Revisiting the Twin Comanche issue...

[Aztec Driver]

To all of you physics and engineering experts out there.

Those that have flown a Twin Comanche know that it can be a little challenging to land sometimes. It can be difficult to land on the mains first, tending, instead, to be a three point "arrival." There are some things that can help, such as a small nose wheel STC.

At the last annual, my A&P and I had the "opportunity" to remove and repair one of the stabilators. When we went to confirm the up and down limit adjustments, we came across the possibility of two adjustments, with two different elevator up limits. One set was for the "unmodified" Twin Comanches, and the other set for ones that were modified.

This modification was the result of a collaboration between the FAA and Piper to combat a series of training accidents many years ago. Most, if not all, of the Twin Comanches have been modified. This modification includes several things, including rudder/aileron interconnect, a stall strip on the leading edge of one wing, a "false" increase in the VMC of the aircraft, and a decrease in the up limits of the stabilator.

The A&P and I reset the stabilator to the original limits to allow for some experimentation. Since the biggest problem I seemed to have with the landing seemed to be running out of elevator before being able to ensure the nose wheel stayed off the runway, I experimented with the new setting. I was easily able to hold the nose wheel off at the new limits. All landings were able to be accomplished at a normal landing speed, while bringing the aircraft to the proper landing attitude.

My question to the engineers is this: What reason could they have had to lower the limit of up elevator travel by 6 degrees?

Stall strips, I can understand. Interconnect springs, I can understand. I can even somewhat understand increasing the VMC value, although it is nigh impossible to keep the aircraft on the ground to that speed. But I cannot see the benefit of decreasing the elevator travel. It almost appears that they were attempting to keep the aircraft from achieving critical angle of attack.

 

[COFlyBoy]

My question to the engineers is this: What reason could they have had to lower the limit of up elevator travel by 6 degrees?

Maybe they were trying to prevent the pilot from being able to drive the airplane into a hard stall which might reduce the possibility of a spin.

We the accidents from way back when mostly stall/spin related?

 

[Aztec Driver]

Maybe they were trying to prevent the pilot from being able to drive the airplane into a hard stall which might reduce the possibility of a spin.

We the accidents from way back when mostly stall/spin related?

Yes. The concensus is they came from the early training curriculum. The accidents went away when they changed the way training was done, but the damage to the aircraft's reputation was irreversible.

 

[weilke]

Maybe they were trying to prevent the pilot from being able to drive the airplane into a hard stall which might reduce the possibility of a spin.

That is my understanding of the 'modification', to reduce the ability to get into a deep stall.

Sometimes manufacturers do that kind of stuff, iirc the Columbia/Cessna 350 has a rudder-travel limiter to reduce your ability to kick it into a bad aerodynamic place. The Diamond DA40 has a limitation in elevator travel that comes with a gross-weight increase.

 

[charlie]

I've never checked stabilator travel but I have the interconnect mod that was part of installing the left ahnd rotation right engine. I can full stall land with full flaps. I've never touched the nose wheel first. I will admit to dropping it on a bit hard in a three point attitude a bunch of times in the 1800 hours I have in the airplane. Installing flap gap seals did tame the landings. At mid to light weights she stalls on at 68 MPH IAS. I can also roll the back of the upwind wheel on with aileron before the stall .

 

[gismo]

I've never checked stabilator travel but I have the interconnect mod that was part of installing the left ahnd rotation right engine. I can full stall land with full flaps. I've never touched the nose wheel first. I will admit to dropping it on a bit hard in a three point attitude a bunch of times in the 1800 hours I have in the airplane. Installing flap gap seals did tame the landings. At mid to light weights she stalls on at 68 MPH IAS. I can also roll the back of the upwind wheel on with aileron before the stall .

A friend who has a nice Twinkie claims that the reason it's easy to touch flat or even nose first is because the elevator will stall when landing with the CG at or near the forward limit. His solution is to trim nose down in the flare which he believes increases the elevator's authority. Personally I suspect that his elevator isn't stalling (it should do the same thing in slow flight if that were the case) but rather is simply unable to generate enough downforce when going slow to hold the nose high and his trick would help with that. He does have counter-rotating engines but I don't know if his plane has a rudder/aileron interconnect.

Is there any easy way to determine if his plane has the elevator travel limited by such a mod?

