Interesting conundrum

[Aztec Driver]

So my mechanic and I have completed the annual. Whilst reassembling the stabilator asm and putting everything back together, I brought up the "unique" landing characteristics of Twinkie and pondered if we might, just for sheets and giggles, check to make sure the stabilator has the correct travel for up and down pitch. So he looks it up in the service manual, we check it, and it is over 6 degrees off in the pitch up mode.

This gets me thinking that maybe I am not such a terrible pilot after all, and that maybe I can't hold the nose off because I run out of elevator authority, which we do. So we go about rerigging the stabilator and now the correct limits are set. All is now completed and ready for pickup on Sat.

But when I get back home, I looked up the service manual and find 2 tables for the stabilator settings. One with the "Air flow modification kit" and one without. With the kit installed, my settings were exactly spot on. Upon further investigation, I find that probably most of the pa30 line likely had the kit installed, as that was part of the "fix" to the problem of stall/spin accidents when training was a bit more cavalier and Twin Comanches were used extensively in training. The kit included stall strips on the leading edge of the wing, rudder and wing root seals, rerigging of the rudder, rerigging of the stabilator, and aileron-rudder interconnects.

I am reasonably certain the extra elevator authority at low speeds during the flare would help keep the nose gear off as the mains settle. These changes, coupled with the artificial VMC change from 80 mph to 90 mph, seem designed to keep a pilot from stalling, and subsequently spinning.

Besides the obvious "not complying with FAA regs", what, in your opinion, would be the ramifications of allowing a slight bit more stabilator travel to aid in the landing flare to a reasonably proficient pilot? (No, it will be returned to its original state)

 

[EdFred]

Can you get more nose up with trim? I don't know the difference between the single and twin versions, but I know with mine, I don't use a lot of back pressure, but I am cranking on the trim handle during landing.

 

[gismo]

Can you get more nose up with trim? I don't know the difference between the single and twin versions, but I know with mine, I don't use a lot of back pressure, but I am cranking on the trim handle during landing.

Actually, nose up trim adds to the problem since there's a trim tab on the elevator. A friend with a twinkie told me he learned that he could hold the nosewheel off more easily if he left a considerable amount of untrimmed up elevator pressure. The idea is that trimming nose up causes the trim tab to be deflected down which reduces the effectiveness of the elevator. Personally I believe a much better solution is to carry enough ballast (water in a collapsible container works well) to shift the CG well away from the forward limit.

 

[steingar]

Can't you just leave a toolbox, or one of those cases of beer, or something else heavy in the back?

 

[gismo]

Can't you just leave a toolbox, or one of those cases of beer, or something else heavy in the back?

The nice thing about water is that you can just dump it out if you don't want the weight.

The toolbox would be a problem should you need to use the plane's payload for people (maybe you could trade the tools for avgas?). The beer would work since you could always drink it to reduce the ballast weight.

 

[Aztec Driver]

So let's talk aerodynamics for a while. I don't profess to be the most versed in all of the calculations required for flight, so I'll glean what information I can from more educated people than myself.

The "airflow modifications performed on these Twin Comanches included the following changes:

1. Leading edge stall strips. Two on the right wing and one on the left. Essentially they are small triangular pieces of metal screwed to the wing leading edge, about a foot long. Articles have suggested that the left wing had a tendency to stall before the right wing. Are these strips an attempt to mitigate that?

2. Aileron-Rudder interconnect. I assume this is to minimize uncoordination when at full deflection at times of single engine stall.

3. Wing and flap gap seals. I assume to hold off stall a little longer.

4. Neutral position of the rudder is now set at 1 degree left deflection.

5. Stabilator limited to nose up 8.5 degrees instead of the original 15.5 degrees. I assume this is to keep from entering an inadvertant stall.

Everything I have read thus far have said something to the effect of, "these changes were made to give more warning of a stall and to improve low speed handling characteristics."

I can say with certainty that having the entire amount of stabilator travel allows the nose wheel to be held off on landing, without any undue problems anywhere else in the flight regime. I don't know what problems would be encountered if you entered a stall or attempted a Vmc demo.

Anyone with more aerodynamic knowledge than I care to elaborate a little and help figure out what these modifications would do?

From all of the people I spoke with who were flying when these problems came around, it seems that the FAA and Piper were trying to fix a problem that did not exist with the aircraft. One was quoted as saying "they were trying to fix stupidity."

If that is the case, then these modifications were put in place, as well as changes to the multi engine training system in 1967, and when the problem went away, nobody ever reopened the case to figure out what fixed it.

Certainly some of these changes are reasonably beneficial, but I don't see limiting the stabilator travel to the point where it is now very difficult to hold off the nose wheel on landing to be a very positive one. One must carry more speed to achieve a smooth landing.

 

[gismo]

So let's talk aerodynamics for a while. I don't profess to be the most versed in all of the calculations required for flight, so I'll glean what information I can from more educated people than myself.

The "airflow modifications performed on these Twin Comanches included the following changes:

1. Leading edge stall strips. Two on the right wing and one on the left. Essentially they are small triangular pieces of metal screwed to the wing leading edge, about a foot long. Articles have suggested that the left wing had a tendency to stall before the right wing. Are these strips an attempt to mitigate that?

Generally, "stall strips" are intended to produce a loss of lift from the roots outward to the tips, leaving aileron control as the last to go. To the extent that pilot's tend to use ailerons to maintain wings level during and after a stall, this would provide some mitigation to a wing drop during a stall. The asymmetry of the stall strips would probably reduce the potential for the left wing to stall first although that is likely to be greatly affected by power (asymmetrical and symmetrical) and rudder inputs.

2. Aileron-Rudder interconnect. I assume this is to minimize uncoordination when at full deflection at times of single engine stall.

That would be one effect but the design intent was probably more related to "feet on the floor" operation during normal flight. A downside to the interconnect is that it opposes control movement when slipping for a crosswind landing.

3. Wing and flap gap seals. I assume to hold off stall a little longer.

Maybe

4. Neutral position of the rudder is now set at 1 degree left deflection.

5. Stabilator limited to nose up 8.5 degrees instead of the original 15.5 degrees. I assume this is to keep from entering an inadvertant stall.

Again, maybe. I suspect that the only reason this was done was to make the plane conform to a certification requirement.