast. This phenomenon has been disused here before so I'll just link back instead of try to rewrite it at 3:30 am
http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/1454-facetmobile-lifting-body-kits-not-dyke-delta.html#post8023

Aircraft Data N6379T, 1961 Dean Herbert Franklin Jr DELT-AIR 250 C/N 001

I would be very interested to hear any new input as I'm sure everybody else would. This is a very important problem that has killed several people
I'm not sure whether I agree with that part:"Now the plane wants to rotate even more and will probably stall if the pilot doesn't immediately push the nose down which he probably won't because all this has happened very fast."

With a properly adjusted CG, there is no tendency for the plane to pitch up further after lift-off. Any pilot with any experience would immediately release back pressure once the aircraft had lifted off.

Regarding delta wings, I can only speak from RC experience. Due to reasons you list, the C.G. range is smaller than that of a regular aircraft. I had a favourite delta plane called a "Lancet" which I flew for many years. It was extremely fast and flew like on rails, yet could also be flown extremely slowly. However, on its first flight, I almost crashed it because I had not added enough weight to the nose. The plane oscillated violently up and down, and I just barely managed to land it in one piece. I am telling this to suggest that the Deans Delt-Air also may have been tail heavy. The main landing gear looks quite far back and quite long, which would have delayed take-off to a relatively high speed, like you stated. Once airborne, any tail heaviness would have lead to the same pitch behaviour my model had experienced. Cutting power would definitely worsen this effect. I doubt the pilot passed out as described by his nephew, he just might have had not enough control authority to prevent pitch up. If he cut power at that moment, he would have simply fallen back to mother earth and crashed either in a nose high or sudden nose low attitude, depending how much control authority he still had. However, with a properly adjusted CG, there should not have been a major issue, simply a delayed take-off. I haven't read any of the related articles, so don't know if this has been discussed.

I do know that a delta wing with the right wing loading and the CG in the right spot is as docile an airplane as one can get, while at the same it has a very wide speed envelope and is extremely manoeuvrable. It seems virtually impossible to cause a spin (unless tail heavy), at least I have never managed to do so, in both pusher and puller designs. Also, a simple symmetric airfoil works very well. However, asymmetric airfoils with up reflex of the trailing edge will improve gliding angle, but adversely effect aerobatic performance. I don't think that a delta wing design requires the complicated mathematical analysis suggested by some here, unless one wants to optimise it in some way.
 
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Kingfisher

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Something for the eye. Most of what I have read and seen on videos is that this was an airplane that flew/flies very well. Landing looks quite docile, although it would have weighed more with the rocket engine. With today's small jet engines and a motor glider style landing gear, this would be an awesome aircraft to have...
 
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Norman

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Landing looks quite docile, although it would have weighed more with the rocket engine.
The rocket motor has to be replaced with ballast to maintain the correct CG but most of the weight of the fuel tanks and plumbing could have been saved.

With today's small jet engines and a motor glider style landing gear, this would be an awesome aircraft to have...]
Then it wouldn't be a replica
 

Norman

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I'm not sure whether I agree with that part:"Now the plane wants to rotate even more and will probably stall if the pilot doesn't immediately push the nose down which he probably won't because all this has happened very fast."


With a properly adjusted CG, there is no tendency for the plane to pitch up further after lift-off. Any pilot with any experience would immediately release back pressure once the aircraft had lifted off.
You seem to have missed the context around that sentence. The weight of the airplane ahead of the main gear produces a nose down moment. As the wing takes the weight off of the gear that moment disappears and the plane rotates. This happens after power is cut but before liftoff. This is a short period of time and and if the pilot didn't anticipate it before reducing power the plane will probably stall before control input becomes effective.
 

JamesG

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What Norm said. Either the plane will sink onto its gear and all will be fine or it will remain rotated for a while (that awkward long flare), either way the aircraft does not have the energy to really get into trouble, baring crosswinds.
 

