D.P. Davies V AP 3456

[SKYTORT]

Hi everyone,

I was wondering about a certain statemet made by the author of the famous book "Handling the Big Jets" and its validity weighed up to statemnts made in the famous RAF training manuals, specifically the flying one - AP 3456 Volume 1, Flying.

On page 149 of HTBJ, three statements are made as to why an aircraft pitches nose-down during the transonic flight regime. The author points out that, for one, the aircraft pitches nose-down because of the flow desturbance over the wing caused by formation of shockwaves on the upper surface of the wing, thereby causing the CoP to shift rearwards. Now, based on the explanations in the AP3456, this is not true. The rearward shift occurs because, eventually, the upper shockwave starts mowing rearward due to lack of resistance to the airflow at a high Mfs number, thereby restoring the lift coeffecient, because at this stage, BOTH shockwaves are at the rear, thus the whole wing is again producing lift.

The second "problem" I have is that, the author says the A/C becomes unstable during this nose-down pitching scenario, whereas in the AP3456, the exact opposite is stated i.e. the aircraft has increased longitudinal stability - I agree with the latter based on my understanding of stability in general.

Could those aerodynamic gurus please clarify the above (I hope I have typed it out rather clearly) since I value both sources, but in this case they are in conflict.

Thanks...

[Dani]

I'm (unfortunatly enough) also studying Davies convolution...
In this instance I think he's right because shortly before supersonic subsonic aircraft experience the so-called "tuck under", the nose goes down and goes steeper and steeper, because of increasing effectiveness.
Maybe your RAF manual, of which I have never heard, is refering to some other aircraft. In which year has it been (re)issued? It basically comes to the profile and other aerodynamical properties of your aircraft. As soon as you have pressure changes over the airfoil you get different reactions.
But I'm no aerodynamical expert - hence I have to continue to study Davies :zzz: :zzz: :zzz:
Dani

[aimscabinet]

As soon as you have pressure changes over the airfoil you get different reactions.
But I'm no aerodynamical expert - hence I have to continue to study Davies :zzz: :zzz: :zzz:
Dani[/quote]

I read Davies some thirty-four years ago, the first edition, I guess.:)
Perhaps Davies is referring to the sweptback wings of jets. As the speed approaches that of the critical mach, the tips begin to lose lift due to the transversal flow. This shifts the CP forward, causing the nose to pitch down (tuck under effect). I am far from being an expert, but this is what I recall from my flight school days.
Some of these Air Force manuals were written many decades ago and never updated; this could be one of the cases. I have one from the USAF and it contains pretty old procedures and theories.

Rgds