## Share

• Wow shaviv, I don't know much (if any) physics, let alone wave physics, this piece is still a reason for applauds

Jan 14, 2008
• Physics was never my forte - however when it comes Bush - well I guess physics was never his forte either

Dec 14, 2008
• "will include one calculation for fun"

Unless I'm misunderstanding the nature of the first set of expressions, there are two. Also, I think you forgot to capitalize Bush a couple of times. Don't shoot.

Interesting info, both on Jerusalem and Helicopter's max speed calculations. And Bush in a quantum entangled state. Sort of an odd combination.

Jan 24, 2008
• 1) Well... I wouldn't call the first set of equations a calculation, but perhaps it sort of is.

2) Yep, I didn't capitalize him a few times. Perhaps it was intentional... perhaps (more likely) it wasn't.

3) Odd science bits... that's the essence here!

Jan 24, 2008
• Actually according to wikipedia the maximum speed of the sea knight is 265km/hr. The seahawk is a bit faster, but those pictures do seem more like a sea knight. At least some of the difference can be attributes to the fact the blades are at an angle of attack with respect to the direction of motion thus you need not add all of their speed to the helicopter speed. However to counter that, the supersonic effects of the blade happen before the blade reaches the speed of sound as the blade causes the air to travel faster over it's top.

Jan 26, 2008
• First, given the window configuration on the second figure above, and the fact that there is something on the sides, beneath the rear windows), it convinced me that it is a sea knight.

As for the maximum speed, I never realized that helicopters are more sensitive to the temperature outside: The higher the temperature, the higher the speed of sound and the faster can the helicopters go. However, unlike planes, because the helicopters' top speed is determined by the difference between the blade speed and the speed of sound, the relative change can be more important.

Thus, if we're talking about 30°C instead of 15°C, the speed of sound should increase by a factor of $\sqrt{(273+30)/(273+15) = 1.052$, i.e., just over 5%. However, it can increase the maximum speed of the helicopter by 63 km/hr, which is not negligible at all compared with the typical speeds of 250 km/hr. Thus, it could be that the maximum rated speed is under favorable conditions, i.e., a high temperature.

– Nir

p.s., didn't realize that you read my blog...

Jan 26, 2008
• p.s., didn't realize that you read my blog...

First time I did...

Still think you are neglecting the angle of attack.

Jan 27, 2008
• or lift in general does introduce a correction, as you suggest. We can of course estimate it.

Using the info on sea knights on wikipedia (and in particular, the drawings of it), one can learn that the rotors have a width of about 40cm. Thus, their total surface area is $A = 0.40m \times 8m \times 3 \approx 10 m^2$ (since their length is 8m each, and there are 3 of them).

Next, we know from bernoulli that the pressure difference between the top and bottom is $\Delta p = (1/2) \rho_{air} (v_{top}^2 - v_{bottom}^2) \approx \rho_{air} v \Delta v$.

Next, this pressure difference should hold the $m=10$ ton helicopter. That is, $$A\Delta p = m g ~~\rightarrow~~ \Delta v \approx {m g \over A \rho_{air} v} \approx {10^4 kg~10~m/sec^2 \over 10~m^2~1.2~ kg/m^3~ 250~m/sec} \approx 33~ m/sec$$ Of course, we have to realize that if this is the velocity difference, then the velocity on top is the average plus half of it. On the other hand, there is probably a range of velocities given that the blade is moving at a different speed at different radii.

So, bottom line, the angle of attack is typically a 10% correction.

Jan 29, 2008
• Knight indeed
http://www.globalsecurity.org/military/systems/aircraft/ch-467.htm

Dec 17, 2009

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