Why are the red/white control surfaces under the wing extended during a take off?

These pictures are more than a little dated as the CF-101 Voodoo's have not been flown since the mid 80's. In these two photos we can see the Voodoo is performing a full afterburner take off. Note the inside red coloured control surface at the back underside of the wing. Why is this surface deployed like this?

and a moment later airborne

The airbrake flap are on the fuselage just aft of the tip of the red and white lightning bolt. They are closed which I would fully expect on a take off. Also the drag chute is not deployed so also another indicator that this is a take off and not a landing.

Here is a picture of a model that shows the control surfaces a little clearer along with the air brakes deployed.

What is this red control surface under the rear aft of the wing called and why is it deployed for a take off?

Note I am not a pilot, nor have I studied aerodynamics much beyond lift, thrust, gravity and drag.

• I'm not an expert, but from a bit of googling, they seem to just be this plane's flaps (albeit a bit odd-looking). If so, they would be deployed to provide extra lift (at the cost of increased drag, so once you're airborne they would be retracted). They're detailed in this page, which is a review of a scale model of this plane — not inherently authoritative, but the author seems to know their stuff! Oct 2, 2022 at 6:24
• It looks like a slipt-flap. This kind of flaps were used because of their simplicity and almost no change in pitching moment when deployed, albeit at the expense of much more drag. If someone else confirms it, I could write it an answer with a couple of plots. Oct 2, 2022 at 6:32
• @sophit The page I mentioned in my comment above identifies it as a zap flap, fwiw. Oct 2, 2022 at 6:35
• You can see a detailed picture here: 109lair.hobbyvista.com/DETAILSITE/US/USAF/f101/… Oct 2, 2022 at 6:37
• "Also the drag chute is not deployed so also another indicator that this is a take off and not a landing." Pretty sure the afterburner should be the dead giveaway that this is a takeoff :) Oct 2, 2022 at 10:26

These are not control surfaces, but a special type of split flap. In a split flap, the rear part (20% to 30%) of the lower wing panels is hinged at the front and can be lowered, thus increasing wing camber.

In case of the F-101, the hinge point moves aft in addition to the flap being lowered, thus increasing wing chord a bit. This type of split flap is called a Zap flap after its inventor, Edward F. Zaparka (invented in 1932).

A split flap gives the highest lift increase for a simple mechanism, but also increases drag considerably. Therefore, it is best suited for landing. For takeoff, it should be opened only partially (10° to 20°) but for landing 45° to 60° are common.

• As per link from @yshavit it is actually a zap-flap, very similar but it moves backward as well slightly extending the chord/surface Oct 2, 2022 at 12:43
• One interesting thing about the CF-101's flaps is that they seem to move not exactly backwards, but back-and-out (ie, they're hinged almost on a pivot, relative to the fuselage). You can see that from an alternate view here. I wonder if that was for aerodynamic reasons, or for something else, like to accommodate some armament. The CF-101 could fly with two external fuel tanks, so maybe this was a way of directing air away from them? Oct 2, 2022 at 18:37
• I read somewhere that the F-101 only had two flap positions: UP and 45 degrees which was used for both takeoff and landing. Oct 2, 2022 at 19:56
• @yshavit -- re "One interesting thing about the CF-101's flaps is that they seem to move not exactly backwards, but back-and-out (ie, they're hinged almost on a pivot, relative to the fuselage)."-- based on this pic, 109lair.hobbyvista.com/DETAILSITE/US/USAF/f101/… , my guess is that the hingeline of the flaps is simply nearly spanwise, as opposed to the marked forward sweep of the inboard trailing edge. So the flaps are much wider inboard than outboard. I think this accounts for all the apparent weirdness that we see in the pics. Oct 2, 2022 at 21:05
• PS your own photo link seems to show same. (PPS A strongly forward-swept hingeline would cause the flaps to create a powerful anhedral effect in terms of slip-roll coupling, when deployed, which might not be a good thing.) Oct 2, 2022 at 21:07

At a first sight it looked like a split flap but as @yshavit found out, it is actually a zap flap. A zap flap is a particular kind of split flap that via a leverage moves not only downward but also backward, increasing the chord as well. Apparently the name derives from his prolific inventor, Edward Frank Zaparka, who patented it in the 1933.

Split (or zap) flap together with plain flap have been historically the first iteration of trailing-edge high-lift devices. They have been superseded by more efficient albeit mechanically more complicated flaps, like the three-slotted flaps seen on virtually all modern jetliners. This picture taken from Wikipedia shows the several types of existing flaps.

Why a zap flap instead of a simpler split flap? The following plots taken from this NACA TN show that moving the flap backward as well, slightly increases the $$C_{l_{max}}$$ practically without any increase in the $$C_d$$:

On the left side there's a split flap while on the right side there's a zap flap. Due to the backward extension of the flap, the chord i.e.wing surface becomes bigger and bigger surface implies higher lift.

From the plots it can also be seen what is in general the effect of a split (zap) flap: both the plots of $$C_l$$ and $$C_d$$ are basically shifted more and more upward the bigger the flap deflection is. With modern slotted flaps the upward shift of $$C_d$$ is more limited.

But why exactly a zap flap instead of a plain or nother kind of flap? According to the relevant wiki entry, the F-101 suffered from "severe pitch-up at high angle of attack". High angle of attack are seen also during landing phase so a kind of flap which didn't changed too much the wing's pitching moment would have been welcomed: split flaps have just this characteristic. So it might be (just speculating here) that the choice of a split (zap) flap was dictated by this requirement.

I think that is a flap used on takeoff and landing. They are normally deployed (partially) on takeoff and (full) on landing. Flaps help in increasing lift at slower speeds. They are called 'high lift devices'. You can see them here(landing)

• In your picture all 3 sections extended downward compose the flap, not only the last one with the arrow Oct 2, 2022 at 7:09
• @sophit i know i will edit the picture Oct 2, 2022 at 7:18
• Note that the flaps in the picture are much more complex than the flaps on the fighter. I assume that is a deliberate tradeoff: flaps on airliners are designed for efficiency and safety, whereas the fighter values space, weight, and simplicity and pays for that with increased drag … which is much less of a problem for a fighter, because they tend to be quite overpowered compared to an airliner. Oct 2, 2022 at 9:50