Aerodymamics...

Originally posted by Canada_EH
You ever read throught the FTGU Kilcoo?

I'll see what I can pry up from mine also. We use the same thing in Alberta here. I've got one version from 2002 and one from '78 (my father's old copy ). Hopefully the illustrations will give a bit of help to explain these phenomena.

Encountering Mach Number Difficulties

Extract from FLY

Until now, you've been flying straight-wing airplanes in a relatively low speed range, one in which airspeed was the primary factor. In the jet, however, you'll be flying at a significant fraction of the speed of sound, called Mach 1 (after Austrian physicist Ernst Mach, who dod much of early reasearch into high-speed fluid and gas flow). As you approach Mach 1, the behavior of the air changes: it becomes more like water, an incompressible fluid, than a gas. Since air can't readily move faster than the speed at which sound propagates through it, in a sense it "can't get out of its own way" fast enough. Instead of flowing smoothly over a wing, it "piles up" to form of shock waves.

The speed at which this occurs, for a given airfoil, is called its critical mach number, and it applies to the speed at which the air moves chordwise, straight from the leading ege to the trailing edge. If the wing is swept, so the air moves over it obliquely, the speed of the chordwise component is reduced, so the airplane can fly faster without encoutnering mach number difficulties.

-My first question is simple: Is there any truth to this extract or does it go in the same drawer with the longer path explanation?

Since air can't readily move faster than the speed at which sound propagates through it, in a sense it "can't get out of its own way" fast enough. Instead of flowing smoothly over a wing, it "piles up" to form of shock waves.

-I got a little confused reading this. How come air can't move faster than sound?

-You have swept wings to encounter Mach number difficulties, but what does the swept wing have to do with this and what are the difficulties caused by Mach 1 flight?

I believe what your talking about here is compressablity. Many of the 1945 fighters like the later model spits P-38 and mustang ran in this problem. When the aricraft was in a high speed dive the air would build up on the wings and stop flowing over it and instead build up on the leading endge. The even more dangerous part of this is that it would stop the control surfaces from responding due to the disrupted airflow and the plane would not be able to pull out of the dive until it hit the heaver air masses at lower alt which was sometimes too late. The stress of the pull out too was huge at almost 9gs in these planes could lose wings or canopys would break off the plane.

I actually found that what I said wasn't 100% true the compressability can be effected by the shape of the wing. The P-38 with thicker wings will reach what is called critical mach speed earlier then the spit which has thiner wings. The critical mach number is when compressability is reached to a factor that it effects the aerofoil. Here is a quote.

The actual speed of critical mach varies from wing to wing. In general a thicker wing will have a lower critical mach, because a thicker wing accelerates the airflow more than a thinner one. For instance, the fairly thick wing on the P-38 Lightning led to a critical mach of about .69 Mach, a speed it could reach with some ease in dives, which lead to a number of crashes. The much thinner elliptical wing on the Supermarine Spitfire, a shape fortuitously chosen to accommodate eight guns within as thin a section as possible, avoided this problem, and had a critical mach of about .89 Mach.

Figher Master FIN

Here is a link to a reasonable explanation of what Critical Mach Number means...
www.aerodyn.org...

As the air passes over the wing it is accelerated due to the airfoil shape, so on any wing shock waves begin to form at the point where the air is moving fastest when the air speed at that point reaches Mach 1 (slightly below Mach 1 actually as the air compresses), so that speed is less than the speed of the aircraft as a whole. The actual speed difference is determined by the airfoil shape - generally speaking the thickness of the wing - actually the ratio of thickness to chord. That's why different aircraft may have a different critical mach number.

Therefore, to delay the drag rise and the onset of shock waves, one must reduce the thickness to chord ratio - that means making the wing thinner. Obviously there are problems with that because it is structurally more difficult to make a thinner wing as strong as it need to be, and a thinner wing makes for less space to accommodate undercarriage, fuel tanks or weapons within it.

The swept wing concept is simply this. By turning a thicker wing to an angle to the airflow, the air has to travel further over the wing surface (in the direction of flight) presenting an apparent lower thickness to chord ratio to the airflow, even though structurally the t/c ratio hasn't changed. Of itself it doesn't solve the problem of transonic drag rise or loss of control effectiveness, but it does increase the speed at which these problems occur.

However, this also induces part of the airflow to travel spanwise to the tip (or to the fuselage with a forward swept wing) which creates handling problems, especially at low speed and high G (called tip stalling).

The early answers to the spanwise flow problem included wing fences, leading edge cuts, and sawtooth leading edges (the latter two creating vortices to help keep the airflow in line with the direction of flight).

The Delta wing solves the structural problem because of the very long root chord, allowing a wing with a low thickness to chord ratio to be quite thick at the root.

I hope that helps.

The Winged Wombat


[edit on 21/6/07 by The Winged Wombat]

Who can teach me that aerodynamical charater of inlet on F-105 and F9U-1?

Thank you Wombat! I got it

I can't really give you an educated guess emile, so I won't even try. But if I'd have to I would say it somehow "captures" air better. More air is sucked in the engine. More air ---> More oxygen ---> Better burn ---> More thrust

If Kilco or anybody out there reads this can you please explain How bound vortex is created? If it's not possibly to do without having great knowledge of something, it's ok I understand

Originally posted by emile
Who can teach me that aerodynamical charater of inlet on F-105 and F9U-1?

Not an awful lot I can really say apart from its all about shockwave control into the duct itself.

There are a few ways of dealing with it, the normal shock inlet (a la F-16), the variable ramped approach of the F-15 (which uses a number of ramped shocks into the duct, ending in a normal shockwave prior to subsonic flow) , or the fixed inlet & bled approach of the F-22.

