F-15 Air intakes...question.

[NY3D]

Ever since I bought lock on I was fascinated with Russian aircrafts,first time flying these bad boys,in fact I enjoyed it so much that when i tried to land the F15 yesterday I crashed...BOOM!!!! I am rusty when it comes to my own countries warbirds shame,shame,shame:doh: Anyway I wanted to ask a couple of questiones regarding the F15.I noticed that the air intakes tilt,sometimes they are straight and sometimes in an angle(downwards)What is this for??? I never did find this out.Thanx in advance.

[Poopskadoop]

I believe it increases intake mass flow rate.

[Pilotasso]

No, they just align with the direction of planes motion (= angle of attack) so that the air that reaches the engines is chaneled with the minimum of flow disturbances. Ultimatly these could cause the engine to stall or loose efficiency.

[Poopskadoop]

Wouldn't it also increase mass flow rate? No? Well, I'm probably wrong.

[Pilotasso]

Depends how you look at it...from my point of view it helps maintain airflow. From the point of view compared to untilted position with a given angle of attack, it does increase the airflow. I think the first point of view is more correct because in reality an airflow thats deflected cannot be bigger than an airflow that goes straight (when AOA=0) in the same duct. And the objective is to prevent shortage (and dirturbance), not to provide excess of airflow to the engine.

[GGTharos]

At high altitudes/AoA, and high mach (in particular supersonic) the intakes move so as to position the shock ramps in a manner that prevents airflow disruption due to these condition. This increases mass airflow over a stationary inlet which is designed to provide good airflow only under certain conditions - this isn't modeled in LO, unfortunately. If it was, the F-15 would have much more power at altitude.

[Pilotasso]

Wait a minute... are we talking about the same thing here?

 

Tilt intake mechanism and shock ramps are not the same thing...are they?

 

I was the impression shock ramps worked inside the duct to change the duct cross section area in the supersonic transition (because a convergent-divergent duct works the oposite way at supersonic speeds)

[Poopskadoop]

Depends how you look at it...from my point of view it helps maintain airflow. From the point of view compared to untilted position with a given angle of attack, it does increase the airflow. I think the first point of view is more correct because in reality an airflow thats deflected cannot be bigger than an airflow that goes straight (when AOA=0) in the same duct. And the objective is to prevent shortage (and dirturbance), not to provide excess of airflow to the engine.

 

But isn't it also used at high altitudes, regardless of the AoA? I always thought this was to provide a larger intake for the purpose of ensuring enough oxygen gets to the engines, considering the air is much thinner up there.

 

Well... err... Am I at least 10% correct when saying it increases mass flow rate? :D

[Pilotasso]

I maybe forgeting that shock ramps mechanism is associated with the tilt mechanism, I thought the 2 were independent and I answered on the context of the first post (i.e normal subsonic manuevering flight). SUpersonic at altitude is another ball game alltoguether, Let GG reply.

[CrazyAggie05]

The moving duct is another way of modifying the shockwave just like ramps do inside a duct. It is going to change where the shock "hits" the inside of a duct. You do NOT want the engine ingesting a shockwave :)

[GGTharos]

It's basically all part of the same system Pilotasso, and its results are increased power at altitude and mach.

[F l a n k e r]

I think the F-15 is the only fighter aircraft that use this "all-moving air intakes" ducts. The inner raps instead are similar to many more planes, like the F-14 and the Concorde for example. I found this, hope it can be of interest:

http://www.concordesst.com/powerplant.html#

[Vekkinho]

This increases mass airflow over a stationary inlet which is designed to provide good airflow only under certain conditions - this isn't modeled in LO, unfortunately. If it was, the F-15 would have much more power at altitude.

 

I don't understand this one...

 

...I can't see the relation between animating 3D model with flight modelling/performance. FM is a scripted thing AFAIK. If someone from ED wants to have 50 G capable wardrobe in game it can be modelled without any aerodynamics or animations for better airflow. Are you suggesting that if F-15 had movable intakes animated properly it would perform better in LO?

 

X-plane is the only simulator available that counts aerodynamics for flight modelling. At least that's what I+ve heard - I haven't tried it yet.

[GGTharos]

No, I said that this is the intake's purpose, and that this isn't modeled.

The question was as to the purpose of the intake's motion. The motion is (somewhat) modeled, the purpose is not ;)

 

As for X-Plane, since ALL it does is model aerodynamics of the aircraft you're flying, it can afford to put all the CPU power into that. LO doesn't have that luxury, but the AFM is -STILL- the best combat flight sim FM produced to date for modern jet fighter sims.

In any case, ED could certainly model the effect of the intakes, and even the effect of their failure should they want to do so.

[britgliderpilot]

No, I said that this is the intake's purpose, and that this isn't modeled.

The question was as to the purpose of the intake's motion. The motion is (somewhat) modeled, the purpose is not ;)

 

 

I thought the non-AFM aircraft had a rather simplified version of relating thrust to speed and height - mostly by tables?

 

Not sure why - perhaps I'd got the idea the AFM aircraft were the only ones to calculate thrust on the fly.

 

 

If the first case is true, then depending on tabular format merely requires you to ensure that the thrust values are correct for speed and height where a variable geometry intake is used. Hopefully can be done in a relatively straightforward manner - somewhere I've seen an equation relating pressure recovery over a shockwave for a variable geometry intake.

 

I'd be surprised if it was missed first time, actually - got more info/links on the way the F-15's dealt with at the moment?

[GGTharos]

I don't think its tabular, but maybe I'm wrong. I think the SFM is lower quality in terms of resolution - ie. it takes fewer variables into account, and does fewer things with them. In addition, it is entirely possible that all the Ps curves weren't modeled, rather they took the top Ps curve and interpolated all the way to the bottom, but I don't really know about that.

[Aeroscout]

I dunno that X-plane is really tha great. Lockon is better in my opinion, though you cant fly GA aircraft. and you can't make your own planes...

[britgliderpilot]

I don't think its tabular, but maybe I'm wrong. I think the SFM is lower quality in terms of resolution - ie. it takes fewer variables into account, and does fewer things with them. In addition, it is entirely possible that all the Ps curves weren't modeled, rather they took the top Ps curve and interpolated all the way to the bottom, but I don't really know about that.

 

 

Could be. Be interesting to find out.

Hardly seem to see complaints about the Su27 and MiG at altitude - but that could be down to any number of reasons.

 

Will take a look at the dev notes at some point and see if they offer any insight into the non-AFM aircraft.

 

 

I can't remember if this is the right curve or not - my brain's fried at the moment - but still, for the casual observer, displays the kind of benefit you can get from variable geometry intakes as opposed to fixed geometry . . . .

 

[GGTharos]

There's a difference in altitude performance also. This isn't the right curve (not one you can model from I think, but rather one to check against) but you get the idea from it. The Ps curve is in the dev notes.