Can thrust vectoring enable use of flaps on pure delta wing?

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Topaz

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As for installing flaps on a tail-less delta or flying wing …. we have seen tilting centre trailing edge on Swift UL glider. We have also seen a few split flaps on late 1940s vintage experimental flying wings. They all seem to be close to the centre of gravity to minimize pitching.
Am I reading this concept correctly?
You've got the gist of it. To be pedantically correct, the flaps are arranged so that the lift distribution and local pitching moments when they're deflected creates the same total-airplane pitching moment as the lift distribution and local moments that exist when they're not deflected. Therefore no trim change. That generally means that the flaps are close to the CG, but not necessarily directly on it.
 

Toobuilder

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I have put some interesting (and reference) configurations into this spreadsheet https://docs.google.com/spreadsheets/d/1SGK_is-R1lgW5lgIff3CNpK6R52SmUzpb6J944VdoS4 where I mark best / good / bad spec. As you can see there is not a single model without yellow field while quite a few are good enough.

If I missed something worth mentioning, please tell, I would add it to comparison.
What I see is a comparison matrix of existing designs. That's good info, but where are YOUR requirements? What is the maximum stall speed you will accept, for example? How about the minimum cruise speed?

What I'd expect this spreadsheet to have is the first column list your requirements, with the compared designs showing a Delta to your requirements instead of discrete numbers.
 

Aerowerx

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As for installing flaps on a tail-less delta or flying wing …. we have seen tilting centre trailing edge on Swift UL glider. We have also seen a few split flaps on late 1940s vintage experimental flying wings. They all seem to be close to the centre of gravity to minimize pitching.
Am I reading this concept correctly?
No.

They have to be inline with the Mean Aerodynamic Center, sometimes called the center of lift.

On a swept wing this is at about 42% of the half span.
 

Dana

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...Mean Aerodynamic Center, sometimes called the center of lift.

On a swept wing this is at about 42% of the half span.
MAC is Mean Aerodynamic Chord, which is NOT the same as the center of lift. Where it is depends on the wing planform, taper, etc.

The center of lift moves with AOA and control deflection.
 

Aerowerx

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MAC is Mean Aerodynamic Chord, which is NOT the same as the center of lift. Where it is depends on the wing planform, taper, etc.

The center of lift moves with AOA and control deflection.
Sorry. My bad.

On a tapered swept wing, locate where the chord is equal to the MAC. That should be at about 42% of the half span. The 25% of chord point should be in line with the neutral point of the wing. THAT is where you put the pitch neutral flap.
 

Thomas Marks

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What I see is a comparison matrix of existing designs. That's good info, but where are YOUR requirements? What is the maximum stall speed you will accept, for example? How about the minimum cruise speed?

What I'd expect this spreadsheet to have is the first column list your requirements, with the compared designs showing a Delta to your requirements instead of discrete numbers.
Sure, have moved the delta to the first column.
I'm totally fine with all its spec, except the stall speed and take-off / landing roll as a result.
I would be more than happy with landmark (value in red) ultralight category limit of 24 knot, while anything under urban area speed limit should also work (varies by country from 30mph to 60 kmh)
 

Norman

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It kind of depends on what you mean by "delta". Lippisch's definition was a wing with at least 55 degrees leading edge sweep and a straight across trailing edge with pointy tips. Such deltas get a lift boost above AoA=8 degrees of about 80% from the leading edge vortices and hang on to AoA>30. A normal plain flap on the trailing edge of a wing like that would produce a huge pitching moment because of the chord length. Instead of plain flaps I'd suggest you look into forward hinged split flaps. The Gloster Javelin used forward split flaps just behind the landing gear. This type of flap has been used on a lot of tailles aircraft including the Me-163 and Fauvel AV-36. If you put the hinge farther forward it can even produce a positive pitching moment as exemplified by the dive recovery flaps developed in WWII for the P-38. On higher aspect ratio swept wings trailing edge flaps can work and are not that difficult to design.
 

