Forum: F-35 Lightning II

Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

dwightlooi wrote:

One of the things which is popular these days is to use a relatively symmetric upper and lower profiles over the first 1/2 to 2/3 of the air foil before you allow the flow speeds to differentiate. This tends to delay shock formation at high sub-sonic speeds further aft on the wing and thereby reducing drag. It is also popular to use a much blunter leading edge. However, some of these things may not be ideal for supersonic flight and the blunt nose is a big no-no for stealth.

It's been too long since my graduate level aerodynamics course for me to say exactly what's wrong with these statements, but I'm pretty sure there are some oversimplifications in there. Firstly, I'm not sure what the term "relatively symmetric" means. It's either symmetric or it's not. Seriously... if you want properties of a symmetric airfoil, you use a symmetric airfoil. Second, you have to understand how an airfoil is defined via geometric equations taking into account things like camber, chord length, and so on before you start talking about symmetry. As for the formating of shock waves at high subsonic speeds, you have to realize that you can screw up "off-design" performance if you try to design an airfoil to perform really well under one condition. Since fighters need excellent performance over a wide range of flight conditions, you can't start tweaking the airfoil to have a "drag bucket" (pronounced area of the drag polar at low Cl values where Cd is nearly constant and quite low as well) or some other special aerodynamic feature. You'll screw up some other part of the performance envelope.

Modern fighters and other supersonic aircraft will basically have thin, diamond-shaped airfoil sections. More modern fighters incorporate things like leading-edge flaps to improve high AoA performance. Trailing edge flaps are nothing new, of course. As for leading edge curvature, the faster you go, the less "blunt" your leading edge can be. You want a sharp leading edge for supersonic speeds, but not necessarily so for subsonic speeds. They can cause an adverse pressure gradient near the leading edge under certain conditions. That's why an F-16, for example, has an automatic leading edge flap system that continually adjusts itself at subsonic speeds. At supersonic speeds it's locked at +2 degree up angle (read, it's not flush with the rest of the airfoil). So variable camber is always a big plus for fighters and the like.

dwightlooi wrote:

One of the things I notice about the F-35 wing is that it has a lot of "curve" to it. This is especially true near the wing tips. There is a hint of reverse curvature on the aft lower surface, but nowhere near as pronounced as a classic Whitcomb style supercritical foil. The airfoil has edges -- leading, trailing and tip -- which are very sharp and almost knife like. From what I gathered, the outer most section of the wing -- beyond where the trailing edge control surface ends -- is largely a lateral housing for an antenna farm to support EW and comm gear. This location has minimal obstructions and integrating the antennas there beats putting them on a tail pod or as tabs sticking out of the fuselage.

What you're seeing is not the airfoil exactly but the finite wing's geometry. The airfoil might be the same shape over the entire 3D finite wing (if you take infinitesimally small cuts over the entire span), but the way in which the general airfoil shape is distributed over the span of the entire wing is what gives it its seemingly curved shape. You're talking about terms like twist, anhedral, dihedral, washin, washout, etc. That's what I was trying to tell you... it's really very difficult to determine what sort of airfoil a fighter is using because 1) it's thin and relatively small and 2) it's embedded in a more complex 3D finite wing shape that twists and turns its way from root to wingtip. I think the pronounced curvature you're seeing is probably an optical illusion due to the wing's twist and other 3D features. I doubt the airfoil itself is very "curved", although I could be wrong on that point.

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Good points and of course in the case of the F-35 CV,... everything has to really really work so it can do safe carrier traps, including useful bringback requirements etc. (off topic CV will also be able to dump fuel) ... thats before any combat perks in the design can be discussed. Wink

Again, good artwork DW. Very Happy

Also it will be interesting to see what stealth qualities an export F-35 will be allowed to have. Will it go through a typical LOEXCOM (or whatever they call it today) review board?

Excellent work, did anybody happen to do a similar frontal profile comparison for the F-35A and F-16C? Just curious.

I wasn't aware the Gripen was such a small plane. That must be smaller than a F-16 easy.

It's in another thread, IIRC.

But seriously, resurrecting a 3-year-old thread...?!

I didn't realize the Gripen was so small.

Its roughly the same size as the F-20.

bjr1028 wrote:

Its roughly the same size as the F-20.

Exactly. Think about it: They each use one (regular size) F-18 engine, and the F-18 isn't exactly monstrously huge.

Forum: F-35 Lightning II

Frontal profile comparisons - F-35 vs other jets