Body lift on the F-35

There are indications that the F-35 body makes significant contributions to lift. This is a "theory" of course. But here are the reasons behind it...


The shaping of the F-35's fuselage -- more specifically the slab undersides with a reverse curve towards the aft 1/3 of it resembles the basic geometry of a typical supercritical airfoil. My guess is that it is not co-incidental. Can we say it is a deliberate attempt at optimizing body lift?



This photo shows the underside of the F-35 with the gears down. We know for a fact that the center of gravity of the aircraft with or without ordnance and/or fuel will have to be safely forward of the pair of main landing gears. If not the aircraft would have toppled backwards on the ground. The center of the weapons bay also provide a clue to the center of gravity as hanging ordnance close to (or on same lateral axis) as the CG is ideal for efficiency as it reduces or eliminates positive or negative trims needed which can be costly aerodynamically. In fact, I'll be very surprised if the center of gravity of the aircraft is not ahead of the vast majority of the wing area if not entirely ahead of the main wing. The overlays illustrates how far forward the center of gravity of the F-35 has to be.

From the picture, it is also very hard to imagine that the center of lift of the main wing being anywhere forward of the main landing gears. That will mean that the center of lift of the main wing is ahead of ~70% of the wing's area and that the airfoil has a center of lift sitting literally on the leading edge! Besides, the typical supersonic airfoil (or even high performance subsonic airfoils for that matter) tends of the aft loaded.



In just about every photo and video of the F-35 in more or less level flight, we can see that the horizontal tails (the elevons) are canted slightly upwards. That is they have a slightly higher AoA compared to the main wing. This indicates that the tails are providing lift to prop up the stern of the aircraft and that in their neutral position the tail will sink and the nose will rise. This is a hallmark of an aerodynamically unstable design (the F-16 is this way too).

Now, given the above facts, things get interesting... If the aircraft is using positive lift on the horizontal tail to stay balanced, this can mean only one thing -- the center of lift of the aircraft has to be at least slightly ahead of the center of gravity! Now, we know that the center of gravity has to be ahead of the landing gear and ahead of most (if not all) of the wing. Hence, the only way that can happen is that additional lift (not provided by the wing) with a more forward bias has to be present. Other than the body, I don't see where it can come from!

Ok, all due respect, that is 99.9% rubbish. Just because a certain profile of a body (A) resembles another body (B) which happens to be lift producing, does not indicate that (A) is lift producing. I know enough about fluids to be dangerous and, you're just noticing a fundemental law of making something "slick" through a fluid. Are the coke bottle F-102's lift producing? No. Is the F-35 a lifting body? Not hardly. In university we had a little box with a square looking model car, and another with a geometry similar to what you're drawing onto the 35.... Turns out that the boxy "car" is a dog.

No way is the F-35 a lifting body. Remember that lift is perpendicular to the forward velocity vector, and if you're always generating a lot of lift, then you'tr slower than the less lift producing fellow. Hence Tomcats sweep the wings and reduce lift to increase speed. Yes, I know there is more to the equation with the Tomcat analogy. But it serves the point.

<i>Ok, all due respect, that is 99.9% rubbish. Just because a certain profile of a body (A) resembles another body (B) which happens to be lift producing, does not indicate that (A) is lift producing. I know enough about fluids to be dangerous and, you're just noticing a fundemental law of making something "slick" through a fluid. Are the coke bottle F-102's lift producing? No. Is the F-35 a lifting body? Not hardly. In university we had a little box with a square looking model car, and another with a geometry similar to what you're drawing onto the 35.... Turns out that the boxy "car" is a dog. </i>

No, just because A bears some resemblance of B doesn't mean that it shares very similar lifting properties. I never said that. I said that it MAY BE an indication of such.

However, if the fuselage is not producing any lift then how can one explain the positioning of the CG well ahead of the center of lift from the main wing? If the CG isn't ahead of the main gear the aircraft will topple onto its tail. We know that doesn't happen with the F-35. We can also be quite sure that the center of lift from the main wing cannot be ahead of the main gear where the CG has to be. If the CG is well ahead of the CL, then the aircraft will need a significant amount of negative trim on the horizontal tail to stay aloft. Yet, in fact ALL in flight photos show that the reverse is true. In other words, unless there is some lift input other than the wing which centers ahead of the wing, everything does not compute!

