Guys,
I'd like to inject a little science into this discussion. The result may be surprising to some, and will demonstrate the truth of Mace's and dtango's excellent posts, and also explains Humble's contention in the original post that the much heavier A20 can outzoom a much lighter aircraft like the NIK2.
The first thing I hear you asking, is how can Mace, dtango, and humble all be right at the same time, they seem to be saying different things?
Just bear with me, I can show how they can all be right, and to do that I'm going to start with Newtons second law. f = ma. and we must resort to a little math, just to maintain clarity and credibility.
The expression f = ma is read as, force equals mass times acceleration. So if you want to figure out how an aircraft will slow down when subject to various forces, Newton's law applied to the aircraft's body axis could be written like this:
T - D - W x Sin(theta) = m a
where T = Thrust, D = total drag, W = Weight of the aircraft, m = the mass of the aircraft, a = acceleration and theta = angle of climb.
If you rearrange this equation you get:
a = g(T-D-W x Sin(theta))/W ft/s^2
So you can work out the acceleration (or deceleration) given the thrust, the drag and the weight of an aircraft for any given angle of climb.
Now assume you zoom climb two similar aircraft at the same angle we can work out how they will decelerate as follows.
Firstly let’s assume a zoom climb at about 64 degrees so that Sine(theta) = 0.9 and also assume that g = 32 ft/s^2 Also for a single engine fighter beginning the zoom at 250mph I’m going to assume a thrust of 2000lbs and drag of 1000lbs
For a 7000lb aircraft
a = 32(2000-1000-7000x0.9)/7000 = - 24 ft/s^2
Which would mean an initial loss of 16mph every second.
Now do the same calculation for the same aircraft at 9000lbs Because this is the same aircraft, the thrust and drag will be similar (drag may be a little higher) the main difference will be the increase in weight, so:
a = 32(2000-1000- 9000x0.9)/9000 = - 25 ft/s^2
Which would mean an initial loss of 17mph every second
This shows very clearly that the heavier aircraft loses speed more quickly in a zoom climb. In practice the situation will be worse for the heavier aircraft, because even if they both pull into the zoom at the same g load, the heavier aircraft will still need to produce more lift to achieve that g and its lift induced drag will therefore be greater and its deceleration will be more than the calculation above indicates, because it ignores that stage of the zoom.
So, heavier aircraft is bad? In that case, yes, but weight isn’t the only factor at play here, so let’s look at what happens if we consider a much heavier twin engine aircraft like the A20.
We now have 20,000lbs of weight, but we also have two engines, two props, and twice as much thrust! We will also have more drag, and in level flight a large chunk of the extra drag would be caused by all the extra lift needed to generate the 1g required to keep that 20,000lbs in the air. However, in a steep zoom at less than 1g, that becomes less important. For example, in a vertical zoom at zero g the induced drag for both aircraft would be zero, so I’m going to assume that the thrust for the twin engine aircraft doubles, while the drag remains the same, just for the sake of illustration. Of course that is far from the case in level flight, and profile drag would be greater, but if you disagree with my assumptions, just re-run the calculations with your own figures.
So, for the A20 we get:
a = 32(4000-1000-20000*0.9)/20000 = - 24 ft/s^2
Which is just as good as the 7000lb fighter and better than the 9000lb fighter.
That demonstrates why, for similar aircraft extra weight is always bad, but if you take the differences in thrust and drag for dissimilar aircraft into account you can sometimes get surprising results.
Hope that helps.
Badboy
