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Elite 1K Joined: Aug 02, 2006 Posts: 1003 Status: Offline

snypa777 wrote:

A question if you will all bear with me, thinking of a delta wing, what does a "cranked" delta do for a pure delta winged aircraft? The Vulcan bomber originally had a pure delta wing which the designers cranked to alleviate wing flutter. How will a cranked delta wing affect low speed maneuverability, if at all?

Thanks in advance!

If by cranked delta you are referring to the F-16XL style wing, it does two things...

(1) The forward portion of the wing near the root is shaped to maximize vortice generation - basically its a large LEX of sorts -- and helps the wing produce more vortex lift than a straight delta.

(2) The outer most span near the wing tips has much less sweep and chord. This behaves much better at high AoA and low speeds and a straight delta. It mitigates some of the problems associated with deltas.

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Dwight is correct about the XL wing. Note also that the XL outer span (lower sweep angle) had leading edge flaps to help even more at higher AoA and low airspeed. During the design phase there was a study made to determine if all-moving outer span (aileron and elevon function) was feasible. performance was good, but weight penalty was too severe.

Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

Thanks for the reply fellas`. My thought was why the cranked delta is virtually done away with in todays` fighters. A lot of modern Delta`s seem to use canards and straight delta wings, such as Typhoon.

Also, on the old Saab Viggen, the outer wing span has a greater sweep angle than the inboard section, it`s a "reversed" cranked delta design. What would be the thinking behind that?

About the Vulcan, I have some doubt the crank (very subtle) was added to alleviate flutter. That big thick wing should be very stiff in twist (key factor in flutter resistance). One web site mentioned it was to reduce turbulence. The wing front spar appears to be unchanged, and the leading edge was simply extended. The new LE appears to be much sharper and cambered, similar to the B-58 cambered delta and F-15 leading edge camber. The camber gives better high AoA performance (something like the F-16 leading edge flaps). Here is a photo where you can see what I'm talking about.

I was lucky enough to see a Vulcan airshow at the grand opening of the Dallas/Fort Worth Airport in 1974, along with a Concorde, a Ford Tri-Motor, and others. The Vulcan was extremely impressive, and even louder than the Concorde, it seemed to me.

The Viggen is a different kind of delta in that there are really two deltas. What appears to be a canard control surface is in fact a fixed surface with a small trailing edge flap, for low speed pitch control only, primarily takeoff and landing. Like a conventional delta, primary pitch and roll control is with main wing trailing edge elevons.

At higher speeds the forward wing acts mostly as a vortex generator to improve the performance of the main wing. It also increases the air velocity over the main wing, resulting in more lift. Sailors will see that the forward wing functions much like a jib sail. To answer the question about why the main wing has a sweep change in the leading edge, it appears to be to provide clearance (physical and airflow) for the forward wing flap. Without the reduced LE sweep of the inboard wing, there could be interference and and airflow choking with the flap down.

Elite 1K Joined: Jul 26, 2005 Posts: 1455 Status: Offline

Thank you John`, that Vulcan picture was the exact same one I was going to upload! You are right, the wing extensions were added to counter buffeting, not wing flutter. I have seen a Vulcan perform and those Olympus engines really make an impressive howl!

Also many thanks for clearing up the Viggen for me and what the forward control surfaces are actually for including the wing sweep change. Thumb

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Interesting "thrashing" of the Typhoon