As far as being inefficient, I am not so sure about it. The CL of an airfoil can change with speed. The lift-drag co-efficient can also change with speed. This is especially true in subsonic to supersonic transitions. In fact, this is exactly what happens if you try to take a traditional high aspect ratio airfoil like those on WWII monoplanes supersonic -- the CL shifts rearwards, plus lift plummets and the aircraft dips nose first into a dive. One possibility is that the fuselage produces lift at subsonic speeds and places the total CL ahead of the wing such that the aircraft is unstable with a rear bias. the tail is used to keep the stern up. At supersonic speeds, it may be the case that the CL of the fuselage lift shifts rearwards and the lift production of the body may also become less effective due to various geometric reasons -- such as flow separation, shock formation or whatever. And as speed increases it may be the case that the trim from the tail becomes progressively neutral and then negative. We just don't know exactly what happens.

No way is the F-35 a lifting body.

First of all, he never said the F-35 was a 'lifting body'. That phrase connotates the very kind of 'wingless' design that you pictured. Rather, he was simply wondering about the amount of lift produced from the body vs the wings. In all fairness, that is a bit of a misnomer. For a blended wing/body type of design such as the F-35 (as well as the F-22, etc) you really don't have the conventional division of wing lift and body (or fuselage) lift. You just have a total lift for the total shape. What is relevant is the forebody lift component in determining the c.p. location. And yes, for the F-35, (just like the F-22) the forebody lift is significant.

And yes, the F-35 is unstable in subsonic flight (c.p. is ahead of the c.g.) - again just like the F-16 and F-22. No one in their right mind would design a fighter aircraft that wasn't in this day and age. Supersonically, the c.p. does move aft, potentially behind the c.g. to some degree.

Remember that lift is perpendicular to the forward velocity vector, and if you're always generating a lot of lift, then you'tr slower than the less lift producing fellow. Hence Tomcats sweep the wings and reduce lift to increase speed. Yes, I know there is more to the equation with the Tomcat analogy. But it serves the point.

Actually, it's kinda the other way 'round. Tomcats increase power (or dive) to increase speed just like every other laws-of-physics-abiding aircraft. The increased speed reduces their CL requirement (they still need the same lift, lift=weight in level flight no matter where it's coming from), so they can reduce the sweep. The reason they have swept wings is so they can fly slow enough to land on a carrier and also fly very fast. (They also reduce their footprint for storage by sweeping back, but that's a side effect).

Back to the original post... A point you didn't mention is that the gear swings backwards, so the c.g. moves aft a noticable amount when the gear comes down. Or inversely, assuming that the line you drew is correct, that line would move even further forward with the gear up.

dwightlooi, your missing something important. The tail only appears to be at a higher "AOA" (your term, not mine). those pictures are at bad angles, can't really be sure.

More importantly, what you are calling angle of attack is more accurately called angle of incidence. AOI is relative to some arbitrary line on the structure....AOA is measured relative to the direction of flight (but can also be referenced to the local airflow). Local, that's an important detail. Now let's expand on that. If the aircraft is in level flight, lift equals weight (and if they are turning, lift = weight*g). That much lift will generate a significant amount of downwash behind the wing. Therefore, if the tail and the wing are at the same angle of incidence, the the tail is in fact at a significantly lower local AOA than the wing. Which of course could mean that it is in fact pushing down.

All that side, of course it generates body lift. Virtually all airplanes fly with some positive deck angle (i.e. AOA)...an aerodynamicist would be a complete moron to not take advantage of that. Is it a significant amount...I hope so. Would hate to think my taxes a

re going to morons. Is this a big deal. Nope...

Let me also add that that just because the CP is ahead of the CG doesn't mean that the body is generating lift. Just means it's longitudinally statically unstable. No big deal. Do you know when the world's first longitudinally statical unstable aircraft flew? Dec 1903, by couple of brothers named Orville and Wilbur. Not exactly a new thing.

dwightlooi wrote:However, if the fuselage is not producing any lift then how can one explain the positioning of the CG well ahead of the center of lift from the main wing? If the CG isn't ahead of the main gear the aircraft will topple onto its tail. We know that doesn't happen with the F-35. We can also be quite sure that the center of lift from the main wing cannot be ahead of the main gear where the CG has to be. If the CG is well ahead of the CL, then the aircraft will need a significant amount of negative trim on the horizontal tail to stay aloft. Yet, in fact ALL in flight photos show that the reverse is true. In other words, unless there is some lift input other than the wing which centers ahead of the wing, everything does not compute!