The F-105 used a variable ramp approach:



You can see the guide groove for the variable ramp. The F-105 is unusual in that the ramp is located on the outside of the duct.

[edit on 17/8/07 by kilcoo316]

Originally posted by Figher Master FIN
If Kilcoo or anybody out there reads this can you please explain How bound vortex is created? If it's not possibly to do without having great knowledge of something, it's ok I understand

I kinda glossed over this back on pg 5.

When a wing first starts moving, the air is is uniform and symmetrical up and downstream of the wing (top + bottom symmetry). However, due to the flowpath forced around the wing, there has to be some pretty fancy manouvering at the trailing edge to keep this uniform/symmetrical flow.

This is physically impossible, hence a vortex quickly rolls up and detaches, moving off downstream.




Thus, the starting vortex is formed. After this happens, the Kutta condition is enforced at the trailing edge -> flow from above and below the wing rejoin at the trailing edge, and there is no abrupt change of direction.

*The Kutta condition is more a theoretical thing for inviscid flow, but its a useful assumption.


Anyway, starting vortex is made - from Newton, equal and opposite reaction blah blah blah, hence starting vortex needs a reacting bound vortex.

Allright, I think i get it. So is it fair to make an assumtion that the use of a Joukowski airfoil make the starting vortex more powerful, hence (according to Newtons 3rd law) the whole bound vortex system in the leading edge of the airfoil will be more effective. This is why the Joukowski airfoil creates more lift than a symmetrical airfoil?

Originally posted by Figher Master FIN
Allright, I think i get it. So is it fair to make an assumption that the use of a Joukowski airfoil make the starting vortex more powerful, hence (according to Newtons 3rd law) the whole bound vortex system in the leading edge of the airfoil will be more effective. This is why the Joukowski airfoil creates more lift than a symmetrical airfoil?

A Joukowski aerofoil can be symmetrical.


A symmetrical aerofoil will not have a starting vortex at zero angle of attack as the top and bottom flows will effectively cancel each other out.


Any cambered aerofoil will have a starting vortex of some description. Sticking a gurney flap on the end (see racing cars) will further increase the strength of both the starting and bound vortices - but at a cost in drag rise.

Thanks for the info

Now that we have covered subsonic lift how about moving to supersonic lift. I believe you said the creation of supersonic lift is totally different from subsonic lift. Wasn't it called Newtonian lift?

Now, what I understand of that is that air particles move so fast and hit the wing at an angle, and (according to Newtons 3rd law, every action has an equal and opposite action) push it in the air. I am not sure if this is correct because you don't really assume that the air particles interfere with eachother? (Are my links rubbish?). Maybe the particles change their character as they move faster.

Originally posted by Figher Master FIN
Thanks for the info

Now that we have covered subsonic lift how about moving to supersonic lift. I believe you said the creation of supersonic lift is totally different from subsonic lift. Wasn't it called Newtonian lift?

Newtonian lift is hypersonic.


Supersonic (and hypersonic too for that matter) is generated through pressure differentials top and bottom caused by expansion waves or compressive shockwaves in the flow.

A piccie probably explains it best.




Above the wing the effective flow area (from the wing surface to "infinity") is expanding = faster flow = lower pressure.

Below the wing the flow area is reducing (from the wing surface to ground), thus slower flow = higher pressure.

I need to digest this a little. It's so different from subsonic lift so I'll get back to you after a while .

Today I was wondering about something rather pointless. According to the Bernoulli prinicple a gas or a liquid has a higher pressure if it moves slower.

The first question is: Wouldn't pressure be highest when the air doesn't move at all? Nothing can move less than actually be in a state where it doesn't move at all .

The second question: You have a ball, which is in rest on a table. You start blowing slowly at the ball, but it doesn't move. When you blow harder it starts moving. Why? Bernoulli states that a gas/liquid has a larger pressure (thus force) when it moves slower.

I am aware that the both questions resemble eachother pretty much, thanks in advande.


[edit on 23-8-2007 by Figher Master FIN]

Originally posted by Figher Master FIN
The first question is: Wouldn't pressure be highest when the air doesn't move at all? Nothing can move less than actually be in a state where it doesn't move at all

.

Correct, kinda, pressure is highest whenever the air is not moving at all.

Originally posted by Figher Master FIN
The second question: You have a ball, which is in rest on a table. You start blowing slowly at the ball, but it doesn't move. When you blow harder it starts moving. Why? Bernoulli states that a gas/liquid has a larger pressure (thus force) when it moves slower.

Well, due to friction it won't start moving until you blow hard enough.

As for pressures, the reason for it moving at all, and in the direction you are blowing is because of stagnation points and pressure distributions.

www.princeton.edu...

See the section on Stagnation pressure and dynamic pressure.

Greetings:

Here is a complicated question share with us.

We've known that shock wave caused by upside board of F-22's inlet covering inlet jaw lip make a low velocity of air flow and high air press inside inlet antrum, which means this atitude of air flow just in what jet engine needed in supersonic working. Otherwisem, the shockwave also lead to a downward flow, which means a little bite additional lift.

That's is a sort of inlet we call it ride wave inlet

Now, the question our faced is why the DSI also be called ride wave inlet?
How the air flow caused by bump cover the inlet, as the shape of shock wave caused by bump is subulate wherease the inlet is multi-side shape?

sorry, correct a wrong spelling word "situation".
the situation of air flow.....

Sorry emile, I don't understand your question...?


edit: If your saying the JSF inlet will produce lift, I think its symmetric (top to bottom) so at 0 deg AoA will not produce lift.

If it is asymmetric, it might.

[edit on 25/11/07 by kilcoo316]

Sorry, Even the bump on F-35 obviously is not symmetrical on top and bottom.

The key point is how the shock wave caused by this Bump cover the inlet.