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Dec 03, 2019, 08:07 AM
Design is everything.
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Under- wing flaps on delta winged aircraft

This topic has been explored before on some other fora. This time I decided to test out the effect of underside flaps on on of my 1:48 scale catapult jet 'gliders' and the results have been quite impressive.

The model is launched straight and level with slight up elevon, to pull up in a few metres, the model being slightly unstable in pitch. With the flaps down, the pull up occurs noticeably quicker, but very clearly due to the flaps. Apparently drag also increases markedly.

These are split flaps, mounted under the wing at about 11 cm from the intersection of the leading edges of the delta wing which is 16 cm in chord. That means it is at 68% from the leading edge. The CG is 7 cm from the reference point, which is 43% from the reference point. The 'flap' actually a split flap is a short strip about 1 cm in chord mounted under the centre section of the wing.
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Dec 03, 2019, 08:14 AM
Design is everything.
Thread OP
The idea comes from the Gloster Javelin, a British fighter jet designed in the 1950s. In this picture the flaps, mounted quite a distance from the trailing edge, are visible.

The flaps work without the powerful undesirable pitch down moment associated with trailing edge flaps on a tailless delta. (I tried this as well). Might this not have been tried on the Mirage III and similar aircraft to reduce landing speeds from the 150 knots or more, and keep the nose down a bit for a better view of the runway?

This model seems to benefit from the effect of flaps, the hard landing is due to wind conditions I would think:

Ultimate RC jets: RARE Gloster Javelin RC Turbine SCALE JET (6 min 45 sec)
Dec 03, 2019, 02:55 PM
An itch?. Scratch build.
eflightray's Avatar
Isn't the device on the Javelin actually an adjustable airbrake ?

Correction: they are flaps --

The wing featured hydraulically actuated flaps and ailerons. There were slotted spoiler-type airbrakes on the top and bottom of each wing; the flap was actually fitted forward of the airbrake under each wing.

Last edited by eflightray; Dec 03, 2019 at 03:02 PM. Reason: Correction
Dec 04, 2019, 03:12 AM
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The Javelin is not tailless though. Pitch control is by conventional elevator, not the wing trailing edge.
Dec 04, 2019, 09:31 PM
Design is everything.
Thread OP
The Javelin is not tailless, granted, however since delta winged aircraft can operate without a tail, and also since there was a Javelin model that was proposed with a butterfly tail, it is reasonable to assume that the Javelin could have been flown without a tail and elevons only.

Model aircraft may behave differently from fighter jets like the Javelin, however it may be worth if for someone to try flaps on their RC model and explore applications for real life aircraft, both military and civilian.

Would flaps help to keep the nose lower down during landing, a problem with delta winged aircraft?
Dec 05, 2019, 01:58 AM
Registered User
If you have a fly by wire system and relaxed stability in pitch, the elevons on a flying wing would be deflected downwards during the approach, roughly proportionally to the plane's angle of attack, pretty much like normal trailing edge flaps would.
Dec 06, 2019, 02:41 AM
Design is everything.
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Yes, I had not thought through on that, but I have never seen a Mirage 2000, an example of a delta with FBW, with its flaps down. A little searching on the you tube netted(!) this video which shows no elevon - as - flap deployment except in the still photos at the end, one could see a few degrees of what might be construed as down- elevon - flap. This is nothing like the 30 degrees or more on tailed airplanes.

Mirage 2000 landing at military airport Malbork - 2014.08.12 - 22 blt (2 min 10 sec)

This video of an Indian Air Force Mirage on a road shows slight up elevon on landing:

Indian Air Force Mirage-2000, Touch and Go on Yamuna Expressway (0 min 58 sec)

In any case, model aircraft do not have the luxury of FBW, and I am quite sure now that the arrangement will work, actually in the example of the Javelin, the FBW system could simply replace the horizontal tail, granted it is a shallow angled delta.

I think it is an absolutely worthwhile configuration to pursue.
Last edited by Designer2010; Dec 06, 2019 at 02:47 AM.
Dec 06, 2019, 01:30 PM
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nmasters's Avatar
The Me-163 had split flaps at mid chord. So did the AV-36 sailplane. They're similar to the dive recovery flaps developed in WWII for the P-38. When you move the hinge of a split flap forward you lose a little bit of the lift increment but the pitching moment also decreases. IIRC the pitching moment goes to zero at about 60% chord and then gets positive forward of that.
Dec 06, 2019, 03:50 PM
Registered User
Originally Posted by nmasters
The Me-163 had split flaps at mid chord...
The flap can be seen in the diagram on this page and also in the attached drawing.

Dec 08, 2019, 10:59 PM
Design is everything.
Thread OP
Thanks for bringing up 163, reputedly the best handling aircraft in the Luftwaffe. The fuel/pilot combination was another matter, really horrific stories. So flaps helped.

So the question is why were they never employed on the Dassault Mirage III, was it ever considered? I know they finally arrived at the canard configuration, which was again reduced to a stub on the Mirage 2000. I am looking forward to testing a small model and then adding these strakes.

Aerodynamics is such a complex thing.
Dec 09, 2019, 04:47 PM
Registered User
Weight, complexity and space for fuel in the wing. Easier to add a more powerful engine to shorten takeoff and a drag chute for landing. Also, I don't think that sort of flap is too efficient for pure deltas compared to a higher aspect ratio swept wing flying wing. Otherwise the Concorde would have used them. Apparently it is more efficient to just use leading edge high lift devices and fitting a taller main landing gear.
Dec 09, 2019, 09:27 PM
Design is everything.
Thread OP
Yes, although it depends on the application - leading edge devices would require more maintenance. In the case of the Concorde you had taller gear plus an entire swiveling nose section, maybe flaps would have helped.

The Mirage 2000 has not only stub canards but seemingly a stub tail mounted high up on the fin, set at a negative angle of attack. Anyone know the effect on pitch of these things? At typical combat speeds these are bound to have an effect: I could find no data on these things.

An antenna?
Dec 10, 2019, 11:29 AM
Registered User
Originally Posted by Designer2010
The Mirage 2000 has not only stub canards but seemingly a stub tail mounted high up on the fin, set at a negative angle of attack.
I learned at Boeing that the flow pattern for Nurflügels, which includes deltas, is downward at the rear center, so the flow well above the rear center would be negative w.r.t. to the wing and thrustline (i.e., aligned with and thus ca. or approaching zero w.r.t. the line of flight and undisturbed air).

Last edited by xlcrlee; Dec 10, 2019 at 11:36 AM.
Dec 10, 2019, 01:23 PM
Registered User
The front strakes on the Mirage 2000 are an aerodynamic device, they are there to further refine the leading edge vortexes. A bit like the strakes on the Atlas Cheetah nose. The two fins on the tail are antennas, possibly part of the IFF or RWR systems. The fixed canards on the advanced Mirage III derivatives serve the dual purpose of refining the high AOA vortexes and shifting the centre of pressure forward, something the Mirage 2000 doesn't need since it was designed from the start with a rearward centre of mass.

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