RV-8 Tail Shake: The Real Story
Part 1: The builder’s perspective
By Randy Lervold
As an RV-8 builder of nearly four years, I have been
active for some time on the various Internet mailing lists. One of the
issues that has arisen recently is an alleged “tail shake” that exhibits
itself at stall on the RV-8. Comments arose on the lists from those flying
RV-8s ranging from “what tail shake?” to “the tail shakes violently”. The
issue got so out of hand at one point that some list lurkers (perspective
builders) became seriously concerned that there was some sort of aerodynamic
problem with the design making remarks like “I hear the tail shakes off the
RV-8”. Ridiculous, and of course this is why Van’s hates these lists —
issues get blown way out of proportion ,and serious misinformation gets
propagated at the speed of light. I can understand Van’s thinking, but of
course in the new Internet age the best antidote to this is real and
accurate information, but that’s another issue. So what is the real story
here? Well, that’s the point of this article: to chronicle one builder’s
experience with it, and compare it to work that Van’s has now done in this
area.
As an RV-8 builder, and recently flyer, who has
experienced some of these symptoms, I was interested in understanding what
was going on. My experience with this issue began this spring while in
Florida taking my transition training. An RV-8 builder who just happens to
be a CFI agreed to spend the weekend with me providing a thorough transition
checkout. On our first flight on Friday evening we were doing basic
familiarization and did some simple stalls. I noticed pre-stall buffeting
coming from the tail, specifically the HS. I looked back there and could see
the tips of the HS shaking with a 2-3” excursion. I thought nothing further
of it, just that it was interesting that the buffet was coming from the tail
and not the wings. My thoroughly enjoyable checkout continued over the next
two days under sunny April skies and I was back home to finish my plane up
and prepare for my first flight.
On May 3rd my RV-8 took to the skies. Other
than a few typical bugs no real problems surfaced. On my second flight
(first one was in the pattern only due to the airspace at my home airport) I
did the usual stalls to ascertain indicated stall speed and felt exactly the
same thing as I had in the plane in Florida. Again, I wasn’t worried about
it at all, but it again struck me as a bit odd that this buffeting/shaking
was coming from the tail and not the wing. In the ensuing flight hours as I
investigated the flight envelope further I came to realize that the tail
shaking was actually slightly pre-stall by about 2 mph.
Discussions on this issue on the Internet list
continued, but now I could participate because I had some actual experience
with the matter. One thing the group quickly learned was that the symptom
was not present on the RV-8A. It could therefore be quickly concluded that
the tail shaking had something to do with the landing gear on the
taildragger RV-8. As I continued to talk with those flying, sometimes via
e-mail and sometimes in person, it seemed that about half of them said “what
tail shake?”, and the other half were quite aware of it. In fact RV-8
builders based out of Arlington, Washington (home of the Blackjack Squadron
and another hotbed of RV activity) were working with Terry Burch, a former
Stoddard-Hamilton employee, on designing and installing strakes attached to
the fuselage intended to somehow manage the aerodynamic influence of the
gear intersection. Van and his staffers saw these strakes for the first time
at the Home Wing’s annual fly-in on June 16th at Scappoose, as
did I. I had queried various Van’s staffers, including Van, about this issue
and they all disavowed any knowledge of it other than the strange phone
calls they had been receiving as spurred by the RV List. While not
acknowledging anything, Ken Krueger, Van’s head engineer, and Van both
assured me they would look into it since they coincidentally had recently
gotten “Delbert”, their new RV-8 test bed ship in the air. (It should be
noted that they had not had access to a regular RV-8 for several years) That
was good enough for me so I put it out of my mind and went about enjoying my
new airplane and the sunny Northwest weather.
While I was willing to be patient and wait for Van’s to
investigate the issue, I was not prepared to simply accept it without
understanding more about what caused it. Frankly, I considered it (still do)
a safety feature. Although I’ve never stalled an RV-6 I am told that there
isn’t much warning. Personally, I think a bit of buffet to warn the pilot is
a good thing, especially if it’s slightly in advance of the actual stall,
and doesn’t effect control in any way. At no time was I ever concerned
about this shake as a safety issue, rather I was anxious to understand what
was going on as part of my overall education of the aircraft.
