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Delta / Canard fighter layout; why?

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posted on Nov, 2 2007 @ 09:30 AM
Euro Fighter, J-10, Gripen, Rafael: They all have the same delta/canard layout while no US fighter has ever made use it. Most US fighters have used swept or diamond wings (like the F 104 or the F 22)

Why? What are the advantages/disadvantages of the delta/canard layout?

posted on Nov, 2 2007 @ 10:21 AM

Originally posted by Number23
Why? What are the advantages/disadvantages of the delta/canard layout?

Simply put (for the designs you mention) - high speed maneuverability.

The US don't seem to like canards from the outside, but looking closer, they didn't design a new fighter for high speed maneuvering before TVC became a serious option.

This allowed them to prioritise radar shape on the F-22 (avoid the canard fuselage junction in the forward quadrant radar 'wetted' area) while using TVC to trim the aircraft for supersonic flight (giving them back good maneuvering capabilities).

There are a number of factors to consider over which is better, some of these are:

1. A canard generates upthrust when pitching nose up, while a traditional elevator generates a downthrust - thus a canard should allow for better sustained turning performance.

2. A canard can generate a little upthrust in steady level flight, allowing for a smaller wing - meaning less inertia & damping, thus better dynamic response rates (in pitch and roll).

3. A canard when pitching nose up will induce a downwash over the main wing, reducing its effectiveness, which conflicts with (1) and (2). Increasing the distance from canard to wing can help offset this (Eurofighter).

4. A canard can be used as a replacement for a LERX for high AoA flight, and its normally more efficient in cruise. Close coupling of canard and wing helps achieve this (Rafale).

5. With a canard, the main wing can be placed further back, hence the c.g of the aircraft is further back, hence the gear is further back, hence more aggressive rotation angles can be used for take-off. The canard also is better at inducing rotation than a tailplane.

6. A canard requires a junction between wing and fuselage - this junction is harder to hide on radar. Its preferred to hide this behind the main wing on an elevator (like the F-22).

posted on Nov, 2 2007 @ 10:26 AM
[edit on 2-11-2007 by Number23]

posted on Nov, 2 2007 @ 11:08 AM
Before addressing the pros and cons of the canard delta with any other configuration, you must understand the reason for the canard in the first place.

A tailless delta wing, when increasing angle of attack (in manoeuvring flight - ie:- turning) must generate a downward force aft of the CG. To do this the elevons must be deflected upwards (which in the case of slow speed flight is the opposite of lowering flaps), and this results in an overall loss of lift from the wing (meaning that to remain level in a turn even more AoA is required). Therefore the tailless delta is at a disadvantage in manoeuvring flight with regard to other configurations. In the slow speed regime, you can see that if a tailless delta were to deploy flaps, they would act is exactly the opposite way to the elevons which would be deflected upward - and this would look exactly like a split trailing edge speedbrake! Hence the lack of flaps on tailless deltas, and a resulting high landing speed (Do you see why there have been no delta shipboard aircraft?)

By adding a conventional tailplane (and elevators) ala MiG-21, the tail can produce the downward force to increase the angle of attack while the wing produces more lift and can also be fitted with flaps to increase the lift of the wing in slow speed flight.

The use of a canard instead of a conventional tail (on both a delta and any other wing configuration) is to provide the pitching moment by an upward force (lift) ahead of the CG rather than a downward force aft of the CG resulting in an overall increase in lift greater than provided by the conventional tail (downward force at the tail=loss of lift which must be subtracted from the increase in lift generated by the wing due to its increased angle of attack). The greater the increase in lift with change of angle of attack vs the increased drag that results, then the better the aircraft will be able to maintain energy in a turn.

The 'downside' of placing the pitch control forward of the wing, is that the airflow over the canard which can disturb the airflow over the wing. Careful design has minimised the harmful effects (which obviously change throughout the flight envelope) and in certain parts of the envelope (especially very high angles of attack) the airflow and vortices off the canard and how they affect the wing have been used to advantage to maintain lift from the wing at angles of attack that would normally result in a stall.

That the US has not gone down that road, has really been that the US never embraced the delta in the first place - with the exception of the F-102 / F-106 tailless delta interceptors. However, of late, the US has become interested in the subject (at least from a research perspective) with the X-31, which then continued into the realm of all axis control by thrust vectoring.

The US did test canard surfaces (retaining a conventional tail as well) on the F-4 Phantom and the F-15 Eagle.

The Winged Wombat

[edit on 2/11/07 by The Winged Wombat]

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