The 'Great Debate' - Spitfire vs BF109

[taildraggernut]

Originally Posted by Crumpp

I agree the elevator is going to be hard to control from the buffet moments and disturbed flow off the wings.

Pretty sure the buffet is caused by flow reversal in the stall portion of the wing. As the flow reverses, it creates suction which creates drag, as our drag goes up, so does our lift. Something has to oppose that drag and the airplane will move to a new equilibrium point.

Something doesn't sound right here, the suction you are talking about is lift....how can there be lift if it is stalled....no lift = no induced drag.

[Crumpp]

the suction you are talking about is lift

No, suction is what happens when the boundary layer seperates.

Watch the tufts, they reverse direction at the stall point for the section. That is a vacuum over that part of the wing creating suction. You can see the tips of the wing over the ailerion is still flying nicely while the inboard portion of the wing is stalled.

Buffet comes from a portion of the wing stalling. It does not mean the entire wing is stalled or the airplane is not in controlled flight.

http://www.youtube.com/watch?v=PxqaQLVZFHg

[taildraggernut]

No when the boundary layer separates the center of lift moves aft and flow on the undeside of the wing is allowed to curl up over the trailing edge, this simly causes turbulence behind the wing.

Light buffet comes from the separation but for most aircraft the significant stall buffet is from the turbulent air impinging on the tailplane.

Washout is what prevents the tips of the wing stalling before the inboard.

as our drag goes up, so does our lift

This is the part I'm most curious about, sounds counter intuitive, increased lift increases induced drag, past the stall then form drag increases but I have never heard of an increase in drag inceasing lift?..

[Fenrir]

Originally Posted by TomcatViP

did you read the doc ?
1. graph is stated with "no washout"
2. the test describe a real flight test

I am not professoring Fenhir. I give explanation. Stall description -> theorized phenomena.

The goal is to make more players aware and immune to evangelist of one side or another.

But I cloud hve completed with more: look af the Re along the wing. You'll understand why ell wing are an illusion.

Or with the wing spar.

Geo Wash out is to prevent stall eh yeah and prevent aileron loss of ctrl. But in the case of that wing planform it's a draggy configuration. And drag and lift I was talking about little Jedy.

but buffet is due to the aero washout (the turbulence hitting the elevator) not the geometrical washout on the wing.

If you had take the time to read the doc linked in my post, you'd see this point. I gave plenty of info for the reader to make his own mind and to draw the same conclusion. You'd see, it's an honest behaviour far far away from a post that pick parts of the info and mud it with sarcasms.

Hve fun

Okay tomcat, my bad, you have my apologies. I only scan read the links and didn't get what you actually communicating. It was late and I was tired but still, should have made sure I understood your post before replying. Interesting stuff, now that I've read it properly.

Sorry again for the flippant response.

[Glider]

Originally Posted by Crumpp

In the turn, the violent buffet is a double edged sword. There is no such thing as a free lunch especially in physics. In the NACA measured results, encountering the buffet represents a change in available angle of bank. The airplane goes from 78.5 degrees of bank to 60 degrees of bank in one second.

****5G @ 147.73KIAS:

ROT = 1091*tan(78.5) divided by 147.73 KIAS = 36.2 degrees a second

****2G @ 141.647 KIAS:

ROT = 1091*tan(60) divided by 141.647 = 13.34 degrees a second

As a quick ballpark using IAS to get an idea of the scope of the effect on turn performance, we see the rate of turn drop from 36.2 degrees a second to 13.34 degrees a second. That means our time to complete a 360 degree turn changes from 10 seconds to 27 seconds!!!

As the Operating Notes relate, you do not want to turn any airplane in the buffet. Energy cannot be created or destroyed, all the energy that was being used to achieve an instantaneous performance rate of turn of 36.2 degrees a second from our ballpark went to warn the pilot of an impending stall, taking the aircraft right down well inside its sustainable performance envelope of 13.34 degrees a second.

The Buffet is not a double edged sword. It is easily and immediately recognisable and resolving the issue is equally easy and immediately effective, all you do is ease off the stick until the buffet goes. It is easy to fly on the edge, you 'feel' for the start of the buffet and ease off as applicable, the normal process is to hold it on the edge with gentle movements once you have found the buffet. You don't fly in the buffet, you can fly to the buffet and then its easy to stay outside the vibration

To pull through the buffet would be dangerous, stupid and also difficult to do, the vibration would rattle your teeth and its impossible to miss it. One benefit of using the buffet is that you do this with your head looking out of the cockpit, you don't need to look at any instruments and because it is something you feel it doesn't matter how noisy it is.

