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Tail buffet

S

smoke

Active Member
:confused: All right all of you experienced aviators, I experienced what I call a "tail buffet" during a demonstrated rudder reversal - the aircraft was at 30 deg pitch, 100 mph ias, throttle closed and alot of left rudder applied. the G forces were very low. After approx 90 deg of heading change there was a very pronounced banging that appeared to come from the tail area. I have experienced this once before (not as pronounced) during a turn from base to final.
Has anyone else had this experience? can anyone attribute this condition to any specific fault?
Thanks!
 
Previous discussion

I experienced what I call a "tail buffet" during a demonstrated rudder reversal
Click to expand...

I remember there was some discussion of this on the Yahoo group a while ago. The majority consensus seemed to be that it was turbulence coming from the landing gear and striking the horizontal stabilizer. My pet opinion is that it is a vortex being shed from the shadowed wing root intersection, but that's just me.
 
Do you have your Intersection Fairings Installed?

Take a look at Randy Lervold's web site - he has a great discussion on this, and after I read it, I decided that I wasn't going to do my flight testing until I had all the intersection fairings installed. Not so much becasue of any danger, just because they seem to affect the configuration enough to make a big differnce!

I have not noticed any real tail shake in all my stall testing, and that is with fairings installed.

Paul
 
Buffet with sideslip

Ironflight said:
Take a look at Randy Lervold's web site - he has a great discussion on this
Click to expand...

One of the issues that was discussed on the Yahoo group was tail buffet in a side-slip, which seemed to magnify the effect. A side slip (such as would be caused by a rudder reversal) can cause vortices and other turbulent wakes to be shed from fuselage, gear legs, wing root intersections, and the root edges of the flaps, if they're deployed.

As for straight ahead power off stalls, I hadn't seen Randy's writeup before. Definitely interesting information, and pretty definitive that the fairings are involved in some way. I still wonder what the exact mechanism is, however. Randy lists stall speeds with and without the fairing, but there are two speeds that I think are relevant to the discussion. First, the speed at which buffet starts, and second the stall speed.

I'm a little suspicious that the fairings reduced the stall by 10 mph, but it could very well have changed the buffet onset speed by 10 mph. Measuring power off stall speed with high accuracy is a little tricky if you're not measuring AOA and logging the data. Looking for the pitch break or buffet is ok from a pilot perspective, but does not really pinpoint the speed where max lift coefficient is achieved.

Given the location of the gear leg intersection on the -8, I could see the wake of the gear intersection going over the top of the wing at high AOA without the fairing and then being forced under the wing with the fairing. This could be enough to keep it away from the tail.
 
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Guys,

I believe I am the one that discovered the buffet you are refering to. This is not approach to stall buffet, and it only occurs with near full rudder during a slip. This buffet is violent. I did some testing, and during the full rudder slip the horizontal stab was moving up and down at least three inches at the tip.

With the flaps down this buffet occured at a slower speed, and was not as violent. I decided to terminate testing as it was hard on the airframe. If full rudder is not used, no buffet occured.

Danny King
Beautiful Doll 80434
 
It hurts...

Patient: it hurts when I move the rudder all the way to the stops while sideslipping...
Doctor: Don't do that.

Is this what we are talking about, or is there a good reason (besides fun) to do a big sideslip in an RV8, particularly one with a CS prop?

I know it's hard to land our club's Bucker without a serious sideslip, since it does not have any flaps, but this does not seem to be the case with the 8.
 
Danny King said:
Guys,
If full rudder is not used, no buffet occured.

Danny King
Beautiful Doll 80434
Click to expand...
Guys,
To Danny's comment that he gets no buffett unless he's using rudder, I did an unscientific study back when I was investigating this and it seems that from polling RV-8 drivers about 50% experience the pre-stall tail shake (straight ahead, no rudder) and 50% don't. I attempted to check what differences might have been between intersection farings and couldn't find any correlation. In the time since then after talking with a few more RV-8 pilots I'd have to guess that the percentage is going up a bit.

I documented everything as thoroughly as possible and don't have much else to add, but if there's new folks out there unfamiliar with the issue you might want to read both my articles that are on this page...
http://www.romeolima.com/RV8/Flying.htm
 
Want fries with that shake?

My -8 buffets straight ahead, clean or flapped, into the stall. It does not buffet in a slip with any amount of rudder or flaps. If the wing isn't stalling, the airflow stays attached enough to not burble over the tail. I guess. Maybe. Who knows? :confused:

I was checking into the One Design a while back and find that it also has reported tail shake issues. Note the gear/wing position relationship. Looks mighty familiar. Some have installed strakes and it fixed the "problem". I kinda like the rumble back there. It's a cheap form of stall warning. It is also, however, not fun to look back there when it's happening and see the HS having a case of the heebie jeebies. :eek:

Brian Denk
RV8 N94BD
annual #6 underway.
 
I built and flew a One Design prior to starting a RV8 (with an interlude affair with a bonanza). The 1-D is an amazing aircraft...+-10G's, 360+ degrees/sec roll rate, etc. And it does typically exhibit a pronounced tail buffet at very high G manuevers and prestall. And this occurs without large rudder inputs. The first few times the buffet occurred, it scared the ... out of me, but with time and experience, the occurrence was a non-event.

In fact several low wing tailwheel acro planes have seen this buffet. In the 1-D case, a simple strake similar to what some folks us on their '8's has been used to remedy the "shakes". I used a fillet but that didn't reduce the problem...but others have shown that strakes are the answer. Unfortunately, I sold the plane before installing strakes.

I believe the leading edge wing root/landing gear airflow interaction and separation at high angles of attack is involved in the problem. It would seem prudent to do your testing will all fairings installed and use a stake if it helps.

YMMY,
Deene

A photo of my 1-D:

http://www.dr107.info/graphics/deene1.jpg
 
My first -8 made all kinds of racket in stalls. It shook, banged, clanged, etc, but I didn't see it as a problem. I've flown other RV models that have no warning before stall, so I figured the -8's shakes were a good thing. You'd have to be completely brain dead to inadvertantly stall an -8 that exibits the shakes.

I never noticed any shakes in slips, but maybe I didn't do them at full rudder, and low enough airspeed.

What I plan to do to "fix" this with the current -8 is exactly nothing. It just doesn't seem like a problem.
 
tonyjohnson said:
Do I understand correctly that the 8A does not exhibit this buffet?
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Correct. The prevailing theory is that the shake/buffet is caused by the gear leg position relative to the wing leading edge. The -8A of course has it's main gear attached more aft.
 
The shakes

Is this the same kind of shake that manufacturers stive to obtain in certified aircraft (the prestall buffet) using stall strips and other aerodynamically dirty devices?
 
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