Aircraft CG (Centre of Gravity) questions

This is a forum where student pilots can post their questions for more experienced pilots and instructors to answer

Moderator: mark

Post Reply
Hawker
Unverified User
Unverified User
Posts: 7
Joined: Wed Aug 22, 2007 8:00 pm
Location: Newbridge, Leinster

Aircraft CG (Centre of Gravity) questions

Post by Hawker » Wed Oct 03, 2007 12:09 pm

Ok, more of a physics questions than aerodynamics, but could some one explain these two to me?

1) Why does the centre of gravity move rearward if an aircraft is overloaded towards the tail?

2) Why does the pilot not point the nose of the aircraft directly towards the destination during straight and level flight during a strong crosswind?

1) I found this link http://www.pilotfriend.com/training/flight_training/wt_bal.htm and had a read.
I understand why the pilot needs to calculate weight distrubution both for the start and end of the flight, and the consequences of overloading etc, but why does the cg move rearward?

2) I know the answer to this (kind of) - like I have the idea in my head - but how would it be explained.
When you get it right, mighty beasts float up into the sky. When you get it wrong, people die.

JFH
Unverified User
Unverified User
Posts: 310
Joined: Thu Jun 23, 2005 1:59 pm

Post by JFH » Wed Oct 03, 2007 12:13 pm

h
Last edited by JFH on Fri Mar 08, 2013 7:09 pm, edited 1 time in total.

Hawker
Unverified User
Unverified User
Posts: 7
Joined: Wed Aug 22, 2007 8:00 pm
Location: Newbridge, Leinster

Post by Hawker » Wed Oct 03, 2007 12:17 pm

I think this sums up the 2nd question:

FACTORS AT PLAY IN A NORMAL LANDING

In a normal landing (defined here as landing on a runway with either no wind, or having the wind aligned directly with the runway) there is no force trying to push your landing aircraft off of alignment. Engine thrust is aligned with the runway and the drag of the aircraft is also. The ailerons are hardly used as the pilot adjusts power (engine) and drag (decreased power and flaps) to lower the landing aircraft on to the runway.

Image

FACTORS AT PLAY IN A CROSS WIND LANDING

In a cross wind landing the wind will push the aircraft off of runway alignment unless the pilot takes corrective action. Obviously, the greater the angle of the wind in reference to the runway the greater the impact on aircraft drift. If the pilot of the landing aircraft does not take any corrective action the aircraft will either land off of the runway, or will touch down while the aircraft has a sideways motion. This sideways motion can result in loss of control or damage to the landing gear.

Image
When you get it right, mighty beasts float up into the sky. When you get it wrong, people die.

Hawker
Unverified User
Unverified User
Posts: 7
Joined: Wed Aug 22, 2007 8:00 pm
Location: Newbridge, Leinster

Post by Hawker » Wed Oct 03, 2007 12:34 pm

JFH wrote:haha, another Carlow avionic student trying to cheat??
who is this????? :lol:
I got the answers out of the book!!!!


Ha, howya.

Not cheating (as such) ... just lookin for some constructive input (sounds better doesn't it)
When you get it right, mighty beasts float up into the sky. When you get it wrong, people die.

junglebob
Unverified User
Unverified User
Posts: 97
Joined: Fri Sep 15, 2006 8:58 am

Post by junglebob » Wed Oct 03, 2007 2:49 pm

What course are you in and what year?
Sorry, I just find this quite basic stuff, but I'll explain the balance one for you as easy as I can, well hopefully! If you need any more clarification, just PM me.

Ah I can't put up the pic! Anyone know how, I don't have anywhere I can host it!
[/img]

mr crow
Unverified User
Unverified User
Posts: 324
Joined: Thu May 25, 2006 7:44 pm

Post by mr crow » Thu Oct 04, 2007 1:08 am

.
Last edited by mr crow on Sun Jan 10, 2010 7:16 pm, edited 1 time in total.

User avatar
Nanolight
Verified User
Verified User
Posts: 602
Joined: Sat Dec 24, 2005 12:57 pm
Location: At a computer Enemy: Gravity
Contact:

Re: Aircraft CG (Centre of Gravity) questions

Post by Nanolight » Thu Oct 04, 2007 11:45 am

1) Why does the centre of gravity move rearward if an aircraft is overloaded towards the tail?

The centre of gravity is an imaginary point in an object through which gravity is said to act. That is to say if you add up the gravitational force acting all over an object, it can be expressed in terms of a single resultant force acting through a single point.

The 'centre of gravity' is a simple way of expressing the sum total of all the gravitaional forces acting on an object.

The see-saw analogy is a good one. Two equally sized people at an equal distance from the pivot will have a CofG in the centre of the see saw... it is in balance.

Image

Move the two people on the see saw and the CofG doesn't change, so long as they are both the same distance from the centre.

From a gravitational point of view, it doesnt matter of the people on the see saw are 2 metres out or right in at the centre, so long as it is balanced the force acting downward through the pivot will be the same.

This is the basis of the CofG principle. Get it?

Now put a fat bastard on one side and a little kid on the other. The fat guy will drop because the CofG of the see saw has shifted to his side of the pivot.

Now picture your plane. Put the fat guy in the back and see what happens. The CofG moves rearward of the pivot. In this case the pivot would be the centre of pressure of the WINGS or the point through which the upward forces holding the plane in the air can be said to act.

It is interesting to note that the same weight further away from your CofG (i.e. further back towards the tail of the plane) will have more serious consequences.

In simple english... fat man over wings = not as serious as the very same fat man in the tail.

The distance from the centre of gravity that you load an object is as important as the weight of that object. This distance is called the 'moment arm' and is based upon the law of the lever.

The moment arm is force which is the combination of this weight and distance.

It's like holding a heavy object at arms length compared to holding it tight to your chest. The very same weight exerts a greater downward force at arms length due to the distance between the point at which the force is applied (i.e. your hand or the aeroplane seat) and the pivot point (your shoulder)

Sorry, I could have explained that better, I am still hung over :cry:

2) Why does the pilot not point the nose of the aircraft directly towards the destination during straight and level flight during a strong crosswind?

Imagine yourself swimming accross a fast flowing river. You want to go straight accross from one bank to the other. If you swim straight for the other bank you'll be carried downstream? Right?

Soloution? Point yourself upstream, or in the case of the aeroplane... into wind.
Smoke me a kipper, I'll be back for breakfast.

Post Reply

Who is online

Users browsing this forum: No registered users and 5 guests