F-15C Question

[Dick Montgomery]

I am working on the Academy F-15C in 1/48th scale. The kit provides two sets of air intakes. When installed, one set will have the upper "deck" of the intake more parallel with the fuselage and wings (or "higher") than the other. The second set has a distinct "droop" to that upper deck.

 

I assume that on the real aircraft that intake scoop moves for aerodynamic purposes....but that isn't critical to me.

 

What I would like to know is, "is one set of intakes more correct than the other for modeling the aircraft on the ground?"

 

Thanks for your input.

[Ralph Nardone]

Most photos of F-15's on the ground show them in the "up" position, although Wikipedia does have one photo with one up and one down. When the engines are running on the ground and when the airplane is subsonic in flight, I believe they'll be in varying degrees between up and down...

 

R

[Dick Montgomery]

Thanks. Further research indicates that I'd be ok by putting them in the "drooping" position while on the ground.That's what I'm going with.

[TimDarrah]

Dick,

 

When the engines are off, they can sit in either position, up or down, mostly up. When the engines are first being started, they raise up in order to get max airflow down the tubes. After it's up and running on it's own, they drop down. It's cool as you can hear the JFS (jet fuel starter) kick off and the intake drops down, that way the crew chiefs know when it's safe to get under the aircraft to pull pins, etc.

 

Hop this helps

 

Timmy

[Dick Montgomery]

Yep. That matches with what I've found elsewhere....up or down....since the kit presents two sets of air intakes I'll test fit them and use the set that fits better. Thanks!

[PeteJ]

FYI- Almost all jets have some method of controlling the inlets as the speed varies. This is because axial flow compressors are very sensitive to inlet pressures and more importantly the shock waves that develope down the intake tunnel even at much slower speeds. This is a very complex problem because angle of attack, inlet velocity, temperature and density altitude all play part in the equation. The air can not be going supersonic when it reaches the turbines or you will get compressor stall which can be deadly on asymetric thrust engines at high speed. E.G. the SR 71. Later versions had automatic idle of the opposite engine if the other side should fail. More than one Black bird was lost at high mach when an engine failed. Believe it or not, the early versions of the YF-12 had the pilots adjust the spike manually. This make flying one a bit like a monkey doing a football in a phonebooth. Sled drivers words, not mine.

 

There have been a number of methods of controling inlet pressures, from the earliest moveable spikes(F-104), to internal ramps(F-4), to bleed valves and other flaps and internal geometry. The dooping hoods on the F-15s are computer controlled and independently controlled because a pilot will not always call for the same thrust from both engines( i.e. battle damage). Theoretically any position is possible after shut down. There is very little control going into the system for the inlets at idle. It is only when they come up to speed that they begin to vary the geometry. At shut down, the engine is generally brought up to speed before it is shut down to scavenge the engine of pooled lubricants and fuels before it is shut down, so depending on where the engine is running when the pilot pulls the throttle to cuttoff the inlets could be in just about any position.

 

For modeling, most people would feel it is wrong if they are not symetric though it would not be. This is just like the high camber on front tires of a current F 1 car. Right but not pleasant to the eye.

[Dick Montgomery]

Well then, that certainly gives me the ability to justify any alignment mishaps that might occur.....thanks!