Norman

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either way the aircraft does not have the energy to really get into trouble
Depends on how fast it was going when the power was cut. If you had barely reached flying speed then it should just be a little hop but if the pilot got frustrated and kept accelerating until the go/no go point he might well have enough momentum to pull up into a dynamic stall that could lift the plane to 30 or 40 feet and drop it on its rear end. Atlantica seems to have done something like this after a bounce during a high speed taxi run in gusty conditions and Witold Kasper also had one of his planes destroy itself this way although he claimed it jumped 300ft straight up. A few years ago I watched the build log of the model shown in the attached picture. I warned the builder about the pitching moment from the pylon mounted prop but before changing it he had to try it out.
Complete 011.jpg
 

Kingfisher

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The problem is not that the high thrust line produces a huge moment but that the control system does not have enough authority to overcome that moment. A tailless delta relies entirely on airfoil pitching moment for pitch control. The thrust line doesn't have to be very high to produce a significant moment. The weight on the main gear of a tricycle also produces a nose down moment and compression of the nose gear shock absorber by the thrust moment decreases the wing incidence. All of these moments stack up to hold the plane on the ground while the power is on then when the pilot cuts power at least half of that disappears. If the plane was above takeoff speed it may well rotate by itself and then lift the weight off the main gear eliminating the rest of the negative moment. Now the plane wants to rotate even more and will probably stall if the pilot doesn't immediately push the nose down which he probably won't because all this has happened very fast. This phenomenon has been disused here before so I'll just link back instead of try to rewrite it at 3:30 amhttp://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/1454-facetmobile-lifting-body-kits-not-dyke-delta.html#post8023Aircraft Data N6379T, 1961 Dean Herbert Franklin Jr DELT-AIR 250 C/N 001I would be very interested to hear any new input as I'm sure everybody else would. This is a very important problem that has killed several people
The problem of involuntary take-off should only be an issue if the thrust line is high with respect to the centre of gravity. In the Delt-Air 250 the thrust line does appear to go through the CG, or close to it. Although the long landing gear will create a large moment pushing the nose down, none of the moments will change when power is cut, and the thrust itself does not generate any significant moment around the C.G., assuming L0 is indeed small (see attached pic, left half, for illustration).
However, the right half of my drawing of a high thrust line aircraft agrees with your assessment for potential lift-off. Of course, a flying wing with a high thrust line, like the model you posted the picture of, would be especially bad, since moment arm l4 is short. So the primary problem for an involuntary lift-off as you describe it would be a high thrust line, which I don't see in the Delt Air.thur530marks 6.jpg
 
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Kingfisher

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The problem of involuntary take-off should only be an issue if the thrust line is high with respect to the centre of gravity. In the Delt-Air 250 the thrust line does appear to go through the CG, or close to it. Although the long landing gear will create a large moment pushing the nose down, none of the moments will change when power is cut, and the thrust itself does not generate any significant moment around the C.G., assuming L0 is indeed small (see attached pic, left half, for illustration).
However, the right half of my drawing of a high thrust line aircraft agrees with your assessment for potential lift-off. Of course, a flying wing with a high thrust line, like the model you posted the picture of, would be especially bad, since moment arm l4 is short. So the primary problem for an involuntary lift-off as you describe it would be a high thrust line, which I don't see in the Delt Air.View attachment 37705
Just read this again, wondered why nobody responded. Tried to use less words and more arrows, maybe they have the same effect?:depressed Still think primary problem is high thrustline...maybe in combination with trimming for take-off, since much more up trim would be needed for high thrust line which could pitch up the plane if thrust is removed.
 

Himat

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Just read this again, wondered why nobody responded. Tried to use less words and more arrows, maybe they have the same effect?:depressed Still think primary problem is high thrustline...maybe in combination with trimming for take-off, since much more up trim would be needed for high thrust line which could pitch up the plane if thrust is removed.
Some may not respond because they did read the post at the time or get around to write a response.

