tail stall?

[DEWATSON]

I've been flying my RV8 for almost 400 hours. I have done acro quite a bit in those 400 hours, but never with a passenger. A friend with acro ability and I put on our chutes today and he flew from the front seat. I have done several hundred split ess maneuvers by myself. He pulled up to slow to 80mph and rolled right to inverted and started the down side allowing the speed to build. There was a violent shake and some strange noise from the tail. Scared me....never happened before. We talked about what happened for a minute and then I did the maneuver to the opposite side. The same thing happened. Scared me again. We landed and took a real good look at the airplane....mainly the tail. Everything was tight and normal. Was the tail stalling because of the aft CG?

[Norman CYYJ]

Probably boundary layer separation on the tail. It also happens when doing loops at too high of a gross weight and too many Gs pulled. Lighten the plane up and reduce the g loading.

[skylor]

In an -8 with mid to aft CG (i.e. passenger in the back) it is very unlikely that you were experiencing tail stall in a split S. More than likely, the pull over was a little too brisk for 80 mph and you were experiencing some accelerated tail-buffet from the wing root boundary layer separation characteristic of the -8 due to the landing gear intersection at the wing root leading edge.

Skylor (lots of acro in the -8)

[PCHunt]

Originally Posted by skylor

In an -8 with mid to aft CG (i.e. passenger in the back) it is very unlikely that you were experiencing tail stall...........

Concur. Tail stall is a very specific aerodynamic term. It is highly unusual, and normally unrecoverable. Barnaby Wainfan did an article on the subject in KitPlanes magazine a while back. If I can find a link or reference, I will post it.

It made for interesting reading.

IIRC, the gist of the issue is: the tail is providing "Lift" in a downward direction at all times. If the tail stalls, the plane will pitch violently forward, which exacerbates the tail stall, and essentially renders recovery impossible. In the vast majority of aircraft, it is not possible to stall the tail before stalling the wing, which is a Good Thing.

Now, would a real aerodynamics expert take over please? I think I am in way over my head!! Steve Smith????

[RVnoob]

http://www.vansairforce.com/communit...ad.php?t=77228

[ao.frog]

Two questions:

1) What was the GW?

2) Where was the CG?

I guess a roll intiated at 70 KNOTS (mph is for cars, knots is for airplanes... ) would result in a low speed entry.
If the CG and/or the GW where near the acro-limit, it would easily result in an accelerated stall if the pull when inverted was initiated too abruptly.

Do you recall or could estimate at what speed he started the pull?

[RVbySDI]

Originally Posted by ao.frog

(mph is for cars, knots is for airplanes... )

Well, actually knots are for boats! Aviators just borrowed the measurement from mariners.

[mannanj]

Happened to me during my phase one testing. Solo, CG slightly forward, don't remember the weight but probably around 1500 lbs.

The Wierd shake and funny noises made me decide, OK----I've done a split ess, don't need to do it again. Sign it off and continue the rest if the testing!

[DEWATSON]

I'm with you. No more acro with a passenger for me. Like I said in the original post.....I've probably done over 200 splits by myself and never had a problem. My RV 8 has never surprised me, but this rumbling and shaking really got my attention. I took it back up today and all was normal. Splits are really fun to me....no more with a passenger though. Thanks for all of the comments and answers.

[g zero]

What amazes me was after the first split S noise that scared you , you decided to do another ! I think I would have determined what the sound was on the ground after the first one .
Tom