Elevator trim tabs

I had some information emailed to me on the Reno crash that has me thinking about the trim system on the RV. My background is aviation safety so I am always looking to avoid single failures that could cause major problems.
As I suspect most now know the P51 accident aircraft had been modified from the stock double tab system to a single tab system. The failure of the trim tab is estimated to have generated a instant 8 G pitchup combined with 150 to 200 lb stick kickback when the tab separated. The single tab system is essentially what the RV's have.
When the weather clears I am going to run a few test to get a idea of out of trim stick forces in the RV. The 6 I have is a bit faster then Vans numbers and I would like to have a idea of what forces I could expect. I realize that the speed range of the P51 is vastly greater then any RV however I wonder about single tab systems on Rockets and Super 8's or even a standard RV in a dive at VNE.
If anyone is aware of tab failures in the RV series and what the results were I would be interested in hearing the stories.

George

Hello George,
You may have seen the following info that was posted by Dave Rogers on the Beech list regarding the Reno event, maybe some of the Aero Engineers here could weigh in with respect to the RV trim system.



>> Typically dual trim tabs, or dual actuators, are fitted, as on the
>> Bonanza,
>> just because failure of a single trim tab at high speed will in fact
>> cause a "snap" to high Gs. The "solution" for the P-51 is, of course, to
>> use dual trim tabs as originally fitted.
>>
>> Why does failure of the trim tab system result in a high G "snap".
>>
>> If an elevator is not fitted with a trim tab, then the elevator
>> "free floats" to some angle depending on design conditions.
>> This is called the elevator free float angle. The elevator free float
>> angle corresponds to a single airspeed. At any other airspeed a
>> stick force is required to position the elevator for that airspeed.
>> This is tiring. Consequently, a trim tab is used to "force trim" the
>> elevator to the angle required for different speeds.
>>
>> The elevator free float angle is usually some small value trailing
>> edge up typically at cruise conditions. For the Bonanza in cruise it is
>> very close to zero, as it should be.
>>
>> If you want to fly faster than the speed corresponding to the elevator
>> free float angle, then you need to trim the elevator trailing edge down
>> with respect to the free float angle. Still with me?
>>
>> If the trim tab fails, then the elevator will go to the free float angle.
>>
>> There are two other parameters that are important. Those are elevator
>> angle
>> per G and stick force per G.
>>
>> Elevator angle per G indicates how many Gs will result for a one degree
>> change in elevator angle. For the Bonanza at cruise it is about one
>> degree
>> per G
>>
>> Stick force per G indicates how many pounds of force are required
>> to move the elevator that one degree to get one G. For the Bonanza at
>> cruise (165 KTAS) it is about 11 lbs/G.
>>
>> Now let's turn to the "accident" aircraft. The typical cruise speed
>> quoted
>> for a stock P-51 is 325 MTAS. To get the estimated 450 KTAS of
>> the accident aircraft considerable trailing edge down elevator trim
>> would be required. Furthermore, I suspect that the accident aircraft flew
>> with an aft CG for more speed and more maneuverability. That increases
>> the elevator angle per G.
>>
>> The stick force per G increases as the square of the speed and the
>> "accident"
>> aircraft speed was a bit more than twice that of a Bonanza. Let's
>> estimate
>> five times or 50+ pounds/G.
>>
>> Let's further speculate that the required elevator angle at 450 KTAS
>> was on
>> the order of 3-4 degrees trailing edge down.
>>
>> The trim tab fails, the elevator free floats, i.e. moves trailing
>> edge up 3-4 deg., the aircraft pitches up in a 6-8G pull up almost
>> instantly, the stick force moves with a force of 150-200 lbs.
>> That's enough to break someones hand or arm. Six to eight Gs is enough
>> to black someone out.
>>
>> What does this have to do with a Bonanza. Well if you, for whatever
>> reason, get to the Vne never exceed velocity of 195 KTAS, then you can
>> exceed
>> the Bonanza design limit of 4.4Gs with a pull of approximately 40 lbs
>> and the ultimate limit of 6.6Gs with a pull of approximately
>> 60 lbs. Those forces are well within your strength limits.
>> So, if you ever get to Vne, be gentle with the aircraft even if
>> you are looking right at the ground.
>>
>> Dave Rogers


David F. Rogers, PhD, ATP
Professor of Aerospace Engineering (Emeritus)
Annapolis, MD

trim

There is info "out there" that the telemetry showed 11G. Separate information that the P51 tailwheel is held up hydrauically and will extend at 8G.
The 6-8G blackout has to be qualified. Factors are:
Physical conditioning of pilot
Angle of seat
Duration of G load.
In a Pitts a pull from horizontal to vertical is normally 6 to 6.5 G max. Pulling harder than that doesn't work well
In high performance monoplanes such as the Zivco Edge or the Sukhoi, 10 to 11G is fairly routine. A 90 degree pitch in the Sukhoi at 11G is not a big deal IF you are well prepared for it. Kirby Chambliss, in a Red Bull race, pulled extremely hard on a half cuban and injured his neck. After that Red Bull established a G limit with penalty points.

trim

I have flown two high performance airplanes with NO pitch trim installed. The stock hershey bar wing Cassutt and the Sukhoi SU26. I flew both in excess of 270 statute ias. The stick forces at high speed in both aircraft are hardly noticeable. There is a lot more involved in stick force vs speed than most people think. The symmetrical and semi symmetrical airfoils seem to greatly reduce stick force thru the speed range. The Sukhoi has a symmetrical airfoil and the Cassutt a semi symmetrical.
Elevator chord vs span also has a huge effect on stick forces on many aerobatic aircraft, but there are exceptions to this. The Sukhoi has a relatively large elevator chord but still has light stick forces at moderate speeds. I believe this is due in part to the symmetrical shaped airfoil of the horizontal stabilizer. On aircraft with a "flat" stabilizer the forces are sometimes quite heavy but can be substantially reduced by reducing elevator chord. I hope this thread will not be considered "speculation" by the monitors. This goes far beyond the Reno accident. Despite the lack of any problems in the RV history, I have some serious misgivings abouit the pitch trim system on most RV's.