BJC

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It kind of depends on what you mean by "delta". Lippisch's definition was a wing with at least 55 degrees leading edge sweep and a straight across trailing edge with pointy tips
Thanks, Norman, for that definition. I’ve looked for a generally-accepted definition of a delta wing platform, but I have not found one. I have seen a 40 degree LE sweep described as a delta.


BJC
 

berridos

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Thanks a lot for that interesting input. The beauty with those flaps is that you dont disturb the laminar flow at the leading edge. So basically the flaps should have the hinge at the center of lift? However they could make ineffective the ailerons.
Do those flaps intenionally have a gap at the hinge for effectiveness or is it due to the mechanics involved?
 
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Toobuilder

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Sure, have moved the delta to the first column.
I'm totally fine with all its spec, except the stall speed and take-off / landing roll as a result.
I would be more than happy with landmark (value in red) ultralight category limit of 24 knot, while anything under urban area speed limit should also work (varies by country from 30mph to 60 kmh)
Ok, I guess I'm confused. Is the "Delta" an existing design?

And if you are not happy with the stall speed, then why consider the design?

Again, what are YOUR performance requirements - forget any existing aircraft and forget any numbers an aero program has provided. Simply state what YOU require the aircraft to do. If it's a stall speed of 25 knots say so in your spreadsheet. That's step one. You must determine performance requirements long before you determine the aircraft shape.
 

Aesquire

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The balancing thrust will do the same thing as conventional tail, take the role of elevator from flaperons at the landing.

.......

Do you mean just a low speed range? This is what I'm aiming for: landing.
That's the problem, it doesn't act the way a tail surface does.

The kindergarten basics is that the center of gravity is ahead of the center of lift, and the down force from the tail/elevons/reflex is behind. Imagine a see-saw. The weight doesn't change ( ignoring fuel burn, etc ) but the down force changes with airspeed. Faster, more force, nose goes up, plane slows down. Slower, less force, nose goes down, plane speeds up.

Substituting thrust for the nose up force and using a nose down force creating flap in the back isn't stable. if you lower the nose and speed up the force on the flap increases and will at a critical speed, overpower the thrust. Long before that happens, you would have to deal with an unstable in pitch aircraft that "wants" to speed up, and keep speeding up, AND slow down, and keep slowing down, when any air movement tries to change the steady state.

And this isn't taking into account that an engine failure will cause the aircraft to violently pitch down, and since you intend this to be a landing aid... do so at too low an altitude to retract the flap & recover.
 

Norman

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Thanks, Norman, for that definition. I’ve looked for a generally-accepted definition of a delta wing platform, but I have not found one. I have seen a 40 degree LE sweep described as a delta.


BJC
Well Lippisch did start using the name "delta" for a series if low sweep airplanes he designed in the '30s (non were good enough to go into prodoction) but he discovered vortex lift on a 60 degree delta sub-scale model that he was working on during the war and that's the only good low speed characteristic of highly swept wings. A low aspect ratio swept wing that won't form leading edge vortices is little more than a lawn dart ie without a stabilized LEV they can't fly slowly because the left and right wing tend to shed vortices alternately which causes hysteresis in the CL. After the war he was recruited by operation paperclip and brought the full scale DM-1 with him. Testing in Langley full scale tunnel showed that the LEV only forms at leading edge sweep>55 degrees and at high Reynolds numbers the leading edge at the root needs to be sharp blending to a blunt nosed section near the tip. The results of that work are described in his 1946 T2 paper "On highly swept wings" (sorry don't know where to get T2 papers and loaned my copy to a dead guy). Anyway he never built a triangular wing with less than 60 degrees leading edge sweep after that and most of the American deltas through the '60s used the geometry described in that paper, not because it's efficient at low speed (IT'S NOT) but because it's efficient at supersonic speed and fairly safe at low speed, if you conciser landing at 30+ AoA safe.
 

Dana

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Sorry. My bad.

