Drag & speeds.

[pixelshader]

I was reading this - http://www.wwiiaircraftperformance.org/me109/Hoerner-Me_109.pdf

 

At the end it is pointed out that if you removed half the drag from a plane, top speed would (obviously) hugely increase, and that there are many places on a 109 where drag reductions could realistically be made.

 

If you take the first equation that appears on http://en.wikipedia.org/wiki/Drag_(physics) and rearrange it to solve for velocity, it would seem to show that top speed is inversely proportional to the square root of the coefficient of drag. So if you reduced the drag of a plane by 10%, assuming that equation is at all relevant to real life, the top speed should be multiplied by the square root of 1/0.9, i.e. about a 5% top speed increase.

 

This got me thinking about the endless argument over aircraft performance, and about the planes that flew in real life, and the crew which cared for them. I think that it is perhaps not completely out of the question that, for example, a specially treated 109 could have been cruising around at speeds better recognized as those of a standard P-51.

 

Am I totally wrong here?

[JtD]

No, you're two thirds right. But you're forgetting that thrust goes down as speed goes up, if engine power is remaining constant, which it roughly is. So instead of square, you need the cubic root, and end up at a 3% speed increase.

 

For reference, Fw reduced the requirements for surface finish some time during the war, and the result was an about 10 km/h top speed reduction (~660 -> ~650), or ~1.5%. This shows at a 10% drag reduction between a 'normal' surface finish and a 'real good' surface finish would be quite a lot, and a really good surface finish is about what you can do in the field. On the bottom line, there's no way you'd bring say a Bf 109G-6 up to P-51 speeds just by giving it some special treatment. Messerschmitt needed a redesign of many components plus a couple of hundred extra horsepower to get the 109 on par with the P-51 speedwise.

[Crump]

Glad you still remember that!

[pixelshader]

No, you're two thirds right. But you're forgetting that thrust goes down as speed goes up, if engine power is remaining constant, which it roughly is. So instead of square, you need the cubic root, and end up at a 3% speed increase.

 

For reference, Fw reduced the requirements for surface finish some time during the war, and the result was an about 10 km/h top speed reduction (~660 -> ~650), or ~1.5%. This shows at a 10% drag reduction between a 'normal' surface finish and a 'real good' surface finish would be quite a lot, and a really good surface finish is about what you can do in the field. On the bottom line, there's no way you'd bring say a Bf 109G-6 up to P-51 speeds just by giving it some special treatment. Messerschmitt needed a redesign of many components plus a couple of hundred extra horsepower to get the 109 on par with the P-51 speedwise.

 

Great answer.