Off-topic aerodynamics spin-off thread

A passable attempt at simplification here:

http://macsblog.com/2011/09/why-the-trim-tab-on-a-racer-matters-so-much/

Moggy

IIRC All (waits to be shot down) tailplanes produce a down force (negative lift) which the proponants of the canard layout will point out as a disadvantage over their favoured concept which produces (positive) lift from both the wing and the canard (it also has a wider cg range).

Don’t forget not all tailplanes create lift, the MU2 (God forsaken pile of poo that used to spit prop blades at the drop of a hat) used to create downforce if memory serves me right.

Curvy Bucc

The curves of the Bucc were to do with the lessening of aerodynamic drag in high-speed and transsonic flight regime. It is called Area Rule, where the cross-sectional area of the aircraft remains approximately constant along its length in the area of the mainplanes.

Anon.

the sexy curvy bit of the rear portion of the fuselage on a blackburn buccaneer, wasn`t it designed the offset the rearward advance of the CofP by making it travel further around the bodywork?.. why dont more aircraft have this shape employed??

I’m no aerodynamicist but:

Increasing the speed of an aircraft does indeed increase lift. However that is for a given angle-of-attack. What they are saying here is that the increasing lift is countered by decreasing the a-o-a, i.e. raising the tail slightly, which in practical terms is true.

In a conventional aircraft the tailplane produces a downforce to counter the forward pitching tendency of the mainplanes. In this case all we are doing to keep the lift at a constant value is decrease the a-o-a but it doesn’t make the tail any lighter. It only reduces the pitching moment (down force) of the tail about the c of g.

Weight is mass multiplied by the local gravitational acceleration. This is a constant value in this instance.

Anon.

Think thats about right . In normal flight the most forward noteworthy piont is the COG , behind that is the centre of lift , meaning the lift of the tail is in a downward direction to balance the other two . It has to be like this to ensure that if lift is lost ( stall ) , the nose will fall and enable recovery . The closer the COG is to the centre of lift , the faster a plane will go ( partly because the tail doesnt need to create so much downward lift to balance things ( lift = drag ) , and the more manourable it will be .It also causes a plane to become uncontrolable , to the piont that modern jets often cannot be flown without computer assistance . I fly RC planes and there is a common saying – ” Nose heavy aircraft fly poorly , tail heavy aircraft fly only once ” .

I do know that I have only a limited appreciation of the the loads we are debating here but as speed increases the loads on the airframe change, the designer tries to make these changes as small as possible, with varying levels of success, and the pilot has to make control/trim changes to compensate for the loads that the designer failes to design out, but I always thought the Cof G stayed the same.
The centre of lift generated by the wings will move foreward or back with speed and power changes will add/reduce loads due to downthrust/upthrust of the engine. And the fuselage will generate some lift/down force at different points along its length also effecting pitch forces.
The elevators on Spitfires nearly always seem to be slightly down in level flight for example but are they level with the tailplane at cruise speed.
Interesting subject that I doubt has one answer.

Richard

Eddie, what you describe as the centre of gravity is what I recognise as the centre of mass – a fixed point dependant on the distribution of the masses in the aeroplane

The description in the link makes no sense to me – where’s the equivalent comment for the wings ‘effectively reducing’ the weight of the wings or the ‘plane?

To quote Wikipedia – “[Centre of gravity and centre of mass] happen to coincide in a uniform gravitational field” (and yes, I entirely agree with the ‘effectively reducing’ comment).

Aerodynamics

I’m no aerodynamicist but:

Increasing the speed of an aircraft does indeed increase lift. However that is for a given angle-of-attack. What they are saying here is that the increasing lift is countered by decreasing the a-o-a, i.e. raising the tail slightly, which in practical terms is true.

In a conventional aircraft the tailplane produces a downforce to counter the forward pitching tendency of the mainplanes. In this case all we are doing to keep the lift at a constant value is decrease the a-o-a but it doesn’t make the tail any lighter. It only reduces the pitching moment (down force) of the tail about the c of g.

Weight is mass multiplied by the local gravitational acceleration. This is a constant value in this instance.

Anon.

That is the crux…..the weight acts downwards at the CofG….it is balanced (in level, unaccelerated flight) by lift from the wing and the tailplane (for a conventional aircraft).

but with more lift you have more drag, putting preasure on other parts of the airframe. the weight is effectively lost in principle on one part of the aircraft but then loaded elsewhere isnt it?.

Ignoring aeroplanes with whirly wings…

Eddie, what you describe as the centre of gravity is what I recognise as the centre of mass – a fixed point dependant on the distribution of the masses in the aeroplane

The description in the link makes no sense to me – where’s the equivalent comment for the wings ‘effectively reducing’ the weight of the wings or the ‘plane?

I suspect I’m missing something here

Richard

its all about balance of forces and definitions. At the end of the day, centre of gravity is static moment distribution on the aircraft……i.e. it is where you put your fulcrum and the thing balances. From an aircraft viewpoint, you can have a longitudinal CofG and a lateral CofG…..not much interested in z axes CofG (unless you are into rockets and motor vehicles and anything that could topple).

When we add aerodynamic forces into the equation you are moving into different territory.
It became obvious to me, at the beginning of my career, that you should keep things as simple as you can……..move to helicopters and it all becomes more and more complex.

“effectively reduces” doesn’t equal “reduces”.

The centre of gravity only changes if some mass moves backwards relative to the reference point on the object. The only way that would happen on an aircraft like a racing P51 would be fuel or coolant moving.