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Tail Shake

P

Pat Stewart

Well Known Member
This weekend me and a friend went out for a little practice. While doing a stall the aircraft had a vibration and when I looked back at the tail I was surprised to see how significant the tail was shaking. Anyone else experienced this, it just does not seem like that should be so severe and I wonder what damage it can create. At a minimum I don't think I will ever stall the airplane again.
 
Yeah... I craned my neck around the other day... while in a deep stall in my (new to me) 9A. The shaking back there made me glad I added the stainless steel straps to bridge the join at vertical and horiz. Hope the rest of the parts hang in there too. Not a habit forming maneuver.
But in the Lancair's some guys never do a stall... for fear that their little tail will let them down and they will die.
 
The RV-10 plane that I used for transition training also did this. I could barely feel it but my instructor told me to look back and sure enough it was bouncing up and down slightly. I have not experienced it in mine as I have not done deep stall testing yet.
 
Go Pro, anybody?

Someone with a flying RV-10 should use their Go Pro camera facing backwards and do some stalls, so we can see the shake. Is this normal? Is the "buffeting" we feel before an airplane stalls caused by the tail shake? Hopefully, someone smarter than me will help describe what's going on, and perhaps even provide video (excellent)..??
 
The tail shake during the stall is likely due to aerodynamic buffeting caused by the separated flow from the wing. Much different animal than the flutter testing on the comanche in the video. There are numerous articles and videos out there that show, quite graphically, the result of flutter. It is something that has happened a little too close to home...years ago, one of my professors worked for Beech when they were doing flutter analysis. He was an engineer, on the ground watching what happens when a divergent flutter mode occurs. Long story short, the airplane crashed and the pilot didn't make it.

More recently, my Dad was flying a C-421 on the first flight after an annual. Everything was fine climbing through 17k when he heard a bang and the airplane was vibrating violently. Long story short, he was able to get it on the ground safely...with one elevator. Apparently the shop had either used the wrong sized cotter pin or didn't use one in the trim tab linkage. It came loose and the tab flew the elevator back and forth until the bolts holding the two elevator together sheared.

Not pleasant. Flutter is nothing to fool around with...
 
I am curious to know what is the difference between a Stall and a Deep Stall. I have never heard this reference before today.
Thanks
 
rocketman1988 said:
The tail shake during the stall is likely due to aerodynamic buffeting caused by the separated flow from the wing. Much different animal than the flutter testing on the comanche in the video.
Click to expand...

That's what I thought. Boy, if 10 pilots were experiencing more "shake" than normal, it could be a real issue. Thanks for clearing this up...
 
Alex at rvtraining

While doing my transition training in January, he had me look back when he was performing a stall. HS bounces quite a bit (Several inches). He said that he had talked to Van's about it and it was normal/expected.

Definitely makes me ready to do close checking on the HS spars and elevator attach areas during annual inspection.

Have not done any stall testing yet on mine.... Second flight completed yesterday, but I will mount my GoPro backwards during that testing.
 
When I started flying my RV-8 I noted that it had a tail shake at the stall. I looked back and the tips of the horizontal stab were vibrating up and down perhaps 3/4". They shake because of the placement of the gear legs just below the leading edge of the wings. At the stall with the high angle of attack, the air coming around the gear legs and up and over the leading edge creates dirty air going back over the horizontal stab, causing them to shake. I stopped the shake by putting on strakes on the fuselage just above and in front of the leading edge which re-energizes the air. I now have no tail shake, no pre-stall buffet, and it lowered my stall speed.
 
I will get a camera looking back there as well. With flaps up and stick full aft, the Ten comes down like a very stable leaf, the 9 handled the same just never fell as fast. A slight check forward and it starts flying right away. There is a slight buffet the whole time but I never looked back. I was told it moved a fair bit but never considered it an issue. I would bet both it and the wings are moving a fair bit on those really turbulant mountain flights, you are just moving around to much to realize.
 
From reading that link, it appears I have used the term deep stall incorrectly since we don't have T tails. Maybe, fully developed stalled wing condition would have been better. Anyway, I am thinking this thread will generate some creative camera mounting now. We should see a body of "evidence" coming forward on tail shaking in all the RV types. I look forward to that.
 
My wife and I flew with Alex a few weeks ago and he showed us the same thing. It is a bit disturbing to see, but he did say it was "normal".

[email protected] said:
While doing my transition training in January, he had me look back when he was performing a stall. HS bounces quite a bit (Several inches). He said that he had talked to Van's about it and it was normal/expected.

Definitely makes me ready to do close checking on the HS spars and elevator attach areas during annual inspection.

Have not done any stall testing yet on mine.... Second flight completed yesterday, but I will mount my GoPro backwards during that testing.
Click to expand...
 
Advice from an old trucker (Applicable)

An old trucker once told me when he was hauling tripples. Never look in the mirrors, because if you do you will pull over to the side and never pull them again. The three trailers are constantly moving back and forth in and out of alignment and looks scary in the mirrors. Tripples are three simi trailers pulled behind one tractor (legal in some States on interstates).

Steve
 
Our Designs..

You may find some interesting reading on deep stalls in the Velocity aircraft...

One guy road one all the way to the ground...and survived!
 
Velocity

There were at least two Velocitys that went all the way to the ground/water out of control. This is an issue that is pertinent only to canard aircraft. The t tail issue is different and is normally where the term deep stall is used. A prototype BAC111 crashed on a test flight in a deep stall.
The DC9 does not have direct control of the elevators. The yoke controls tabs which in turn move the elevators. To deal with the deep stall there is a hydraulic assist feature that moves the elevator directly in the elevator down position only, and only if there is a "disagreement between the yoke command and the elevator position. The low wing T tail aircraft have a problem with disturbed airflow from the wing affecting the tail. In low wing aircraft with a low horizontal tail, the horizontal is well below the disturbed airflow.
 
