Mig21 is a delta wing , so why do not use the ailerons like any other delta's ? For lowering the approach and touch down speed. It is not possible that?
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$\begingroup$ Do you mean flaperons? $\endgroup$– SanchisesSep 28, 2019 at 12:22
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$\begingroup$ Yes , the delta use all the trailing edges for pitching. $\endgroup$– George GeoSep 28, 2019 at 16:26
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1$\begingroup$ I know. But you seem to be conflating pitch authority (elevators / elevons) and high lift devices (flaps / flaperons). Only the latter is used to lower approach speed (unless the elevators have insufficient authority which I doubt on a high performance plane). So which one would you like to know about? Or both? $\endgroup$– SanchisesSep 28, 2019 at 17:14
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$\begingroup$ I want to know if it was possible to use the ailerons like high lift device (camber changed) for lowering the approach speed. There was some wind tunnel trials? $\endgroup$– George GeoSep 28, 2019 at 18:01
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$\begingroup$ Please edit your question accordingly, then, comments can be deleted at any time. An elevon is not a flaperon so leaving this terminology in is confusing. $\endgroup$– SanchisesSep 28, 2019 at 20:26
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2 Answers
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I'm no expert on the plane, but wikipedia claims the Mig-21 has separate flaps and ailerons.
On the trailing edge there are ailerons with an area of 1.18 m², and flaps with an area of 1.87 m².
The common reasons for not combining flaps and ailerons are that it reduces aileron response and increases the chance of stalling the wing tip (which results in a violent roll in the direction opposite to what you want).
As the Mig-21 has a separate horizontal tail for pitch control, there's no need to use elevons.
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$\begingroup$ So how you could alleviate the landing speed? $\endgroup$ Sep 28, 2019 at 20:31
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$\begingroup$ Like the F18 Hornet putting all these control surfaces from the trailing edge in down position. $\endgroup$ Sep 28, 2019 at 23:16
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1$\begingroup$ @George, presumably the designers considered that the flaps were large enough to reduce landing speed on their own. The F18 has to land on carriers, so it needs a lower landing speed. It uses the whole span of the wing for flaps and uses the horizontal tail for roll control. $\endgroup$ Sep 30, 2019 at 7:46
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$\begingroup$ So the Mig21T WILL NOT use the horizontal tail for roll control?I believed that the ALL fighters use differential deflection of the H tail for rolls $\endgroup$ Sep 30, 2019 at 8:07
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1$\begingroup$ No, MiG-21 didn't have a differential stabiliser. The next one, MiG-23, had it, but it had no ailerons at all. $\endgroup$– ZeusOct 3, 2019 at 1:25
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The reason to use ailerons as flaperons on delta wings is twofold:
- Delta wings reach maximum lift at a much higher angle of attack, which entails high pitch angle on landing, which is impractical. A way to generate enough lift at a reasonable angle is required.
- With low aspect ratio, space on the trailing edge is very limited, so there is a desire to use all of it.
However, there were several considerations against using flaperons on MiG-21:
- Overall, MiG-21 is still a 'classic' airplane with conventional mechanical (though boosted) controls. It is not a pure delta which would require special controls by definition.
- It is not only classic, but cheap and simple by design. Adding control mixers would complicate the system.
- To compensate for the lift problem at landing, two things were used:
- Most variants had the 'blown flaps' system, which used compressed air from the engine to blow the boundary layer from above the flaps. The air was injected in the (single) gap of the flap. Like any fighter, MiG-21 had a significant excess of engine power available on landing, and it could be put to use this way.
- A somewhat higher landing speed was simply tolerated. The landing speed was about 280 km/h (~150 kt). As one of the results, main tyres were wearing out quicker (I heard the typical lifespan was 5-10 landings).*
(*) That said, the contemporary F-104 was reportedly even worse in this regard, without being delta.
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$\begingroup$ I read that the landing speed is 320 ,340 km/h $\endgroup$ Sep 30, 2019 at 7:35
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$\begingroup$ No, that's gross. Speeds over 300 km/h happened when that blown flaps system (SPS) was not used, but officially normal landing speed was as low as 250 km/h on some modifications. $\endgroup$– ZeusSep 30, 2019 at 9:42