On a tapered swept wing, locate where the chord is equal to the MAC. That should be at about 42% of the half span. The 25% of chord point should be in line with the neutral point of the wing. THAT is where you put the pitch neutral flap.
Your 42% number would only be valid for one particular taper ratio. With no taper at all it would be 50%, with 100% taper (triangular planform) it would be 33%, anything else, somewhere in between 33 and 50. It might happen to be 42, or it might not.

The sweep doesn't matter at all.
 

berridos

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The beauty of the split flap proposed by Norman is that in a tractor delta you have the prop blowing on the flap, as the flap is right below the pilots ass in a central position. So at the end you got thrust vectoring without pitch side effects. Sounds pretty ideal addon on the verhees. Maybe i need to lengthen the landing gear to allow for propper deflection of the flap.
I wonder if the flap should have some tapering build in, to form a beneficial vortex. It could be 2m wide and centrally located as there the dettachable wings start.
 
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Jay Kempf

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Not very familiar with split flaps implemented in that way. Flaps normally act like changing the trailing edge location so that the airfoil acts like it has more camber and that changes the pitching moment. So having the trailing edge brought forward like that changes the camber profile like the airfoil has a lot more thickness in the central area and all below the undeflected camber line? That just changes the pitching moment and it seems it would be a big addition of drag. How does it increase CL deflected? It is basically like a hinged spoiler but only on the lower surface... If the goal is to slow down you need to get more CL... out of the same area...
 

Riggerrob

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Even if your belly flap only increases drag, that can come in handy when you want to fly steep approaches - over trees - when landing on short airstrips.
A pure drag belly flap is worse than useless when trying to climb out of that same airstrip.
 

Riggerrob

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If you look at Facetmobile and listen to Barnaby Wainfain's lectures, the primary advantages of small, slow deltas are light weight and large Reynolds numbers. Because his Facetmobile is so thick, he can use very light-weight structure. Remember that the weight of a wing spar is inversely proportional to its depth. IOW the thicker the spar/beam, the lighter you can build it.

Large Reynolds numbers increase lift, while reducing drag. A common problem with homebuilts (e.g Rutan Quickie) is their tiny wing chords, which are difficult to build accurately enough to retain smooth airflow. The larger the Reynolds number (wing chord) the less precise you have to be.

Another major advantage of delta is reduced parts count (e.g. Verhees). Reducing parts count simplifies construction while reducing fastener weight.

When I was a teenager, I wanted a Dyke Delta. When I grow up, I want a Verhees Delta 2.
 

Topaz

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Not very familiar with split flaps implemented in that way. Flaps normally act like changing the trailing edge location so that the airfoil acts like it has more camber and that changes the pitching moment. So having the trailing edge brought forward like that changes the camber profile like the airfoil has a lot more thickness in the central area and all below the undeflected camber line? That just changes the pitching moment and it seems it would be a big addition of drag. How does it increase CL deflected? It is basically like a hinged spoiler but only on the lower surface... If the goal is to slow down you need to get more CL... out of the same area...
Even if your belly flap only increases drag, that can come in handy when you want to fly steep approaches - over trees - when landing on short airstrips.
A pure drag belly flap is worse than useless when trying to climb out of that same airstrip.
These. A "belly flap", well forward of the trailing edge, has another name: "Drag Brake." The LongEZ has one under the fuselage for that purpose. No increase in lift, even on a delta wing because, unlike a conventional split flap near the trailing edge, there's no increase in circulation resulting from the flap being that far forward.

Fundamentally, a delta wing is a poor choice for a STOL aircraft design. Might be fine for landing, where vortex lift and the attendant high drag could be used to effect for a steep approach and short flare out. However, wherever you land, you have to take off from again and, for takeoff, vortex lift (and the attendant high drag) are not good. This is why you don't see delta wings used for "professional" STOL designs. Occasionally for VTOL designs, where lift engines completely off-load the wings for takeoff and landing, but not for STOL.
 
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