I once got caught in a thunder cloud in a strong thermal. Water got into my instruments, the only one I had working was an audio vario that told me by sound if I was climbing and an indication as to how fast I was climbing. To stay in the thermal in the cloud, I had to rely totally on the audio vario and the buffet giving me warning of the stall as I had no idea how fast I was going. By staying on the edge of the stall, flying outside the buffet just feeling for it every now and then, I was able to say in the thermal and ensure that I didn't exceed the Vne.

I say this as it proves that its effective in the most difficult situations

[Crumpp]

The Buffet is not a double edged sword.

Sure it is and you understand you don't fly in it for maximum turn performance.

It is easily and immediately recognisable and resolving the issue is equally easy and immediately effective, all you do is ease off the stick until the buffet goes. It is easy to fly on the edge, you 'feel' for the start of the buffet and ease off as applicable, the normal process is to hold it on the edge with gentle movements once you have found the buffet. You don't fly in the buffet, you can fly to the buffet

Absolute 110% agree. You are absolute right.

It is easy to do in a stable airplane.

To pull through the buffet would be dangerous, stupid and also difficult to do, the vibration would rattle your teeth and its impossible to miss it.

Not really, In fact accelerated stalls are a testable maneuver for a Commercial rating, now.

The amount and energy of the buffet depends on the airplane design too. Some airplanes don't even buffet before the stall. You get just a nibble and its gone!

[Crumpp]

No when the boundary layer separates the center of lift moves aft and flow on the undeside of the wing is allowed to curl up over the trailing edge, this simly causes turbulence behind the wing.

Not sure what you mean here. The aerodynamic center does not change with AoA.

The Center of Pressure moves forward with increased angle of attack and disapates at the stall.

Is that what you mean? The CP moves forward, the AC is stationary and when the CP is gone, down the wing comes about the AC.

Cause otherwise you have it backwards, the CP moves backwards in infinity as we approach zero lift AoA.

One of the reason's why CP is obsolete.

Light buffet comes from the separation but for most aircraft the significant stall buffet is from the turbulent air impinging on the tailplane.

Washout is what prevents the tips of the wing stalling before the inboard.

All true. Some airplanes the turbulence is the predominate factor. As a general rule though, it is the buffet.

In fact, it is really not good design to put the tail in the wings wake. It can lead to an unrecoverable stall condition. That is the big issue with T-tails.

Near the stall incidence, in most airplanes, the tail by design is in clean air.

I have never heard of an increase in drag inceasing lift

Well it is the co-efficients not the forces. In level flight we are lucky our lift force remains constant.

The co-efficients have a direct relationship. So as the airplane's co-efficient of lift increases, the co-efficient of drag increases.

Drag, unlike lift does not remain constant. So when our CL increases lift force stays the same but our drag force goes up.

[gimpy117]

Originally Posted by TomcatViP

@Jamz

For me the spit is more gracious and the 109 more viril (obvious seeing that nose). Just like comparing a dolphin and a shark.

to be honest, I'm used to BnZ tactics from Rise Of flight with the spad and other aircraft, so I tend to not really fly the spit as well as I could. I turn with it too much

[Glider]

Originally Posted by Crumpp

Sure it is and you understand you don't fly in it for maximum turn performance.



Absolute 110% agree. You are absolute right.

It is easy to do in a stable airplane.

Clearly I have no experience in a Spit but I do have some in aerobatic gliders which are close to the edge. The best are borderline unstable as are fighters, it isn't difficult to recover and they also have very light controls. All you have to do is ease off the stick a touch and you will immediatly recover. Its an almost instinctive reaction.

Stable gliders and I assume fighters are hard work, the physical effort involved can be very tiring and that in itself can make flying close to the edge of the envelope difficult. Some of my first aerobatic lessons were done in a Twin Astir and you had to really haul it around, she was too stable. Close to the edge the lighter touch gives you a better 'feel' for what is going on with the aircraft.

[Crumpp]

The best are borderline unstable as are fighters,

It is a complete fallacy that manueverability and stability are linked by an inverse relationship.

"Just statically stable" has nothing in common with "borderline unstable".