Strakes

Hi Stu,
Do you happen to have any pictures of the strakes that you installed?
Thank you,
Mike
 
Two Times!

rocketman1988 said:
You may find some interesting reading on deep stalls in the Velocity aircraft...

One guy road one all the way to the ground...and survived!
Click to expand...

There are two documented incidences where deep stalls were encountered. Both of the experienced test pilots indicated that the airplanes stalled at an angle of attack of about 40 to 60 degrees, and "locked" in a non-rotating descent with an almost flat angle to the horizon. Despite efforts to push the nose down by the control stick, rocking the wings and even manually attempting to manipulate the center of gravity forward by shifting their body weight forward, neither pilot was able to recover from the stall. As both airplanes collided with terrain, both pilots remained unharmed due to the slow 1,000 foot per minute descent rate. In both cases, the airplane was within its normal weight and balance envelope.
 
crabandy said:
Not sure about the RV, but during my multi training I had several hours facing backwards (club seating) in a Baron and the tail shook a LOT during stalls.
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As pointed out above, tail shake when an aircraft is held in a stall is not unusual. It occurs on a lot of certificated and experimental aircraft. It is prominent on the RV-10... probably because of the huge slotted flaps coupled with a specific relation ship of the elevation of the horizontal stab above the wing trailing edge. It occurs to some degree on most all of the RV models if you hold the airplane in a stall.
My point... it is nothing particularly unusual about the RV-10.
 
Velocity

Post 23 is correct. Both airplanes survived relatively intact and both flew again relatively quickly. Both were based at Ft. Lauderdale Executive around 1989-90.
The blue airplane was purchased by a gentleman from Germany. He intended to ship it to Germany. I contracted with him to prepare the airplane for shipping. My recollection is that we found the airplane would not fit in a container and would have to go as deck cargo and would not be insurable. I then started building tanks to ferry the airplane but he decided to sell it in the US.
I flew the airplane several times. I would not take one as a gift, well maybe I would for the engine and junk the airframe. A horrid airplane.
I am not anti canard, I flew the Long Eze and liked it, however the Long does not meet my personal requirements for runway performance and baggage space.
 
Carl J. Niedermeyer said:
I'm not Stu, but thought this might help. My understanding is that the cause of the excessive tail shake at high AoA's on the RV-8s is the formation of two vortices; one from the wing root/fuselage junction, and the other from the landing gear/fuselage junction. These two vortices may coalesce into one larger, stronger vortex. The strake seems to capture and reduce the the strength of vortex or vortices.

Some folks have modified the wing root/fuselage fairing and the landing gear/fuselage fairing into one fairing that appears to eliminate the excessive buffet. I believe Randy Lervold's excellent RV-8 website discusses this.

My strake:

RV-8Strake.jpg
Click to expand...

I am Stu (Falcon), and my strakes are essentially same as pictured. I got them in 2001 from Robertson in Arlington WA and reworked them to make them parallel to the wing. You can see the placement parallel to the wing root in a position to capture the dirty air coming off the landing gear upper fairings at high angle of attack and energizing the air so it flows smoothly back over the Horizontal Stab. It completely eliminates my previous Tail Shake.
 
More than two?

strahler13 said:
There are two documented incidences where deep stalls were encountered. Both of the experienced test pilots indicated that the airplanes stalled at an angle of attack of about 40 to 60 degrees, and "locked" in a non-rotating descent with an almost flat angle to the horizon. Despite efforts to push the nose down by the control stick, rocking the wings and even manually attempting to manipulate the center of gravity forward by shifting their body weight forward, neither pilot was able to recover from the stall. As both airplanes collided with terrain, both pilots remained unharmed due to the slow 1,000 foot per minute descent rate. In both cases, the airplane was within its normal weight and balance envelope.
Click to expand...

I believe there were more than two of these accidents. I helped recover the aircraft involved in the following report. It was flown by a co-worker. He was definitely not an experienced test pilot. The descent rate was at least 2000 fpm, and if it hadn't been for some very large cottonwood trees sheering energy he likely would not have survived. He lost an eye from this accident. Long story short, the main wing stalled and the canard did not.

To see the report go to the link below and type in N57V for the registration it is the first report in the list:

http://www.ntsb.gov/aviationquery/index.aspx

Please bear in mind that this was a purchased aircraft that he did not build and it was a very early model. At least one of the above mentioned crashes, perhaps both, happened prior to this crash and the Velocity factory had incorporated wing changes, "Vortilons" IIRC, that had not been completed on my co-workers aircraft.
 
In this case we were at 4000 feet headed into a 16 knot headwind. I first did what I would call a mild slow flight flaps down stall and felt a little buffet in the stick at around 62 mph and stopped, probably short of the stall, my friend who is also a very high time pilot with a CFI then did a little more aggressive stall with a high AOA, the last time I looked at the airspeed it was dropping through 60 mph when I looked back at the tail. I would guess we were about 56 mph. The aircraft never dropped the nose or wing. We stopped the event after seeing the tail shake. I guess I was a little nervous about the cracks some of the other aircraft have had and did not want to push it any longer.
 
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Alex showed me this during my transition training back in 2008. I usually show it to my transition training customers. I have flown and stalled several -10's and they all do it. As I recall, SB 08-6-1, that Van's released in June of 2008 to add doublers to the F1010 bulkhead, was due to cracks on the bulkhead on Van's demonstrator aircraft. Presumably, that airplane has been stalled many, many times during flight test and years of demo flights.

Many years ago I had the opportunity to sit in a rear facing seat on a Citation II while it was being stalled. The amount of shaking and shuddering on the horizontal stab and elevators was very impressive!:eek:
 
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