Forum: General F-22A Raptor forum

I have been reading quite a bit on the F-22 lately but as expected there is little aerodynamic information out there and I had a question.

Does anyone know why there is a 3-4 inch gap between the fuselage and the air intakes? Is this for some sort of cooling? Or is it something to do with lift? Perhaps it speeds up the air going under the plane?

F-16.net Sponsor

Jet engines can't breathe supersonic air. The separation insures that the - air meets an edge, which generates shock waves and slows the air to subsonic speeds.

exorcet is right, it's to avoid the boundary layer air moving along the fuselage. Many, many aircraft have this same separation (F-4, F-15, F-16), it just manifests itself in different ways.

exorcet wrote:

Jet engines can't breathe supersonic air. The separation insures that the - air meets an edge, which generates shock waves and slows the air to subsonic speeds.

Nope. Those gaps are for ACFC cooling and RAM air cooling and they're more like 4 or 5 inches wide. Aero into the motors is accomplished in a totally different manner.

I'm sure they could be multipurpose. As Prinz said, it's a very common method of preventing supersonic engine flow.

Hmmm... we need another expert. Boundary layer separation was my understanding.

It could be multipurpose. In other aircraft there are similar gaps to avoid the boundary layer which is turbulent.

You're right, I have no idea what I'm talking about. It's not like I work on the thing every single friggin day.

RaptorDCTR,
You may work on the thing every single friggin day, but you just may not understand all the finer details of the design. The air diverted by the gap may well be used for cooling as you say (many airplanes do this including F-16), but shingen, Prinz Eugn, and exorcet are correct in saying the boundary layer air is removed to improve engine inlet performance. Air flowing along the forward fuselage (called boundary layer) slows down and becomes tubulent, neither of which is good for best engine performance. The F-35 does not use a splitter to remove the boundary air. The bump just forward of the inlet re-energizes the boundary layer air.

I was speaking from my knowledge that most high-speed aircraft have a distinct separation between the inlet and the fuselage, and that the explanation I remember is boundary layer air. This site even names the F-22 as an example: http://www.airspacemag.com/military-avi ... amp;page=2

The reason I'm still skeptical is that the F-111 has an air intake for something (can't remember what) between the inlet and the fuselage, but that's not the reason for the gap. In fact, they jacked up the separation every iteration (Triple Plow I to Triple Plow II, for example) because there was so much trouble with airflow problems and the infamous TF30. Ex:http://www.f-111.net/models/inlets/IN009a.jpg

EDIT: johnwill beat me to it

The 111 is an extreme example of a wide boundary layer. As Prinz Eugn says, the splitter was moved outboard several times. The duct visible between the fuselage and the inlet in the photo is also for cooling air.

In an attempt to reduce drag, the inlet was made very short, resulting in a long run down the forward fuselage for boundary air. So the layer became very thick and turbulent. The TF-30 was quite intolerant of uneven pressure distribution and would stall at certain conditions. That behavior continued even on the F-14, which had straight inlets, widely separated from the fuselage.

It sounds like the designed the gap to lessen the effect of the boundary air but they found they could cool the fuel with that waste air. Brilliant engineering. A lot like Formula One in many ways. I know very little about this subject but the more I login here the more I learn.

Thanks for the info guys!

I'll jump in and confirm boundary layer control.

Primary purpose for the 'gap' is boundary layer control for the inlets; secondary purpose is ram air for ECS or other uses in the aircraft.

The gap would be considered a "inlet diverter" and the "ram air inlets" are contained within the inlet diverter 'gap' to utilize the pressurized 'dirty' air that isn't good for jet engine consumption... See references below;

http://www.f22-raptor.com/team/index.html wrote:

http://www.f22-raptor.com/team/index.html
Lockheed Martin Aeronautics Company (Palmdale, CA)
Responsible for developing and constructing the low-observable edges of the F-22 including the flaperons, ailerons, leading edge flaps, wing tips, wing stubs, aftbooms, vertical stubs, vertical leading edge, inlet diverter lips, rudders, CNI antennas and the Integrated Forebody. Also, Palmdale is one of two locations responsible for F-22 modernization work.

