[brianscharp]

With the exception of free-flying (fowler or Junkers flaps), flying wings do not do well with flaps.

I'm not aware of a flying wing hang glider that outperforms the SWIFT, with or without free-flying (fowler or Junkers flaps).
http://aero.stanford.edu/reports/swiftarticle1991.html
http://aero.stanford.edu/reports/swiftaiaa004336.pdf

[Frank Colver]

That's some of the heavy brain work I said was needed - solving the problems with flaps and flying wings is one. Remember, Rich, we're talking rethinking the whole thing here. No small task!

My guess is that flaps on all wing designs might have a pitch down moment. Unlike tailed aircraft where they produce a pitch up reaction. If that's the case then the outboard twist would need to be increased with the lowering of the flaps. It would be interesting to experiment with current design flex wing glider models and flaps.

One thing I can say for sure is that all rigid wing with tail stabilizers hang gliders should have flaps. No ifs, and's, or but's, about that.

I'm going to e-mail Jim Marske and see what he says about the subject of flaps and all wing designs. I don't, off the top of my head, know if any of his numerous flying wing sailplanes had flaps or not. They may have all just used spoilers.

Frank C.

[ARP]

Flaps on a flexwing are not very practical so I use a slotted wing to achieve slow flight. Much higher angle of attack is possible before the stall.

[KaiMartin]

Paragliders have flaps - in fact that is all they have - 2 Independent flaps that are called brakes in PG speak. It is their only method of control which is a disadvantage in certain situations but it does make for slow landing speeds if used correctly.

While I was typing away on the not immediately obvious problems of big flaps on flex wings, I forgot about my main argument:

Flaps can reduce minimum airspeed. As a by-effect, they increase the glide angle. Both facilitates navigation to the desired spot. But it does not address the fundamental reason why hang gliders are prone to whacks. Unless there is strong laminar wind down to the last metres, minimum air speed will never mean close zero ground speed. This leaves the pilot with two options:

a) Fly with minimum speed at touch down and roll, run, glide until all speed is burnt by ground friction. This comes in several variations including the Greblo Moonwalk. In any case, this option requires a clear run(!)way.

b) Put the wing in a deep stall when sufficiently slow and sufficiently close to ground. The wing acts as a very efficient air break to almost immediately achieve close to zero airspeed. When the wing wants to accelerate in reverse direction, the legs touch the ground with no steps and the pilot walks away with a grin on his face.

Traditional flaps obviously help with type a) landings. That's why they are an integral part of the design of regular aircraft . But flaps hardly affect type b) flare landings. Birds prefer to land with their wings in full stall. This may be a reason why nature did not bother to give them flaps.

Option b) would be ideal if it wasn't for the difficulty to actually put the wing in a deep stall. The wing is deliberately configured to try and return to regular flight state when disturbed. This is what pitch stability and stall recovery is about. The wing "wants" to drop the nose when too slow to support the pilot. In particular, there is range of angle of attack at which the centre wing is stalled but the tips are not. Since the tips are a decent amount behind the centre both conspire to induce a nose-down moment. And a very strong one at that. Stay in this state for longer than a split second and the wing will go on its nose no matter how far the pilot moved himself to the back → WHACK!
The trick to pull of a decent three-stepper is to push yourself to the back of the wing fast. With the weight in the back, the wing gets pitch unstable while the air flow is still attached to the wing. With an unstable wing a small positive deviation of pitch accelerates itself until the flow finally separates. This raises the attitude of the wing so fast that it pushes through to complete deep stall in split seconds. The requirement to be both sufficiently slow and sufficiently low adds a critical timing aspect to the already physically demanding action.

Paragliders still routinely do their non-issue no steppers with their brakes. They literally pull them all the way down until the wing stalls. Unlike hang gliders there is no sweep and consequentially no strong pitching torque while the wing is partially stalled. It does not drop its nose. Instead, right after stall is in effect, the pilot looses height and is delivered standing on his feet. With the pilots weight on his feet rather than on the lines the canopy looses its driving power and falls to the ground.

