I used to work on F-4s and one interesting feature was the inverted airfoil on the horizontal stab. Also the stab leading edge had built-in slats. I am curious why the design took such drastic considerations. Would a symetrical airfoil for the stab develop too much drag while creating negative lift compared to the current design?

Well, I don't know the exact answer to the question but I think I can make some progress. The first consideration is that most full scale airplanes are built with tail down force into the structure. With the center of lift behind the center of gravity the tail down force equalizes the torque of the center of lift, putting the center of lift on the center of gravity can cause some unwanted flying stability characteristics.

As for the inverted airfoil, a fully symetrical airfoil cannot generate lift at zero AOA, if the the AOA of the stabilator in level flight is zero then you would decrease drag. With a "negative" airfoil, the longer route for the air to travel across the curved bottom of the stabilator creates a high pressure area on the bottom of the surface and creates the needed tail-down force. A non-symetrical airfoil can create lift at zero AOA, therefore decreasing drag compared to a full symetircal. There is still drag induced by the surface creating lift but it is not as great as an AOA other than zero.

As for the LE slats, they're there to keep the stabilator from stalling during maneuvers, a pretty self explanitory theory.

-Kelly

Inverted airfoiled horizontals are common. L-1011... C-130.. used to get the download without having the larger structure a conventional symmetrical shape would require, as well as the trim drag benefits from not having to trim far from neutral.

Thanks Paul, thats what I was thinking. Would the slats create multiple shock waves during supersonic flight? Seems to me not being a supercritical airfoil design would do that but I guess with enough thrust you can overcome anything.

I think the F4 just shames the air into getting out of its way! And bludgeons it in the process!
The former EAFB example mounted on a plinth in Lancaster has a horizontal without the slot..
But a fancy shape topside..

I flew the C's, D's & E's in the Air Force. The C & D model had the non-slotted stabs. On take off the normal procedure was to hold full back stick and go full burner. When the plane rotated, you eased off the back stick after lift off when you reached the climb attitude. On landing you usually hit full back stick just at touchdown. For in-flight maneuvering there was plenty of elevator authority, but on and near the ground, you were limited.

The Air Force E model had the slotted stab with more downward lift. You could make a normal burner take off and pull back stick at take off speed. You could also land the plane normally with a flare.

Kelly you are double dipping. The aerodynamic center or neutral point of a conventional symmetrical airfoil planform is aft of the CG, however it includes the lift of the wing already. Make a drawing of wing lift and tail lift about the CG. The lift of the wing is forward of the CG and to balance it the lift of the tail must be up. You have used the tail lift twice. You can say either "center of lift" or "wing and tail lift" but not "center of lift and tail lift".

When you include airfoil camber effects, flap effects and fuselage effects (all which seem to pitch the airplane nose down) then many times the tail loads shift to down. There is where the supercritical inverted airfoil is a valid use to keep the drag to a minimum.

In the case of the F-4 with flaps and slats, etc., there is a big nose down pitch. With the slatted tail it is keeping the horizontal effective enough at the low speeds to keep the nose up. As was noted by Ed (now known as Lucky Ed who I am very jealous of for him getting to fly lots of one of the meanest, good looking, airplanes of all time (while I flew a desk to and from the coffee pot, big thrill!)) during normal maneuvering tail effectiveness should not a problem. You put up with the drag of the horizontal slats at high speeds because you need them more somewhere else.

Paul, shame on you, don't talk bad about the F-4.

Ben

Ben, how fast did your desk fly? Was it a prop job or a jet? If I can get my desk to fly I wouldnt get up but to go to the bathroom!

The F-4 was big and it was ugly unless you like functional, but it carried more bombs than a B-17! You could load 18 500 lb bombs on board and we regularly carried 16. That's 4 tons. It had its quirks, but it brought me home every time.

I flew the F-4E's out of Korat Royal Thai AFB into Laos and North Vietnam. I remember getting off the C-130 which flew me up from Bangkok and getting my first sight of the E's. They all had shark's teeth and eyes on them. My first thought was, "I'm in the Flying Tigers." They were the most beautiful planes I had ever seen. My plane was tail number 283 with JV squadron letters.

Everyone who comes back unscathed tends to look the war they were in and recall the fond memories. Except at rare times we tend to forget our buddies who didn't make it or those who were broken, both mentally and physically.

One last thing, combat flying is the greatest game alive. You bet your life and the loser can die. When you come back, it's the greatest feeling there is. You don't want to leave. You become addicted to the after-combat high. Well, enough of an old guy talking about ancient history.

Funny as it may....we, younger guys enjoy the war stories older guys tell. So ancient history can still be quite exciting. I was a crewchief for the 4th wing when they had F-4Es and during the transition to F-15Es. Although the F-15 is far better aircraft it seems that even our commander couldnt stop saying good things about the phantom.

Ed-
When were you there? I was at Ubon w/ the 555th in '68- perhaps we crossed paths.
John

69-70.

The thing I say about the F4 Phantom has nothing to do with aerodynamics, but is that in appearance it`s up there with the Chance Vought F4-U Corsair as one of the meanest looking aircraft ever built. Maybe the F4, common to both, has something to do with it. There`s an F4 on display in Hanoi that has more holes in it than a kitchen strainer. Got onto the ground though in one piece. That`s a quality it also had in company with the Corsair; its ability to take a lot of damage and still get back on the ground.

I was able to start at McDonnell Aircraft Co in 1965, there were lots of F-4 airplanes around. They always struck me as just a mean airplane, especially with the teeth on some. I would have made it a requirement on all F-4's.

Then we built the F-15 and I remember the first flight - sitting with the other guys in St. Louis watching the flight on TV. It was on tape flown back I believe. I thought what kind of killer did we make? It looked like a ballerina tip-toeing out to the flight line! It is a smooth sleek but not mean looking airplane. Even with lots of bombs and as unshiney as you can get with some of the drab color schemes it doesn't look mean.

No other airplane from any country beats the F-4 for mean.

I agree, but not an easy plane to model properly.

I believe the mean gene was passed directly from the F-4 to the A-10[>:] Even has the outboard dihedral kink.

Anyway, I never thought much about the F-15 appearance until I was flying thru Albequerque and saw one of the Edwards test F-15's taxiing. No tanks, no stores, not even the underwing sidewinder pylons that every other F-15 flies with. I wouldn't call it dainty, but it did look like the sport jet you would take out for vertical cliimbs in 15 minute bursts.

I gotta say though, the F-15E with the FAST pack nacelle-side mounted fuel tanks looks like a plane flexing it's biceps.

It doesn't say "I'm here to eat you" Like an F-4 though. I will always remember with the hair on my neck standing up, the after-dark AB close-pass I saw in Daytona in '90. That's a unique color of purple, especially when reflected in the vapor cloud around the fuse.