http://www.fas.org/man/dod-101/sys/ac/docs/f-22-emd-paper.htm wrote:

http://www.fas.org/man/dod-101/sys/ac/d ... -paper.htm
During first flight, Paul got an AIR COOLING ICAW... ...What really happened was that at this test condition, 12 degrees AOA, insufficient airflow was available through the inlet diverter duct for cooling the (ECS) primary ram air heat exchangers. This results in hotter than normal air to the air cycle machine. As a result of this problem, one of the modifications made to the aircraft during the down-time was a modified inlet diverter duct Subsequent testing has shown that this modification was successful. The figure shows the air flow on first flight and the airflow with the diverter mod at the same condition. Mass flow is improved with a subsequent increased cooling effect and no AIR COOLING ICAW - at least not for this reason.

Also read about inlets in general this article from Air & Space Magazine. (Not too technical to read.)

http://www.airspacemag.com/military-aviation/Supersonic_Inlets.html wrote:

http://www.airspacemag.com/military-avi ... nlets.html
(on page 2)
With a top speed of Mach 1.6, the Lockheed Martin F-35 Joint Strike Fighter has an inlet design that is far simpler than that of the Mach 3-plus SR-71; the single-engine JSF inlet cannot vary its geometry. The F-35’s engineers could get away with a less complicated design because at vehicle speeds up to about Mach 2, the shape of the inlet itself can slow down much of the supersonic air before it enters the inlet. The JSF inlet is, however, a breakthrough design: It has no diverters. Traditional fighter inlets, such as those found on the F/A-18 and F-22, have slots, slats, and moving parts to divert or channel airflow. The F-15 inlet has ramps and doors that alter its external and internal shape to adjust airflow as needed.

Many other currently operational fighters also have boundary layer diverters. Air that clings to the surface of an aircraft in flight is known as boundary layer air, and it tends to cause turbulence in the air flowing into the engine, especially when it interacts with shock waves. Inlet designers try to keep out as much boundary layer air as possible, frequently positioning the inlet several inches away from the surface of the fuselage and its boundary layer air and employing a duct system to whisk the undesirable air away. (The SR-71 inlet rids itself of boundary layer air by sucking it in through slots on the spike and passing it through ducts that exit the nacelle.)

The F-35 inlet, however, is positioned flush against the fuselage, and just in front of the inlet opening is a raised surface, or bump, that pushes much of the boundary layer air off to the sides and away from the inlet. The bump serves another purpose: During supersonic flight, it compresses and slows the air passing over it into an oblique shock wave. The air is still moving supersonically, however, and it is slowed down to subsonic speeds after passing through a normal shock wave that forms at the mouth of the inlet. The simplicity of the JSF design makes for an inlet that requires less maintenance, reduces aircraft weight by 300 pounds, and costs $500,000 less than a traditional fighter inlet.

For VERY technical explanations of what we're discussing in 'boundary layer control' read this from NASA: http://ntrs.nasa.gov/archive/nasa/casi. ... 036970.pdf
(there are pictures in the last few pages)

So what you see as a 'gap' on the F-22 is the same as the 'gap' between the cockpit and upper inlet lip of the F-16.

Keep 'em flyin' Thumb

TEG

Quote:

Traditional fighter inlets, such as those found on the F/A-18 and F-22, have slots, slats, and moving parts to divert or channel airflow.

Maybe I don't understand it correctly but I've always thought that F-22's inlet has fixed geometry and it's not equipped with any moving elements which control airflow (like ramps).

Regards

You're reading into it too far; it says 'traditional fighter inlets, such as those found on...' and gives a few examples. The F/A-18 has fixed inlets too, right?

The F-22 may have a fixed inlet, but has 'slots and slats' to divert airflow, where other 'traditional fighters' (not mentioned specifically on their 2 whole aircraft list) have 'moving parts'

It wasn't a very technical explanation; nor did it give specifics on what types or methods each inlet uses.

Doh

TEG

Forum: General F-22A Raptor forum

Gap Near Air Intakes