# Tag Info

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### Is the Centre of Gravity moved by an aircraft travelling at “High Speeds”?

Changing the aerodynamic forces on any part of the aircraft CANNOT change the distribution of mass within the aircraft structure. The center of mass, (AKA center of gravity) can ONLY be changed by ...
• 6,110

### What does %MAC mean?

%MAC (Percent Mean Aerodynamic Chord) is a mathematical calculation that shows where the center of gravity is over the wing. The calculations is rather straight forward and the generic formula is the ...
• 12.8k

### Is the Centre of Gravity moved by an aircraft travelling at “High Speeds”?

The author of the book makes a correct introduction: "..movement of the centre of gravity is due to a change in weight. The distribution of the aircraft’s weight can change for three reasons"...
• 61.9k

### What is the weight that is used to balance an aircraft called?

747s in everyday operation typically use fuel for ballast when needed. Ballast fuel is considered to be part of the zero fuel weight and you are not allowed to burn it, although obviously if you were ...
• 39.2k

### Why is the CG envelope wider when the aircraft is heavy?

So if I understand that correctly the CG can be more forward when the aircarft is heavy. Not really, although I guess you could say the index can move "forward" because of the way the form ...
• 39.2k

### What is the weight that is used to balance an aircraft called?

I am pretty sure you are thinking of ballast. These kinds of weights are used aircraft and in many other types of vehicles as well, particularly in ships, to maintain proper balance.
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### Is the differential between the forward and aft CG limits indicative of the overall stability of an airplane?

Normally, the aft limit is governed by static stability, while the forward limit is governed by the elevator control capacity (but sometimes, dynamic stability: excessive static stability may cause ...
• 9,103

### Why is the top left corner cut off on the cg location envelope for small airplanes?

With "top left corner" you mean the combination of a heavy airplane and a forward cg location. This is to limit maximum stick forces. Manoeuvring (= flight at a specified load factor ≠ 1) needs ...
• 234k

### How do I calculate the centre of gravity for the below question?

To find the position of the centre of gravity, you need to find the point where the sum of the moments of the weights on the wheels is 0. \sum_{i=0}^n \left[W_i \times \left(x_i-x_{c.g.} \right)\...
• 83.6k

### What does %MAC mean?

As @abelenky points out in his comment, it is the Mean Aerodynamic Chord of the wing. This is not exactly the mathematic mean of the wings' chord, but a size which includes the damping effect of a ...
• 234k

### Is the Centre of Gravity moved by an aircraft travelling at “High Speeds”?

NO. The "center of gravity" is where the total weight of the body, or alternatively it is total mass, may be thought to be concentrated. It is often seen as the internal physical feature of ...
• 12.1k

### Is the Centre of Gravity moved by an aircraft travelling at “High Speeds”?

There are two separate issues here. Firstly, as long as we are not physically redistributing the mass of the aircraft, then a change in the aircraft's pitching tendencies, as speed is varied, ...
• 22.8k

### Is the Centre of Gravity moved by an aircraft travelling at “High Speeds”?

You should not rely on Mr. Bristol's book to ace the Technical Piloting Interview. He makes three errors in the quoted section: Mass is not weight. The mass of something can only be changed by adding ...
• 234k
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### Aircraft CG limitations: what drives constraints beyond max weight & a single forward (and aft) limit?

The "aircraft C.G. location" is indeed the C.G. location of the loaded aircraft. At some given (constant) speed for final approach, in the "cut off" upper left hand corner of the ...
• 22.8k
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### Are there additional structural measures in airliners with asymemtrical seating arrangements?

There are lateral center of gravity limits, mostly to deal with fuel imbalance and to deal with controllability and trim limits in a worst-case flight configuration,like an engine failure right after ...
• 132k
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### Is there a reason why the vertical lines of some airplane envelope graphs are not parallel?

This kind of presentation is called a weight vs. moment 'fan chart'. Although the labeled and drawn x-axis is in CG position, it is really calculated in terms of moment (inch-lb). The reference ...
• 14.3k
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### How can I calculate the C.G height for an Aircraft?

No that's a really good question. Weight and balance calculations for flying are only concerned with the location and value of the variable weights - fuel, pax, baggage. The C of G of the "empty" ...
• 132k
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### Is fuel in trim tank usable?

Here's a good pilot write up of the A380's fuel systems: https://captaindave.aero/2017/09/07/the-a380-fuel-system/ Of particular note is this paragraph: Towards the end of the flight there are ...
• 9,358

### What does %MAC mean?

A document said that the optimum CG position is from 13% MAC to 33% MAC. A possibly better phrasing of that statement is that the "allowable CG position is..." or some such. Typically the term "...
• 39.2k
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### Controllability vs. Stability in PHAK. Is it an error?

Your understanding of controllability and stability is correct, for the following definitions: Controllability: the response of an aircraft in steady flight, on pilot control inputs. For instance ...
• 61.9k
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### CoG Envelope Calculation - Am I Doing Something Wrong?

Yes, you are doing something wrong. Your biggest mistake is using 154.75 as the empty weight moment. As shown in my last image (from the Icon A5 user manual), 154.75 is just a reference datum marking ...
• 37.8k

### Sensitivity of aircraft stability with respect to the CG location

It sounds like it has a horizontal tail, so you basically need to increase the tail volume which will allow you to move forward CG limit forward, allowing you to move the median CG forward and ...
• 132k

### CoG Envelope Calculation - Am I Doing Something Wrong?

First, the reference datum for the spar is of no consequence in W&B calculations. For Empty Weight and Moment, you have to refer to the actual W&B record for the specific aircraft. It is part ...
• 20k

### Aircraft CG limitations: what drives constraints beyond max weight & a single forward (and aft) limit?

When designing an aircraft -- specifically sizing the horizontal tail and elevator -- we use an 'X-Plot' or a 'Scissor Plot'. This plots tail size (often as volume coefficient) on the Y vs. CG ...
• 14.3k

### Longitudinal/Lateral CG envelope for hoist operations

Presuming that the winch is in a fixed location, (not on a track that allows it to move fore and aft or sideways) the longitudinal and lateral center of gravity of the helicopter would not change as ...
• 27k
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### Does center of gravity change between ramp weight and takeoff weight?

The difference would be from fuel burned during taxi, which is generally a very small fraction of the total fuel, so the impact on CG would be tiny. And, since burning fuel tends to bring the CG more ...
• 51.9k

### How can I calculate the C.G height for an Aircraft?

You can calculate it the same way they calculate the vertical CG on astronauts in space suits. First calculate CG with the aircraft level. Then raise the nose of the aircraft so that the deck angle ...
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### What is the weight that is used to balance an aircraft called?

It is called ballast - part of the inherited ship terminology- or counterweight. Placed where it produces the necessary torque for balance. Usually in the nose or tail. I don't have data specific to ...
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