AV_Vibration

AeroVantage ~ Vibration Analysis

Overview:

Note

Notes: To eventually be moved.

There is a concern with the AeroVantage that during transition a portion of the aft proprotors will be subjected to the higher velocity and vortices of the front proprotor's stream tube. This is a valid concern. However, will not this same problem occur on the V-44? The above sketch suggests that 1/2 to 1/3 of the forward proprotor's streamtube will pass through the disk of the aft proprotor during cruise. And of course , the time in cruse will far exceed the time in transition. It was mentioned that they might go to 4-blade rotors.

A clearer understanding of the V-44 will be beneficial.

Christian;

The first 4 paragraphs in your posting #7 cover your concern about the aerodynamic interaction between the front and rear proprotors during transition, and the resulting vibration.

I think that your concern is a valid one and have therefore been trying to look into it. The following are observations;

The Xantus has very small diameter proprotors and it looks like the forward streamtube may not interact with the rear proprotor.

The same probably applied to the Bell-X22A

However, the streamtube and proprotor on the V-22 appear to have a 1/2 to 1/3 overlap. Picture of V-22 mock-up and 3 view drawing. In addition, this overlap is applicable during all cruise not just transition. Bell has mentioned going to 4-blade proprotors, which may increase the frequency and reduce the amplitude.

The question therefore is 'What is Bell's approach to this potential problem?'

Dave

Potential Improvements:

Use 4-blade proprotors for greater frequency and reduced amplitude. However this will increase the needed power. Perhaps start with 2-blade.

Concerns:

Could there be a self-exciting aerodynamic created oscillation between a fore and aft proprotor in the cruise mode?.

Could there be a self-exciting aerodynamic created oscillation between a single rotor and the ground due to an automatic blade control?

Working Papers:

DESIGN:

A154

MAKE:

Secondary Group:

All:		Information relevant to both Full-size and Scale Model.
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Full-size:
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Scale Model:
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Last Revised: May 15, 2009