# Helicopter whump-whump

**URL:** <https://boards.straightdope.com/t/helicopter-whump-whump/517441>\
**Category:** Factual Questions\
**Created:** [November 13, 2009, 2:44pm UTC](https://boards.straightdope.com/t/helicopter-whump-whump/517441 "2009-11-13T14:44:26Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\
**Post date:** [November 18, 2009, 8:36pm UTC](https://boards.straightdope.com/t/helicopter-whump-whump/517441/21 "2009-11-18T20:36:50Z")

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> [@SanDiegoTim](#):
>
> As an interesting aside to all of this a “huey” has a very distinctive “whop whop” because its rotor has only two blades. It’s very distinctive.

Two-bladed rotors tend to have worse BVI since the strength of the interaction is proportional to the strength of the vortex, the strength of the vortex is proportional to the lift on the blade, and the lift on the blade is inversely proportional to the number of blades.

Basically, fewer blades doing the same work means bigger noise.

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**Author:** ![Don\_t\_Call\_Me\_Shirley](https://avatars.discourse-cdn.com/v4/letter/d/f4b2a3/32.png) [@Don\_t\_Call\_Me\_Shirley](https://boards.straightdope.com/u/Don_t_Call_Me_Shirley)\
**Post date:** [November 18, 2009, 9:35pm UTC](https://boards.straightdope.com/t/helicopter-whump-whump/517441/22 "2009-11-18T21:35:39Z")

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> [@flight](#):
>
> Two-bladed rotors tend to have worse BVI since the strength of the interaction is proportional to the strength of the vortex, the strength of the vortex is proportional to the lift on the blade, and the lift on the blade is inversely proportional to the number of blades.
> 
> Basically, fewer blades doing the same work means bigger noise.

Why do hueys only have two blades? My mind is dredging up some knowledge from long ago that says that fewer blades = more maneuverable. But hueys didn’t need to be more maneuverable, and Apaches, which you would think they would want to make as maneuverable as possible, have four, so maybe I’m remembering backwards.

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**Author:** ![SanDiegoTim](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@SanDiegoTim](https://boards.straightdope.com/u/SanDiegoTim)\
**Post date:** [November 18, 2009, 9:53pm UTC](https://boards.straightdope.com/t/helicopter-whump-whump/517441/23 "2009-11-18T21:53:59Z")

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I asked a good friend and former-Marine pilot this question. The short answer is that the two-rotor configuration on the Huey was the best they could do with existing (1960’s) technology on that air frame.

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**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\
**Post date:** [December 4, 2009, 7:31pm UTC](https://boards.straightdope.com/t/helicopter-whump-whump/517441/24 "2009-12-04T19:31:43Z")

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> [@Don\_t\_Call\_Me\_Shirley](#):
>
> Why do hueys only have two blades? My mind is dredging up some knowledge from long ago that says that fewer blades = more maneuverable. But hueys didn’t need to be more maneuverable, and Apaches, which you would think they would want to make as maneuverable as possible, have four, so maybe I’m remembering backwards.

Two-bladed rotors use what is known as a teetering rotor configuration. That is where the opposite blades are rigidly attached without lead-lag motion. This is a much simpler design, but has a lot of issues that non-teetering rotors do not. They aren’t worse for everything, either, though they are worse for BVI noise.

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