View Full Version : Leading edge devices

12th Oct 2004, 12:11
Please indulge me in this question I have pondered for quite some time. Leading edge devices....

With the 707 and 720, Boeing decided to merely use Kreuger Flaps as the leading edge device - no slats / slots.

With the 727, Boeing opted for a combination of Kreuger Flaps and slats - lift generation and stall protection. The 737 has a similar arrangement.

Now, as I understand them, Kreuger's merely create lift - on the A300B4, I believe the relatively MINOR inboard Kreuger they had generated approx. 5700 KG of lift. Slats, conversely, do not create lift per se, merely prolong flow seperation at high angles of attack (in fact, at low angles of attack, slats create drag out of proportion to lift generated).

So....... what is the best all round? A leading edge device that creates lift directly ala Kreuger, or slats that create negligable lift at low angles of attack, but prolong flow seperation (allowing higher AoA), or a combination of both like in the 727, 737 etc?

I'd like to hear from old-timer's that may have flown the 707 and compare the performance in relation to a combo-aircraft like the 727.

Sorry if the question seems daft!

12th Oct 2004, 13:35
Slats increase Cl max and hence will increase lift at a given IAS at alpha values beyond those available without slats.

12th Oct 2004, 16:09

While I have not flown the B727 (and am certainly not an aerodynamics expert...just a pilot), the older models of the 707 were certainly a challange with regard to runway performance.

These early aircraft (straight pipe powered -300 series, with minimal leading edge devices) would roll...and roll....and roll, seeminging right to the very far end of the runway, before rotation was possible, at heavy weights.

A large part of the problem was the non-fan engines, but also the lack of full-span leading edge devices.
Only one airline opted for full-span leading edge devices on early -300 series aircraft...South Africian.
I flew one of these for another operator after it it was sold, and the runway performance was greatly inproved.
Along came the fan engines, and full span leading edge devices were fitted as standard, on the long body aircraft.

The only aircraft that I have flown with slats was the Lockheed TriStar, so I really can't compare with others.

Mad (Flt) Scientist
14th Oct 2004, 00:17
If we're discussing field performance, a big issue will be what drives your field numbers. Are you Vmu-driven, in which case, unless your geometry is truly odd, you're likely limited to, say, 15 degrees pitch. So anything that affects in-ground-effect lift at, say, 20 degrees and above only is useless.

Whereas you may be stall-speed limited - in which case anything that improves CLmax is good (provided the lift is usable of course).