Right after the Scappoose fly-in I spoke with Ken
Krueger again about it, and since I was about a 15 minute flight away, we
agreed that a test flight some sunny afternoon was in order. (Ha, you can’t
call that playing hookey, that would be work) As happens, the
Arlington fly-in got in the way, then Oshkosh, and before you know it it’s
mid-August. I called Ken once again one Friday and said “hey, remember that
conversation about taking you up to demonstrate the tail shake? Well,
whatareyadoin at about two today?”. “Uh, I think I’m going flying with you”.
“Good, see ya then”.
I touched down at UAO at around 1:45, proceeded up to
the counter to pick up my parts (just because you’re flying doesn’t mean
that you stop buying parts) and asked for Ken. The parts were stowed and we
were off. I leveled off at a safe altitude and proceeded to do some stalls
first without flaps. Sure enough, at about 71 mph indicated the buffeting
began, only it behaved a bit different this time. First off, for the first
time ever I was getting some wing rocking. Second, although the HS could be
seen shaking, the buffeting seemed to be the whole plane buffeting at once
rather than just the tail. Hmm. Another series, this time with flaps. 60-62
mph indicated brought on the same thing. We climbed back up and this time
Ken took it through. Same results. We concluded our testing on this flight
by also trying an accelerated stall in a level 3-4G turn. Same thing, but a
slightly higher frequency buffeting, probably due to the higher airspeed.
Since it was a sunny day, since we had the time, and
since we had the airplanes, he said “wanna fly Delbert and your plane
side-by-side to check stall speeds relative to each other so we have one
more data point?”. Of course, so off we went to do stalls both with and
without flaps side by side, calling airspeeds on the radio as we went. Since
Delbert has a fixed pitch prop, I left just a touch of power in with my
constant speed Hartzell at fine. I believe our conclusion was that stall
speeds were identical with my ASI reading 2 mph higher than Delbert’s under
all conditions.
As we debriefed Ken commented that the
consensus of the Van’s staff who had now flown Delbert was that indeed there
was a bit more buffeting (they would not call it “tail shake”) at the stall
than the other RV models, but that it felt more like the whole plane than
just the tail, and that they felt it was actually better than having little
warning such as an RV-6 exhibits.
Van’s has done some additional preliminary
testing that they will comment on in this article, and determined that the
buffet is worse with the upper gear leg intersection fairings installed —
mine are still not installed (too busy flying!). I will let them explain it
aerodynamically, but my understanding is that at high angles of attack and
slow airspeeds the gear leg intersections are creating turbulence that goes
up and over the wing root and hits the horizontal stabilizer, hence “tail
shake”. They have installed a preliminary set of strakes on Delbert and plan
on doing further testing. I’m sure that testing will allow them to design a
set of strakes that smoothes the airflow and reduces or eliminates the tail
shake. I’m not sure however if that is a good thing or not. Frankly, they
will necessarily add drag, and could possibly eliminate a valuable bit of
feedback to the pilot when about to stall.
Needless to say this was a very educational
day. I can say that I came away from the afternoon with several conclusions.
First, I am quite pleased that my plane seems to stall at exactly the same
speed as a factory built ship. This tells me I must have built and rigged it
correctly (whew). Second, now that I know what is causing the tail shake I’m
not sure eliminating it is a good thing. And any sort of strake to eliminate
it is likely to add drag. We’ll wait for Van’s official analysis on this,
but I wouldn’t trade any top speed away, and that shaking is kind of a nice
wake-up call if you get too slow, and I would think especially useful in an
accelerated stall situation. As I’ve learned my airplane more and
experimented with different landing techniques I’ve now felt the onset of
the shake twice in the landing flare… instant feedback through my backside
that I’m just above stall, and at a time when I really don’t want to put my
eyes inside the cockpit to check airspeed. Seems to me that this is a good
thing.
So there you have it, my take on the “RV-8 tail
shake”. Other RV-8 pilots around the country may have different opinions on
it — this story is presented only as my own experience and my own
perspective.
(see the same issue of the RVator for the article by
Van on this issue)
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