Delta Wings and Flap

[tjg]

Hi all

Can anyone explain why Delta wing A/c dont have flaps- I think I know the answer but not sure.

Something to do with no corrrection for wing pitching moments usually sorted by the horizontal stabilator. Also they probably dont need them due to the high ampunts of profile drag at low speeds.

Any takers?

:confused:

[Tinstaafl]

Quite correct.

Not just delta winged a/c either. Canard types can't have them either - unless the canard also has some lift enhancing mechanism so that the overall CoP (canard+mainplane) doesn't move significantly eg the Beech Starship canard/pusher turboprop.

Delta winged types can often make use of much higher AoA than conventional aerofoils, along with a method of lift production using a large vortex shed from the forward wing root area.

[ 19 July 2001: Message edited by: Tinstaafl ]

[Genghis the Engineer]

Agree with tinstaafl, although the reasons vary between deltas and canards as he says.

With a delta, frankly it's not needed. They keep lifting to higher and higher alpha, eventually stalling around 30-35°. There's no need for flaps, the main problem is still seeing the runway when you're that nose-up (hence the famous Concorde drooping nose).

With multiple wings you have to match the lift of each or you get a huge, and potentially uncontrollable pitch change with flap selection. This is very hard to mechanise successfully, and I'm not sure that anybody's ever really succeeded.

I believe Darrrol Stinton's done quite a lot of work on the latter subject, although I'm not sure where he published it.

G

[John Farley]

tjg

I hope you don’t think it a nitpick, but I suspect you mean tailless delta wing aircraft.

The Javelin (a delta winged aircraft if ever there was one) most certainly has flaps and as a result has a bundle of lift in the flare without a high nose attitude.

The Tu-144 that crashed at Paris (the version with a canard) had an effective flap as all the trailing edged surfaces were well down at touchdown unlike an ordinary tailless delta where all the trailing edge surfaces are necessarily up to flare. As a result it landed some 25 kts slower than the original version without a canard.

So I would not go so far as Genghis and say deltas don’t need them as I see that as a tad simplistic. Its no good having all the Clmax in the world if you can’t use it. Yes deltas can use a very high AOA and get a tremendous max lift coefficient as a result but at the expense of a very high nose attitude. This leads not just to view problems (as solved by drooped noses) but even more important (and not much talked about) the very high available Cl is quite unusable on the approach as the back end hits first with any reasonable length of undercarriage leg. As a result the approach speed has to be stuffed right up and you sit there thinking I wish I had some flaps on this thing. It was nothing to have to land the HP 115 at 90 kts to avoid clobbering the back end - despite a few minutes earlier being quite happy with the handling at 60kts.

Cheers

JF

[con-pilot]

John Farley is quite correct. The US Navy A-4 had a delta wing, flaps and tail with elevators. I guess it depends on where the delta wing is on the airframe.

[tjg]

Thanks for your comments- I am sure that I over simplified the situation and am now aware that certain delta wings have flaps.

Cheers

[Genghis the Engineer]

Yet again the old and bold put youngsters like me into our places.

Yes John, I was thinking in terms of tailless deltas, which wasn't entirely the subject under discussion. The Javelin, if memory serves, had a tailplane, which could balance trim change due to flaps and allow a reasonably flat approach attitude.

A subsidiary issue, is that although they generate lift at very high AoA, deltas generate huge amounts of drag. This is almost certainly beneficial when trying to stop a monster like Concorde, but might be another reason why you needed a high initial approach speed in the HP115 - to stop you running out of flying speed. When I fly my flexwing microlight (essentially a tailless delta) I have to fly an approach at about 1.7Vs otherwise you end up with no control in the flare (and can potentially stall). Also lateral stability becomes excessively high in a tailless delta at high-AoA, offering severely reduced roll authority and excessive rolling gust response. Neither of these will be very helpful when trying to land on a less than perfect day.

I wonder though if the Tu-144 didn't use the flaps to shift CP aft, so it was more similar to the 50% chord supersonic position, thus reducing the trim issues as it changes speed regimes. (It also, perhaps on a different tack, had a retractable canard, which would allow control with the pitch changes with flap). Not a simple subject really.

G

[ 24 July 2001: Message edited by: Genghis the Engineer ]