Newps wrote:
: but speed goes up as the cube root of power.... i.e. doubling the power gives you
: 2^(1/3) = 25% more speed for a given airframe. So a Cherokee 180 at 135 mph will go
: to at most 170 mph if you could strap on a 360 hp engine.

: That formula has always been taken as gospel but the real world truth is
: if you could get a 360 hp engine under the cowl you would go a lot
: faster than just 170 mph. My buddy and I both have 182's. I have the
: standard 230 hp engine, he has the IO-550 300 hp engine in his. Even
: though he has more drag because he has much larger tires than I do as
: well as bubble windows on both front side windows he typically indicates
: 30 mph faster thah I do, and he lost 5-7 mph with the 3 8.50 tires over
: the 8.00x6 mains and 6.00x6 nose like I have. And while he goes
: virtually straight up he cannot legally haul more.

While I won't argue with your specific results, they really can't be
considered the norm. Even though two planes are identical models, they cannot be
directly compared from differences beyond control (mis-rigging, different weights, the
things you mentioned, etc). Take a look through a multi-purpose POH (e.g.
PA-28-150,180, or PA-24-180,250,400)... the numbers line up.

e.g. Book: Ratio^1/3 (relative to 250)
PA-24-180: 139ktas 140ktas
PA-24-250: 157ktas base
PA-24-400: 185ktas 184ktas

Awfully close numbers. Again, I'm not saying your numbers are wrong, just
that it holds remarkably well to published numbers. According to hp, he should
indicate 9.3% more speed than you. I doubt you're doing 323 mph in a 182...

-Cory

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************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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