stealth chinooks? your thoughts

[PELHAM 10]

It's physics, that loud whomp whomp you hear when a helo is near, that is the tips of the rotor-blades breaking the sound barrier as they spin through the air.

No it isn't, it's caused by one blade passing through the shed tip vortice from the blade in front of it. It's called blade vortex interaction noise. While blade tips ocasionally do go supersonic it's not the usual case and is undesireable for many reasons, the sound is completely different from BVI.

There is much research into reducing this effect as can be seen in this video from Eurocopter, the worlds leader in helicopter design.

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Edited May 31, 2012 by PELHAM

[rksl-rock 1301]

Ugghh... supposed experts and armchair generals.

Guess what? It is not the engines that are audible. It's physics, that loud whomp whomp you hear when a helo is near, that is the tips of the rotor-blades breaking the sound barrier as they spin through the air. The only way to avoid that is to slow the turbine rotation down, and thus to an extent, making the aircraft more vulnerable to SAF in its various forms and spotting by enemy combatants.

As a "supposed expert" i have to ask what your qualifications are? Mine are pretty well documented on here. ;)

The "whomp whomp" is caused by the huge surface area of the blades passing through displaced air, creating the characteristic slapping sound. As Max Power correctly points out the blade tips dont go supersonic at all. They are actually designed so that they don't/can't/won't go supersonic as a safety precaution at normal operating speeds. And its the reason for the VNo (Velocity Normal Operation) & VNe (Velocity Never Exceed) figure in the operations manual.

Can I asked what you mean by "SAF"? I'm not really sure what the Slovenian Air Force 'in its various forms' has against Chinooks but i would like to know :D

Stealth Chinook ? - i don't think so , it would be alot work to make it "stealth"

More reasonable option should be Osprey , because it can carry heavy stuff and also it is more faster than normal helicopter

I agree with the first part. But not the second. A stealthy Chinook well I dont think its really that viable. Atleast not cost effective for the resulting purpose - see my previous posts. The Blackhawk is a more obvious choice especially since they've done it eg the lost in OBL raid.

Osprey is somewhat stealthier in so much as the noise footprint is less immediate but in a hover its still a very loud beastie. IE the sound is focused behind the airframe not below it. But it carries a much lower payload and in Hot and High areas such as Afghanistan its use is restricted by conditions quite significantly. The operating margins in the Afghan summer mean that its payloads have to be limited during hot daylight hours.

[max power 21]

You're absolutely, 110% wrong. That said, so am I.... to an extent.

The transonic region of flight is between mach 0.8 and 1.2. Even the P-38, with it's very thick chord wing, only started getting shock stalls at mach 0.7. The huey's blade tips in the hover travel at mach 0.6, and all surfaces inward of those tips travel more slowly. Since the huey has quite a lot of blade slap in the hover as well, I think we can conclude that the blade slap phenomenon is not cause by supersonic helicopter blades.

[Serclaes 0]

Iirc it's caused by the way the rotor head compensates the dropping of the retreating blade (with the UH-1 at least). I believe they switched for lighter/more flexible blades much later that made the characteristical "thump" much weaker.

Can I asked what you mean by "SAF"?

I think it means small arms fire.

[dekster 1]

Just had an idea: blow air trough the rotor-ends (face it forwards), by that you can stall the ends and reduce the noise.

Don't know if it would work, but I like my thinking