Compressibility in the works?

[Kieren]

One issue that no one has directly hit is control reversal. I seem to recall reading accounts where pilots had to trim opposite controls to recover from a high-speed dive?

WRT the 109 (as per funked's description) the whole aircraft doesn't have to fly transonic, only the airflow over a particular component of the aircraft needs to be transonic. One possible difference between compression and plain ol' heavy control surfaces might be control reversal.

[bloom25]

The 109s funny behavior above about 450 mph is not due to compression.  Rather the speed of the air passing over the control surfaces caused a normal force on them large enough that the pilot could not overcome them.  Remember the 109 cockpit was TINY compared to planes like the p51.  As a result the pilot wasn't able to get much leverage on the stick in order to move it.  (Remember a 109 is about 1/3 the size of a p51.)

Compression is an entirely different phenomenon due to shock waves forming on parts of an aircraft exceeding the speed of sound.  These shock waves could interupt the normal flow of air over the control surfaces.  Compression effects would also be different for different airplanes.  Someone above asked what the reasoning behind swept wings is.  Basically a swept wing allows you to keep a relatively large wing area, while at the same time reducing the span of the wing to keep it out of the shock wave coming off the nose of the aircraft.  Perhaps the best way to understand this is to picture an aircraft with long straight wings and a pointy nose.  As the airflow over the nose exceeds the speed of sound a shock wave is produced that looks like a ">" off the nose.  (This shock wave is what you hear as a sonic boom on the ground.)  If your wings were long enough to intersect a part of this shock wave airflow over the wing would be drastically altered.  (Not to mention the increase in drag.)

Propellors are also negatively effected by compressibility.  As the length of the propellor blades increase, so to does the speed of the blade tip at a given rpm.  (Picture a record:  Even though the record is spinning at a constant speed, its outer edge is moving much faster than its center.)  (Interesting fact:  The sound that a helicopter makes is due to the blade tips moving faster than the speed of sound.  In effect you are hearing a series of sonic booms.)

Please feel free to add to or correct me here, as many of you are far more experienced in this area than I.



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bloom25
THUNDERBIRDS

[funked]

Great stuff Bloom.

Santa, I have read some stories that the K-4 could get into compressibility at high altitudes in level flight!

Basically, bad things happen when you put a 1945 engine on a 1935 airframe.  


[This message has been edited by funked (edited 05-06-2000).]

[StSanta]

mm, I know that it isn't compressibility but rather the flow of air over tiny control surfaces that make the 109G10 hard to control.

Did some checking; apparently the 109G10 can go up to 600kts (303m/s) and still have some control, through use of trim.

Due to the way air flows over a moving body, it should be possible for the air to have a larger speed in localized areas, meaning the that G10 would possibly be susceptible. But I wonder; can the difference be as large as 40m/s? I honestly have no idea, so please enlighten me.



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StSanta
II/JG2

[Sharky]

Citabra,

Now way you get compressibility until I get prop drag  

Can you imagine the lawn darts in the arena if compressibility were introduced with the current nose 20 degrees down=550ias with idle throttle  

Sharky

[funked]

Santa, the local speed can be much faster because the air has to speed up to go around the airplane.  If you have a very blunt feature (e.g. 109 windscreen), the local air velocity will be much higher than the free stream velocity.  (Free stream velocity is what the pitot tube tries to measure)