Message 6539

From:     "sheffield_ma"
To:         x-plane-tech
Date:     Mon Jul 26, 2004 6:42 pm
Subject: Re: Prop Wash onto the Wings

I appreciate the comments on my forum query. I am including some clarifications below. Also, I invite you to download two version 7.41 .acf files that I put up on my web site:

http://sleibowitz.home.att.net/MyV22.acf

http://sleibowitz.home.att.net/MyV22BigWing.acf

Both files have injected recip. engines locked in the vertical (VTOL) position. The first file has the standard Osprey wings, while the second has extremely large wings. I would expect a more pronounced prop wash effect on the big wing model. Yet both seem to have the same maximum takeoff weight (slightly revised) of approximately 34949 lbs. from San Diego International

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Andy Goldstein wrote the following and his understanding is correct:

> Interesting... Just to make sure I understand correctly what you’ve
> done - you have the prop shafts vertical so the props are acting as
> helo rotors, and they’re blowing down onto the wings, right?

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Matt Bailey wrote:

> On the V-22, I’m not sure I understand what you’re doing. Sounds like
> you’re leaving the wings more or less horizontal, which means propwash
> from the vertical props is going to be hitting the wing from the top
> (extreme high negative AoA). In this situation, the wings are producing
> lift, but the lift vector is *down*. The wings are effectively
> blanketing part of the propwash. Max weight should actually come down in
> this situation, but to what extent depends on how much of the wings are
> in propwash, distance of props from wings, etc. Also X-Plane may not be
> fully considering the blanketing effect of the wings on propwash.

Stephen: This is a good analysis. The prop wash is beating down on the top of the wing. (Actually, the wing has a 5º angle of incidence.) I did not see any reduction in the maximum weight, and am not sure if that is a correct result, or if I made a mistake, or if there is a limitation in X-Plane.

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Matt Bailey also wrote:

> Oh, I got the impression he was trying to increase efficiency by
> increasing wing area exposed to propwash. Looking at the post again, he
> doesn’t specifically mention this.

Stephen: Actually, my intention was nearly the opposite. I was trying to get a sense of what the lift penalty is when prop wash beats down on the wings during VTOL. On the Osprey, the engines and prop hubs are at the lateral ends of the wings. Also, the wing chord is less than the propeller radius. (A look at the Osprey explains this easier than words.) So, the prop down wash against the wings on the Osprey during VTOL is probably not very much. But I have been thinking of other VTOL designs where it would be more substantial, particularly where the engines and prop hubs are not located at the lateral ends of the wings. During my experiment, I was not able to see any reduction in maximum weight, even when I used a design that exposed an extremely large wing surface to prop down wash during VTOL.

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Correction: The original version of this post contained an error concerning the wing’s angle of incidence.