Jump to content
Sign in to follow this  
Luis Hernandez

CG before the wings

Recommended Posts

Hi,I think so too.Bottle half full or half empty..."It's a draw".Remember first flight lessons.Jan

Share this post


Link to post
Share on other sites

In an aircraft, the center of gravity can move. Weight distribution, fuel burn are factors that change the location of the center of gravity.The center of lift is a non-moving point. It's a point somewhere in the chord of the wing. I say 'somewhere' because the only time it changes is if you have wings that can change their shape (F14 as example). However, because the center of lift is actually defined by the physical wing... it's not a moving point.


Ed Wilson

Mindstar Aviation
My Playland - I69

Share this post


Link to post
Share on other sites

I originally said "For a trimmed aircraft the centre of lift is at the cg by definition." Re-read it and understand it before saying it's wrong - because it isn't. You are confusing the centre of lift of the aircraft with the centre of lift of the wing. They atre not the same.What's artificial about the lift on the tail? It's real and has to be taken accountf in trimming an aircraft. Also, what other centre of gravity is there other than that of the "actual weight"? The centre of gravity doesn't have to be ahead of the centre of lift of the wing. See the picture showing a trimmed aircraft. As long as the wing and tail pitching moments are equal and opposite the aircraft is trimmed.Of course lift and weight are vector quantities - they have position, magnitude, and direction as shown in my pictures.

Share this post


Link to post
Share on other sites
Hi,I think so too.Bottle half full or half empty..."It's a draw".Remember first flight lessons.Jan
Yeah that is the basic four forces of flight
I originally said "For a trimmed aircraft the centre of lift is at the cg by definition." Re-read it and understand it before saying it's wrong - because it isn't. You are confusing the centre of lift of the aircraft with the centre of lift of the wing. They atre not the same.What's artificial about the lift on the tail? It's real and has to be taken accountf in trimming an aircraft. Also, what other centre of gravity is there other than that of the "actual weight"? The centre of gravity doesn't have to be ahead of the centre of lift of the wing. See the picture showing a trimmed aircraft. As long as the wing and tail pitching moments are equal and opposite the aircraft is trimmed.Of course lift and weight are vector quantities - they have position, magnitude, and direction as shown in my pictures.
I think this is where are misunderstanding is from. There is a Center of Pressure for the wing and Center of Lift that acts on a location on the aircraft. I was using CoL for CoP so maybe this is where the misunderstanding came from. Here is the center of pressure and showing it move back and forth with the pitch of the aircraft:pressure_distribution_airfo.gif

Share this post


Link to post
Share on other sites

The centre of pressure (lift) of a wing doesn't necessariy change with incidence. There is little change with a symmetric wing (upto the stall) - see the left-hand figure below. With a positively cambered wing the centre of pressure is an infinite distance aft of the wing at zero lift - see the right-hand figure, where it can be seen heading off to infinity. To complicate matters further, it then moves instananeously to an infinite disance ahead of the wing as the lift becomes negative.

Share this post


Link to post
Share on other sites
The centre of pressure (lift) of a wing doesn't necessariy change with incidence. There is little change with a symmetric wing (upto the stall) - see the left-hand figure below. With a positively cambered wing the centre of pressure is an infinite distance aft of the wing at zero lift - see the right-hand figure, where it can be seen heading off to infinity. To complicate matters further, it then moves instananeously to an infinite disance ahead of the wing as the lift becomes negative.
Yes what you said is what wikipedia said almost exactly... There is a change in location of lift which is important to make sure you have the center of gravity far enough in front of this force to aid the tail in lowering the nose and angle of attack to regain flight from stall.

Share this post


Link to post
Share on other sites
Yes what you said is what wikipedia said almost exactly... There is a change in location of lift which is important to make sure you have the center of gravity far enough in front of this force to aid the tail in lowering the nose and angle of attack to regain flight from stall.
Wikipedia is sometimes correct.. If the centre of wing lift is ahead of the centre of gravity that will give a nose up pitching moment. Reducing the wing lift as the wng stalls reduces that, causing the aircraft to tend to pitch down. On the other hand, if the centre of lift is aft of the centre of gravity that will give a nose down pitching moment. Reducing the wing lift as the wing stalls reduces that, causing the aircraft to tend to pitch up.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
×
×
  • Create New...