Glide at different masses same AoA Bristol DB Q 1459:

[raul.jetpilot]

Bristol DB Q 1459:

Two identical aeroplanes at different masses are descending at idle thrust. Which of the following statements correctly describes their descent characteristics ?

Correct answer:

At a given AoA, both vertical and forward speed will be greater for the heavier aircraft.

I think I can understand why foward speed would be greater for the heavier as Vmd increases with Mass increase. But for the vertical speed I used the ROD in glide fomula (zero Thrust) as D/M and as mass increases ROD would decrease.

Can anyone explain this one for me.

Kind Regards

Thanks

[keith williams]

Try looking at the problem from the "Glide Endurance" aspect.

In a glide an aircraft uses up its stored kinetic and potential energy to push itself forward through the air.

The glide endurance is the time it takes to consume all of this stored energy.

Increasing weight increases drag, which increase the power required.

Power required is the rate at which the energy will be consumed.

So a heavy aircarft consumes its store of energy at a faster rate than a light aircraft.

This in turn means that it runs out of energy, and reaches the ground sooner.

To reach the ground sooner the heavier aircraft must have a greater rate of descent.

[vlkyplky]

gliders

I guess you havent had the chance to fly a lot of gliders, otherwise you would know why do they use watter ballast. Think about that, a race glider pilot will fill his aircraft with additional 200 litres of water to have heavier plane. The answer is similar for both questions.

[Fliegenmong]

Glide distance will remain constant though. regardless of weight

[good finish]

Glide angle is unchanged
The increased mass moves the polar curve to the right
In simple terms the aircraft have the same descent profile but the heavier one is going down the hill quicker
This is why gliders flying cross country add water-it makes them go faster with no penalty on glide angle.