If I get some VFR weather tomorrow I am going to run some checks with the tab neutral just to see what G forces are generated at various speeds. It may be its not a real issue. It may also be that even if G forces can exceed design limits that the tab system is strong enough that failures are not a issue. We are not dealing with the very high speeds of the P51. That is why I asked if anyone had heard of failures. Please note that trim failure where it is stuck in one position is quite different then a loss of the tab or it fairing instantly. I am concerned about the later. I am sure there have been many instances where the electric trim motor has burned out ect.. freezing the trim.

George

trim

I know of an EAA Biplane which is more or less a 100 m/h aircraft where the elevator trim actuator failed and the tab fluttered. Also a Pitts S1 derived homebuilt, same thing. Vne on the Pitts S1 is 203 statute. The elevator fluttered on the Pitts with the stick moving arround so much it beat up the pilots arms. My biggest concern on RV's is previous discussions on what appears to be excessive play with the manual trim. This does not appear to be a potential problem with electric trim.
Having survived a fully developed aileron flutter on a homebuilt and having lost two friends to aileron flutter on homebuilt aerobatic monoplanes, I take this issue very seriously.

trim

One issue David Rogers does not mention is the disparity in behavior based on airfoil. The original Wittman W8 Tailwind used a modified NACA 4309 airfoil. This airfoil behaved just the opposite of Dr. Rogers description of the Bonanza. The 4309 has a large center of pressure travel and the faster you go the more NOSE HEAVY the aircraft becomes. This same upper curve with a NACA 0006 lower curve has a minimal center of pressure. There is minimal pitch trim change from minimim speed to Vne.

The weather cleared this afternoon and I did a few trials in the aircraft with the trim tab neutral. It looks like the loss of a trim tab in a RV should not be a big issue. With the tab neutral at 160 KIAS the aircraft was generating about 2.2 G's. At 180 KIAS it was 2.7 to 2.8 G's. The instant snap up would get your attention if you lost a tab however it should not present any real problems. I suspect the IAS would have to be well passed any conceivable RV speeds to ever become a issue.

George

sailvi767 said:

The weather cleared this afternoon and I did a few trials in the aircraft with the trim tab neutral. It looks like the loss of a trim tab in a RV should not be a big issue. With the tab neutral at 160 KIAS the aircraft was generating about 2.2 G's. At 180 KIAS it was 2.7 to 2.8 G's. The instant snap up would get your attention if you lost a tab however it should not present any real problems. I suspect the IAS would have to be well passed any conceivable RV speeds to ever become a issue.

George

Click to expand...

George, can you explain this in a little more detail? Not sure I understand what you mean by neutral and 2.2 G's etc.
Thanks!

Since the RV is a single tab system I wanted some idea of what would happen if the tab departed the aircraft or some type of linkage failure caused it to go neutral. In essence I wanted to know with a zero trim setting what would the aircraft do at the moment of failure. Its actually quite easy to check. I was in a RV6 solo with half fuel. Since these are home built aircraft results could be different between aircraft. There would also be different results between types. CG would have a effect. I checked on the ground to insure the tab was dead neutral. I then took off and accelerated to 160 knots. I rolled the aircraft to 90 degrees and then relaxed the stick pressure letting it go to its trimmed position hands off. At 160 knots the aircraft maintained a 2.2 G turn. Less then I had anticipated. At 180 knots it was around 2.7 to 2.8 G's. This was with the stick released. With these values I don't see any issues in the RV's. It would be well under any design limits.

George

jrs14855 said:

There is info "out there" that the telemetry showed 11G. Separate information that the P51 tailwheel is held up hydrauically and will extend at 8G.
The 6-8G blackout has to be qualified. Factors are:
Physical conditioning of pilot
Angle of seat
Duration of G load.
In a Pitts a pull from horizontal to vertical is normally 6 to 6.5 G max. Pulling harder than that doesn't work well
In high performance monoplanes such as the Zivco Edge or the Sukhoi, 10 to 11G is fairly routine. A 90 degree pitch in the Sukhoi at 11G is not a big deal IF you are well prepared for it. Kirby Chambliss, in a Red Bull race, pulled extremely hard on a half cuban and injured his neck. After that Red Bull established a G limit with penalty points.

Click to expand...

While those factors could be contributors... None will tell the whole picture. I routinely pull 7 or greater at work. If you prepare your body for it properly prior to the onset, you're fine, especially with training. I've greyed out at as little as 4 g's in the break when I was being lazy and just snatched the stick back, no physical prep, rapid g onset.

From watching the video (haven't seen the telemetry) the rapid onset of 8-11 g's would have been so surprising and so quick that even the most seasoned and fit pilot would likely have g locd. Even with a g-suit and a reclined seat, you can't just hang on and go for a ride expecting to maintain consciousness in that kind of situation... The stick most likely banged back and the g onset so rapid, he more than likely g locd before he ever knew what was happening. Just like voodoo.

As far as losing a trim tab in an RV at high speed, while I wouldn't be so concerned about the stick force... What I would be worried about is the trim tab fluttering and causing further damage with a linkage failure.

It's been argued on here over and over... Vne is established for a reason.

Reno

Word now is that the ground crew had the telemetry data before it was released to the NTSB. 11.6G on the pullup and 105" mp on the way down. I totally agree on the trim tab. Tab itself is not the issue. The issue is possibility of a disconnect in which case the tab and probably the elevator will flutter.