# Air resistance and jet engines

1. Apr 13, 2004

### kurious

When a jet aircraft is flying at high speed doesn't the air impacting on the front of the engines compress the air before it gets into the compressor.
And how fast does the combusted fuel leave an engine compared to the speed of airflow at the intake?

2. Apr 13, 2004

### Staff: Mentor

Yes. On a plane like the SR-71, the cone in front of the intake provides the vast majority of the compression. In a pure ramjet (not sure if there ever were any), it provides all of the compression.

Not sure about exhaust velocity - it varies.

3. Apr 14, 2004

### Clausius2

Surely it depends on the intake shape. Usually compressor intake has a diffusor (subsonic flow) that makes possible a pressure increasing before the compression stage.
I have been thinking about this and wondering me what occurs when the sound barrier is traspassed. Don't we need variable intake geometry in this case?. How is it implemented?. I have heard that compressor and turbine stages are not important in supersonic flow due to shock waves in their vanes. So that, does a supersonic jet a variable geometry in it engines?. I am not talking about scramjets (does it works subsonically?) but about combat aircrafts for instance, or the Concorde.

About the exhaust speed, is easy checking it by the momentum equation. The force exerted on the engine structure must be compensed with a variation of momentum flow. You sure be able to do this, and you will see that the force is:

F=(m(air)+m(fuel))*W - m(air)*U

where m's are the mass flow, W is the relative velocity of the gases exhausted, and U is the forward speed of the engine.
Thus, if m(air)/m(fuel)>>>1 then W>>U. By the way, it satisfies the common sense.

4. Apr 14, 2004

### kurious

Would it be possible to eject combusted fuel over the top of a wing and if so would this increase lift? The flow might be turbulent but I understand that vortex generators
can increase lift by speeding up airflow on the top of a wing.

5. Apr 14, 2004

### Clausius2

What do you mean by ejecting the fuel over the top of the wing?. Is the engine fixed at top of the wing?. Please be more explicit in order I could be able to answer you.

6. Apr 14, 2004

### kurious

jet engine mounted on fuselage

If an engine could be mounted on the side of the fuselage slightly above and in front of the wing could it increase lift? Presumably the wing would have to be made of materials resistant to the heat and the shape of the engines and perhaps tailpiece would have to change to keep the ensemble aerodynamic.Such an aircraft would generate more lift at lower speeds - useful for cargo flights on short runways-but then I
suppose a biplane did the same job.

Last edited: Apr 14, 2004
7. Apr 14, 2004

### enigma

Staff Emeritus
For low supersonic speeds, a normal shockwave forms in front of the diffuser dropping the flow velocity subsonic, and then the diffuser slows the flow down further.

For higher supersonic speeds, a nosecone is located out in front of the engine intake which causes oblique shockwaves. These shocks slow down the flow to low supersonic numbers with a lower total pressure drop than a normal shock. There is then a weak normal shock right before the engine intake bringing the flow subsonic, and the diffuser slows the flow down further.

For scramjets, the nose of the vehicle causes an oblique shock which touches the low end of the engine intake, so the entire intake rides behind the shock. There are no normal shocks, and the flow is supersonic throughout the combustion chamber.

8. Apr 14, 2004

### enigma

Staff Emeritus
This isn't my area of expertise, but I'll make a guess:

When the fuel is burned, it increases the total temperature of the flow (obviously). That temperature increase will cause the pressure to rise as well.

AFAIK, vortex generators increase lift because they force a transition to turbulent flow which pushes flow seperation farther back along the wing.

Using an engine on the top will cause the temperature to be much higher which would need much more resistant materials, which of course weigh more.

9. Apr 14, 2004

### Staff: Mentor

I did once read (Popular Science probably) of an idea to spray fue over the wings - but for thrust, not lift. The article noted that the passengers might object...

Also, one iteration of the aerospace plane used the underbelly of the fuselage as an inake and exhaust ramp.

10. May 9, 2004

### jimbot

Engine intake geometry

In the attached picture, you can see how the F-4 Phantom is able to change the intake geometry. The verticle flaps along side the nose of the aircraft can move in and out, thus changing the amount of air entering the engine. On the SR-71, the cones in front of the engine intake move forward and back, thus achieving the same thing.

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11. Jun 8, 2004

### Owen

The principal of using jet exhaust over the wings is used on the C17 cargo aircraft. the engines are mounted under the wings. but when full flap is deployed there are slots in the leading edges of the flaps specifically designed to capture the engine exaust and produce lift over the flaps. This enables very low operating speeds allowing for good STOL performance from a 4 engined jet

12. Jun 8, 2004

### Staff: Mentor

Separate issue (used for lift, not propulsion), but still a very important innovation.

13. Jul 4, 2004

### fulcrum

Yes. I remember Bill Gunston writing in one of his books that the SR-71 produces only 17% of is total thrust because of the engines alone at max speed (M2.8+ or ~M3). The rest is because of the effect that the inlet has on the air coming in.

As for the question on placing the engine on top of the wing: the effect is known as the Koandau effect (not sure about how it is spelt, but presumably after the person who discovered the effect). And the Boeing YC-14 uses the same. As a matter of fact, the principle here is easily understood if you consider how a blown flap provides much more lift than an ordinary split or fowler flap.

Intake geometry is a very widely researched subject I guess. If I remember right, General Dynamics worked for two years in their windtunnel just to perfect the asymmetric inlet lip of the F-16! However, simple parallepiped intakes like on the MiG-25 or the XB-70 Valkyrie have always impressed with their simple efficiency...

Regards,
Fulcrum
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