# Simple acceleration question

1. Aug 11, 2007

### Diresu

Does the weight of an aircraft effect acceleration on a flat trajectory? Obviously the Earth is round making a truly flat trajectory impossible but I'm trying to simplify this.

Example) Would an aircraft that weighs (Force*Mass) 10,000 Kg accelerate slower with an additional 5,000Kg of weight stored inside? I know the top speed is the same, it's the time to the top speed I'm after.

The obvious answer is a resounding yes but sometimes the obvious answer is incorrect.

THANK YOU!!!!!

2. Aug 11, 2007

### rcgldr

Force = mass x acceleration, power = force x speed, so given the same power, more mass results in reduced acceleration. In addition, it will require more power to fly at the same speed, because more lift is required, and correspondinly, more drag will occur.

3. Aug 11, 2007

### Danger

Nice summation, Jeff. The top speed will not be the same without an engine upgrade (unless it's an airframe issue such as trying to go transsonic in a subsonic bird).

4. Aug 11, 2007

### Diresu

Thanks Jeff and Danger!!

Danger, I thought drag was only thing that reduced top speed.

Extra weight reduces top speed only if it changes the way the plane sits and creates more drag. If the plane sat the same in the air the extra weight wouldn't reduce top speed. Realistically any extra weight is going to alter the way the plane sits and ultimately mess with the aerodynamics. But in theory it is the extra drag that reduces top speed, not the extra weight. Am I right or is that confused?

5. Aug 12, 2007

### Staff: Mentor

And it does.
You seem to understand it, but are also trying to draw a distinction where none is necessary. More weight means more lift is required, which means more drag is produced.

6. Aug 12, 2007

### Danger

I wasn't very clear about what I meant by the airframe issue. I was envisioning a situation wherein the aeroplane has more than enough power already to carry the load in question up to the rated speed. For instance, say you toss a couple of sandbags into a C-130. It will still reach its top speed easily, but will require slightly more fuel to do it. There are definitly restrictions upon how fast a particular airframe can go before the stresses overcome the structural integrity. The most prominent of those is the case of a plane having enough power to go supersonic, but isn't the right shape to do so.
In any event, if the added weight is in high enough proportion to the available power, then top speed won't be attainable.