As far as being inefficient, I am not so sure about it. The CL of an airfoil can change with speed. The lift-drag co-efficient can also change with speed. This is especially true in subsonic to supersonic transitions. In fact, this is exactly what happens if you try to take a traditional high aspect ratio airfoil like those on WWII monoplanes supersonic -- the CL shifts rearwards, plus lift plummets and the aircraft dips nose first into a dive. One possibility is that the fuselage produces lift at subsonic speeds and places the total CL ahead of the wing such that the aircraft is unstable with a rear bias. the tail is used to keep the stern up. At supersonic speeds, it may be the case that the CL of the fuselage lift shifts rearwards and the lift production of the body may also become less effective due to various geometric reasons -- such as flow separation, shock formation or whatever. And as speed increases it may be the case that the trim from the tail becomes progressively neutral and then negative. We just don't know exactly what happens.

Exactly.

The lift that produces the tailplane makes the aircraft to have positive static longitudinal stability.

NOTE1:If the CG is infront of the Lw(Lift produced by the wing), the Lt(Lift produced by Tailplane) must have negative value.

Just remember the simplified equations (DCm/DCl)=h-ho-V*k and it has to be <0 for positive static longitudinal stability.

OK, I think let me try to clear things up a little...

(1) We can be 100% sure that the Center of Gravity of the F-35 is somewhere in the region denoted by the GREEN BOX. (see picture below) That is, it is ahead of the main landing gear. It has to be or the aircraft will fall on its a$$ on the ground! (Yeah, I know the gears all fold forward and probably push the CG even further forward, but thats not really important to the essence of argument)

(2) I think we can also be 100% sure that the Center of Lift has to be in the region denoted by the RED BOX. That is, it has to be ahead of the Center of Gravity. This is evident from the up-trim posture of the horizontal tails during subsonic flight.

(3) We can also see from the picture that the Center of Lift of the Main Wing (YELLOW BOX) cannot be in the RED BOX region, but has to be aft of it -- you cannot have an airfoil generate lift centered practically right on its leading edge (or in front of it) even if you tried!

(4) Therefore, we can conclude that other than lift from the main wing there has to be significant lifting forces coming from somewhere. The Hypothesis is that it is generated by the fuselage, and that it is very significant.

dwightlooi wrote:(2) I think we can also be 100% sure that the Center of Lift has to be in the region denoted by the RED BOX. That is, it has to be ahead of the Center of Gravity. This is evident from the up-trim posture of the horizontal tails during subsonic flight.

(3) We can also see from the picture that the Center of Lift of the Main Wing (YELLOW BOX) cannot be in the RED BOX region, but has to be aft of it -- you cannot have an airfoil generate lift centered practically right on its leading edge (or in front of it) even if you tried!

The angle of the picture makes any definitive conclusions tough, but the way I look at it, I'm not sure I can agree with (3). Traditionally, subsonically, the c.p. of a wing can be approximated to be at about 35% MAC. From looking at the yellow box as drawn, and taking a stab at where the MAC is, and looking at about 1/3 of that, and connecting the lines, looks like it goes through the back half of the red box. Also, again the picture angle is tough, but I think the green line should be further aft than drawn. Looks to me like it is ahead of where the main struts attach to the structure.

Also, regarding (2), the airplane is not necessarily unstable at all times. It certainly is at the nominal, midweight fuel condition, but the c.g. range on this aircraft (due to all that fuel spread out everywhere) is way, way wider than previous, similar type aircraft. The forward c.g. loadings may be neutral or slightly stable. Also, with gear down the flaps schedule move the c.p., so you may (or may not) be unstable in that configuration.

One word on c.p. When talking about things like stability and margins, you have to be specific. In those discussions c.p. refers to the center of all lift except that from the tail (or other stabilizing surface). By the definition of trim, when you add in the lift from the tail, the c.p. goes exactly to the c.g.

None of this changes my basic points: forebody lift is significant, the c.p. (excluding the tail lift) is normally near or ahead of the c.g., and you can't talk about wing and body (fuselage) lift as seperate entities.

