# Skidding, Compressiblity, and Mushing



## GatorDude (Feb 16, 2010)

I've been reading a couple of books on the P-40 (Donald Lopez's _Into the Teeth of the Tiger_ and periscopefilm's reprint of the_ P-40 Warhawk Pilots Flight Operating Manual_) and I've run into some new terms: Skidding, Compressibility and Mushing.

Apparently aircraft can skid in a turn, encounter compressibility and mush at high altitude. Can anyone explain what these terms mean? Thanks!


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## B-17engineer (Feb 16, 2010)

Skid has to do something with the rudder I think...the other two are beyond my most wild guess


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## Markus (Feb 16, 2010)

Compressibility | Flight Journal | Find Articles at BNET


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## Njaco (Feb 16, 2010)

I can assume - as I'm no expert - that skidding would refer to when a plane goes sideways on a level which I would assime decreases the airflow significantly (compresability) so that it negatively affects the flying characteristics of the plane and the bleed off of speed is drastic (mushing). I guess.


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## billswagger (Feb 16, 2010)

you might google the terms and get a better explanation but i will offer my thoughts.

Skidding / slipping : The plane is put into a skid with the rudder and orients the nose of the plane away from the direction of travel. Its used as a way to bleed off speed but it also can change the flight path on the horizontal plane with out having to bank the wings. Pilots also skid to place their shots while shooting. 

compressibility: In a nutshell. as the speed of the plane increases the air moving over certain parts of the plane begins to travel in the supersonic realm. Compressibility is what occurs when air is compressed from the shock wave that forms over these parts of the aircraft. Compressibility can also effect control responses depending on the severity of the shock wave and the disruption of the air flow. Its more trouble for an aircraft at high speeds particularly at higher altitudes.

mushing: A plane usually mushes in a dive pull out or sudden changes in elevator deflection. I think it may stall the wing as well, but doesn't put the plane in a spin. If you can imagine putting a plane in a dive and pulling out to get your nose horizontal yet see the aircraft continue to drop another 200-300 meters. Some planes have better mushing characteristics than others. A plane that really flattens out while mushing can black out the pilot or even put the plane into a spin. 



Bill


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## renrich (Feb 16, 2010)

Skidding is when an AC is not in a coordinated turn which means that the ailerons are trying to make the airplane bank into a turn but aileron drag is forcing the nose of the plane to go opposite from the intended turn. This can be countered with the rudder.

Conpressibility is when an AC is approaching the speed of sound and the air in front of the wing cannot get out of the way and flow smoothly over the wing'

Mushing is when the wing is nearing the stall and is not providing lift.


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## vikingBerserker (Feb 16, 2010)

I'd never heard the term "muching" in regards to aircraft, thanks for the education fellas.


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## Colin1 (Feb 16, 2010)

Mushing

Typhoons were notorious for it.
As I understand it, pulling out from a dive eg a strafing pass, you point the aircraft away from the target and into your exit route. The Typhoon being an ungainly beast, will continue into the direction of the target even though he's pointing in the direction of his exit route. This will continue until everything 'bites' and normal flight service is resumed.

Sometimes, all too often with the Typhoon, it wasn't resumed...


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## timshatz (Feb 17, 2010)

Haven't run into compressability or mushing much. Those are problems for high performance birds with big engines and big weight. 

Skidding is still something that happens to pilots now. As Naj and Bill noted, it is when the plane slides (or skids) in a turn. Usually the result of an uncoordinated turn (cranking the stick over to turn but not using the rudders or using the rudders too much). 

When you play video games with a stick attached to your computer (like in the IL2 series) and you want to see what a skid is like, either turn off the stick/rudder link and try playing/flying that way or just crank in a ton of rudder. In both cases, you'll be all over the sky in a skid.

One other point about skids, sometimes they are useful. If you are setting up to land (and this is in flying privately) and are a little high (or even way high), the best way to get down into the groove for a landing is just to do a "forward slip". It is essentially a skid, nose down. You kick the rudder (usually the left one in my experience). That skids the airplane sideways. You'll dump ailtitude down to where you want to be. Then, take your foot off the rudder and it straightens out. The trick is to keep flying towards the runway while you are pointed off to the right.