Anyway, I think you are mostly correct. I would have worded it different, the pitch up/down with power changes are caused by a large offset between the thrust and the drag vector. The whereabouts of the CG really does not matter, the culprit is that the thrust vector have a moment arm around the point the drag vector originates when it do not apply any moment on the airframe.

One secondary effect I have not seen discussed is if a high mounted propeller if mounted over the wing modify the flow pattern in such a way that the wing's moment change with applied thrust.
 

Norman

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the pitch up/down with power changes are caused by a large offset between the thrust and the drag vector. The whereabouts of the CG really does not matter, the culprit is that the thrust vector have a moment arm around the point the drag vector originates when it do not apply any moment on the airframe.
Inertia doesn't matter? Try pushing a ball with a lever attached to the periphery.dumbell_on-a_skateboard.pngEven airplanes with long tails feel some pith tendency during acceleration if the thrust line does not pass through the CG but the horizontal stabilizer compensates quickly enough that it's not a problem. This thread has been about tailless aircraft which do not have such high levels of static stability.
 

jedi

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New to this thread but the subject appears to be important. With regard to the serious problem of excessive pitch up on take off there are many examples of fatal accidents due to this on aircraft without high thrust lines.
Without going into all the details I will try to name a few.
Dean delta Flint Michigan
Mark Stull (sp?)
two Kasper wings
UAL DC 8 Detroit
These demonstrate issues other than the hi thrust lines. Excessive nose up at the trim is an example.
 

Kingfisher

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Some may not respond because they did read the post at the time or get around to write a response.

Anyway, I think you are mostly correct. I would have worded it different, the pitch up/down with power changes are caused by a large offset between the thrust and the drag vector. The whereabouts of the CG really does not matter, the culprit is that the thrust vector have a moment arm around the point the drag vector originates when it do not apply any moment on the airframe.

One secondary effect I have not seen discussed is if a high mounted propeller if mounted over the wing modify the flow pattern in such a way that the wing's moment change with applied thrust.
The location of the C.G. definitely matters, there I agree with Norman. If you cut power, all that keeps the plane moving is its inertia force, which acts at the C.G. Where I don't agree is that the moments generated by the drag forces, including the landing gear, do not suddenly get "halved", they gradually get lower as the plane decelerates. The drag vector may, however, suddenly move up in a high thrust line plane, like I drew it, if the prop acts as an airbrake when power is reduced. This would then cause a pitch up moment caused by the inertia force still acting on the C.G., since it is below the prop. This would not happen if the prop centre line is going through the C.G. I have experienced flying an RC plane with a pylon mounted engine, and the up-trim needed to keep that high thrust line air plane flying level under power (F tail x L4 balancing F thrust x L0 in my sketch) certainly resulted in noticeable pitch up when power was reduced. Since the plane was also nose heavy, it simply dropped the nose back down after loosing speed. In a tail heavy plane, this would be a different story and could well result in a stall and crash, unless the pilot deliberately pushed the nose down against the trim.

Your second point regarding the air flow, I'm not sure about. I do think one can angle the high thrust line so it "points" to the C.G, so to speak. If the engine is in front, the prop should then point up, if it is rear mounted it should point down. If your pylon is on the C.G, you are out of luck, as the prop would have to point straight up or down. But it can blow on the tail, which then creates a speed dependent compensation. I just saw this was discussed here:
http://www.homebuiltairplanes.com/forums/general-auto-conversion-discussion/4520-thrust-line.html
 
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Kingfisher

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Inertia doesn't matter? Try pushing a ball with a lever attached to the periphery.View attachment 40627Even airplanes with long tails feel some pith tendency during acceleration if the thrust line does not pass through the CG but the horizontal stabilizer compensates quickly enough that it's not a problem. This thread has been about tailless aircraft which do not have such high levels of static stability.
This hits it on the head. Even if moving at steady speed, just overcoming drag from wheels, your picture applies. To keep ball from rolling you could also apply the thrust load to the lever, but add a horizontal lever with a vertical downforce. What would that system represent ;)?
ball skate board.JPG
 

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