As published in the RVator, Fourth Issue 2001...

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)

Click the "Back" button on your browser to return to