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## drgondog (Feb 17, 2010)

The definitions are pretty much dead on.

Skidding is simply applying rudder input without using ailerons. If you ski it is the difference between a stem christie and a carved turn.

Mushing is a continued descent with nose in horizontal or even positive angle of attack, but a condition in which the force of gravity contines to exceed the lift on the aircraft.

Compressibility is local transonic flow, usually on the wing, but could also occur on a canopy or other surface on the aircraft.

It is characterized by an increase local density and velocity as the flow over the wing approaching the velocity of sound - and in those days associated with a steep increase in drag rise also. 

When the shock wave first appears, it forms normally on the region of the wing with highest velocity over the airfoil.. when the a/c goes completely supersonic, the shock wave moves to the leading edge, the velocity and pressure is different aft of the shock wave (from freestream) until it reaches the trailing edge where another shock wave forms as the velocity transitions back to free stream speed and pressure - and this transition is what causes the 'sonic boom'


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## renrich (Feb 17, 2010)

Bill, I thought about using the ski analogy in the skid definition but was afraid not many knew about skiing. It is a good analogy though because a carved turn allows one to not only control the track but also speed whereas a skidding turn controls neither very well. During WW2, during a guns attack, if the attacker is in a turn but is also skidding, the guns will not be pointing where he thinks they are. I read about some RAF pilot, can't remeber which, who always checked his turn and bank indicator before firing to make sure he did not need a touch of rudder to straighten up.


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## Colin1 (Feb 17, 2010)

renrich said:


> ... I read about some RAF pilot, can't remember which, who always checked his turn and bank indicator before firing to make sure he did not need a touch of rudder to straighten up.


Robert Stanford-Tuck


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## timshatz (Feb 17, 2010)

Colin1 said:


> Robert Stanford-Tuck



Talk about your great shots...


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## renrich (Feb 17, 2010)

Many thanks, Tuck sounds right. I have read about so many Johnny Johnson, Tuck, Deere, Bader, Townsend, Malan, I can't keep them straight.


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## drgondog (Feb 17, 2010)

My dad made me train and solo in a glider to understand unpowered skid and co-ordinated turns... it does make a difference to understand both as well as sideslips and crabbing - all closely connected.


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## MikeGazdik (Feb 17, 2010)

An ugly, but I think accurate demonstration of "mushing". If you search Youtube you will find an older video of a Spitfire (Spitfire Loop Crash) performing a loop at too low of an altitude. Its a sad video with obviously fatal results, but you can see the plane is in a horizontal when it hits.


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## billswagger (Feb 17, 2010)

drgondog said:


> The definitions are pretty much dead on.
> 
> It is characterized by an increase local density and velocity as the flow over the wing approaching the velocity of sound - and in those days associated with a steep increase in drag rise also.
> 
> When the shock wave first appears, it forms normally on the region of the wing with highest velocity over the airfoil.. when the a/c goes completely supersonic, the shock wave moves to the leading edge, the velocity and pressure is different aft of the shock wave (from freestream) until it reaches the trailing edge where another shock wave forms as the velocity transitions back to free stream speed and pressure - and this transition is what causes the 'sonic boom'



Once the A/c has exceeded the speed of sound, do the drag ratios drop back down?

When the plane exceeds the speed of sound, does the pilot feel an increase in acceleration if throttle is kept in the same position?




Bill


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## drgondog (Feb 19, 2010)

billswagger said:


> Once the A/c has exceeded the speed of sound, do the drag ratios drop back down?
> 
> *No - you move from drag rise due to compressibility to Wave drag based on a lot of factors. There is another phenomena in which a Critical Mach CL point is attained in which the Lift coefficient peaks, then drops after the Mach number increases.
> 
> ...



No, for the reasons given above, although you will not experience a deceleration


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