Let me also add that that just because the CP is ahead of the CG doesn't mean that the body is generating lift. Just means it's longitudinally statically unstable. No big deal. Do you know when the world's first longitudinally statical unstable aircraft flew? Dec 1903, by couple of brothers named Orville and Wilbur. Not exactly a new thing.

True, but not a fair comparison. Canard configurations (the Flyer) don't inherently have the very low time-to-doubles that aft-tailed, unstable configurations have. (Obviously, or it could never have been flown by any human, let alone an amateur, rookie pilot).

<i>None of this changes my basic points: forebody lift is significant, the c.p. (excluding the tail lift) is normally near or ahead of the c.g., and you can't talk about wing and body (fuselage) lift as seperate entities. </i>

This is one thing I don't get (or agree with). Yes, obviously as an aircraft in flight only total lift and where it is centered matters. But, if you take the wings off and put them in a wind tunnel I am sure it produces lift. If you put the wingless fuselage in the wind tunnel it can also produce lift if it is shaped in a certain way. Now, when the two are joined and the aircraft is whole the interface between the two may interfere with their respective lifting properties a little, but probably not by much.

As far as the discussion goes, I am separating the two simply to illustrate the point that it appears that if the body does not produce any lift, the wing alone would have placed the aircraft's Cp well aft of the Cg. Since this does not appear to be the case, therefore the body should have contributed some significant lift to the aircraft and it should be centered ahead of the Cg. This is the only way the total Cp can be ahead of the the Cg.

The boxes are not intended to be very precise. They are merely there to indicate the relationship between the Cg and Cp.

Dwight... you're in over your head on this one. I suggest you put down your mouse in photoshop and pick up an aircraft stability and control book and an aerodynamics book. People are trying to explain things to you that you obviously haven't studied in any detail if at all. This is a complicated subject.

dwightlooi wrote:But, if you take the wings off and put them in a wind tunnel I am sure it produces lift. If you put the wingless fuselage in the wind tunnel it can also produce lift if it is shaped in a certain way. Now, when the two are joined and the aircraft is whole the interface between the <i>two may interfere with their respective lifting properties a little, but probably not by much</i>.

Your assumption more or less works for your generic 777, but is not valid at all for a blended wing-body configuration like almost every current day fighter. The interference terms are way too significant to ignore.

dwightlooi wrote:As far as the discussion goes, I am separating the two simply to illustrate the point that it appears that if the body does not produce any lift, the wing alone would have placed the aircraft's Cp well aft of the Cg. Since this does not appear to be the case, therefore the body should have contributed some significant lift to the aircraft and it should be centered ahead of the Cg. This is the only way the total Cp can be ahead of the the Cg.

This is totaly wrong.
You dont have only the fuselage and wing, you have and the tail equations.
And keep one think in mind, the CG is not a fixed point.The CG moves in a margin.Expect changes.

May I be allowed to bring this topic back for a comment? I was deeply involved in the development and flight testing of the F-16 for many years, so have first hand knowledge of this topic for that airplane. You may be surprised to know that the lift provided by the F-16 fuselage is a very significant portion of the total lift. Every flight condition is different, but at one critical condition, 0.95M 10,000 ft, the fuselage provides 45% of the total lift at 9g. The percentage is even higher at some other conditions. Considering the F-35 wide flat-bottom fuselage, I would guess it provides at least as much lift, probably more.

Fuselage lift is extremely important in reducing the weight of an airplane. If the wing had to provide all the lift, its weight would be much greater. Similarly, the fuselage weight would be much greater if it did not have lift to help support it. The structural weight could easily double without fuselage lift.

So dwightlooi has it about right, in my opinion.

One other comment, concerning wing lift and fuselage lift in a wind tunnel. The lift on the combined wing/fuselage will generally be greater than the sum of the wing-only lift plus fuselage-only lift.

Ya'll are making this ol'crewdawgs head hurt.

This is nothing new, its been arround for a while. F-14's pancake shape fuselage accounted for 40% of the up lift of the TOMCAT. F-16 made the wing-body blend so hat there was no clear distintion as to wich portion of the aircraft is providing up lift and wich is not. Same for Su-27, Mig-29 etc etc.