My conclusion, just adding big flaps to a flex wing won't give us ease of landing comparable to the PG. We'd also have to get rid of sweep and twist. This would of course open a whole can of stability worms.

---<)kaimartin(>---

[Rick Masters]

Figure 8. The inboard flap produces nose-up trim despite the large negative section moments.
DESIGN AND DEVELOPMENT OF THE SWIFT: A FOOT-LAUNCHED SAILPLANE - Ilan Kroo

Kroo points out that the SWIFT is an acronym for Swept Wing with Inboard Flap for Trim.

"One of the fundamental ideas behind the SWIFT control strategy is to deflect an inboard surface downward, rather than an outboard surface upward to achieve trim at higher CL. Such approaches were discussed by Lippisch and others many decades ago, but have been used to great effect on the SWIFT. By deflecting the inner flap downward one increases the effective wing washout as desired, but also increases camber and maximum lift. (Fig. 8). Because of the moderately tapered planform, the SWIFT can exploit a trimchanging flap that covers about 45% of the wing span. When deflected down for higher lift, the glider noses up slightly and trims at a lower speed. It may be deflected downward as much as 55° for landing and approach, reducing the L/D to a manageable value and slowing the glider down for stand-up landings."

----
Kroo refers to the control reversal problem I mentioned.
http://www.desktop.aero/library/OAW_Publications/Published_Documents/Kroo_tailless.pdf
This is a function of the proximity of the trim flaps to the centroid of lift. Note that on the SWIFT, the combination of sweep, aspect ratio and taper allow the flaps to be almost centered on the CL. When the flaps are lowered, a positive pitching moment results. But if the flaps are placed aft of the CL, the flaps must be raised to achieve positive pitch. This problem has plagued designers for generations. Don Mitchell found that free-flying elevons placed, by necessity, aft of the CL on flying wings of lesser sweep, provided greater stability than conventional control surfaces.



Kroo's work is a breakthrough but it's success depends on a rigidly constrained shape that is not applicable to flex wings nor, by percentage, to the vast majority of rigid wings currently being flown. The cost of the modern variants prohibits popular use. For these reasons, and the fact that added complexity increases risk, flaps on flex wings are unlikely to proliferate. More likely, imo, will be a more significant range of simple variable geometry.

But I ask again, what's wrong with a drogue chute? They cost almost nothing, are reliable, work well, demand little additional skill and add little to pre- or post flight duties.

I use a slotted wing to achieve slow flight.

More?

[ARP]

Rick,

You ask for "More?" so I put this up for your interest:- https://www.youtube.com/watch?v=BDUrX_o727Y showing a glider from the mid 70's. A number of different versions were built with varying amounts of overlap of the wings. A picture of paper cut out of the latest design (not yet built), was put up by JoeF on the 5' pack length thread, for me. The airframe is much lighter than a conventional HG as there is only one wing spar which supports both wings and acts as the leading edge of the main wing. This latest version has a much higher aspect ratio than the one in the film but with the original glider I was able to take off from a 1:10 slope and land most times with no step.

[KaiMartin]

so I put this up for your interest:- https://www.youtube.com/watch?v=BDUrX_o727Y showing a glider from the mid 70's.

Hmm, I was curious but the the link gave me:

Unfortunately, this video is not available in your country because it could contain music from UMG, for which we could not agree on conditions of use with GEMA.

---<)kaimartin(>---

[ARP]

Kai,

Yes unfortunately, when the film was put onto video, my brother put a music sound track on it and that breached the copyright rules in Germany. If you provide me with your e-mail address I can send pictures of the glider direct to you.

Tony

[ARP]

Kai,

I see that I have your e-mail address from 5 years ago. If it has not changed I will send some pictures to you. For some reason I cannot post pictures on this site.

[KaiMartin]

I see that I have your e-mail address from 5 years ago. If it has not changed I will send some pictures to you. For some reason I cannot post pictures on this site.

Should still be good. You seldom switch emails if you run your own domain

---<)kaimartin(>---