 

[N801BH]

A friend who has a nice Twinkie claims that the reason it's easy to touch flat or even nose first is because the elevator will stall when landing with the CG at or near the forward limit. His solution is to trim nose down in the flare which he believes increases the elevator's authority. Personally I suspect that his elevator isn't stalling (it should do the same thing in slow flight if that were the case) but rather is simply unable to generate enough downforce when going slow to hold the nose high and his trick would help with that. He does have counter-rotating engines but I don't know if his plane has a rudder/aileron interconnect.

Is there any easy way to determine if his plane has the elevator travel limited by such a mod?

Maybe a digital inclinometer placed on the stabilator and the spec sheet for the mod ?

 

[gismo]

Maybe a digital inclinometer placed on the stabilator and the spec sheet for the mod ?

Got the digital level, don't know where to get the mod data. And I assume the thing to measure is the difference between the main wing and the stabilator (don't Twinkies have stabilitors rather than elevators?).

 

[N801BH]

My guess would be Piper should have specs for the non modded planes and the later version. The difference between full up and full down will be 5 degrees less.. Am I thinking wrong ? And... you question of where to obtain that data is a valid one.... I am gonna try the google method now and see if I get lucky.

 

[Aztec Driver]

My guess would be Piper should have specs for the non modded planes and the later version. The difference between full up and full down will be 5 degrees less.. Am I thinking wrong ? And... you question of hwere to obtain that data is a valid one.... I am gonna try the google method now and see if I get lucky.

The difference is 6 degrees. The up limit for the unmodded ones is 15 degrees. The limit for the modded ones is 9 degrees. Down limit is the same, I think around 6 degrees, but that one eludes my memory. I have the chart in my service manuals, have to go look it up again.

I should have clarified my landing situation a little. Most of the problem was landing flat, not necessarily nose wheel first. And it was a problem of not enough stabilator authority left at slow speed to keep the nose high. At the 15 degree setting, there is plenty of authority left to keep the nose high.

Just wondering what kind of aerodynamic issues would be introduced by resetting it to the original values, instead of the modified values. It is certainly a lot easier to land in the proper landing attitude at the original setting.

 

[taters]

His solution is to trim nose down in the flare which he believes increases the elevator's authority.

The PA-31 Chieftains have a spring in the elevator to give this effect. Personally I dont think it does much but I am still pretty new to the plane...amazing how everything Piper seems to favor a forward CG

 

[N801BH]

The difference is 6 degrees. The up limit for the unmodded ones is 15 degrees. The limit for the modded ones is 9 degrees. Down limit is the same, I think around 6 degrees, but that one eludes my memory. I have the chart in my service manuals, have to go look it up again.

I should have clarified my landing situation a little. Most of the problem was landing flat, not necessarily nose wheel first. And it was a problem of not enough stabilator authority left at slow speed to keep the nose high. At the 15 degree setting, there is plenty of authority left to keep the nose high.

Just wondering what kind of aerodynamic issues would be introduced by resetting it to the original values, instead of the modified values. It is certainly a lot easier to land in the proper landing attitude at the original setting.

I Love POA.. With vast amount of experience in here the wealth of knowledge is truly "priceless" ...

Ben.

 

[Fearless Tower]

A friend who has a nice Twinkie claims that the reason it's easy to touch flat or even nose first is because the elevator will stall when landing with the CG at or near the forward limit. His solution is to trim nose down in the flare which he believes increases the elevator's authority.

Interesting - when I was flying the Twinkie, I found that I got my best landings by trimming slightly nose UP on final (to where I would hold a slight forward pressure on the yoke to maintain blue line on short final. Then pull power add slight back pressure on the yoke and I could consistently grease it on the mains.

 

[flyingcheesehead]

The difference is 6 degrees. The up limit for the unmodded ones is 15 degrees. The limit for the modded ones is 9 degrees. Down limit is the same, I think around 6 degrees, but that one eludes my memory. I have the chart in my service manuals, have to go look it up again.

According to the TCDS:

Control Surface Movements:
Ailerons 19° Up 15° Down
Stabilator (T.E.) 15½° Up 4½° Down
Rudder 27° Right 27° Left
Anti-Servo Tab 9° Up 9° Down (with stabilator in neutral)
Flaps 27° Down

When a PA-30 has been modified in accordance with Piper Service Letter
No. 558, the following control travel apply:
Ailerons 19° (+ 1°) Up 15° (± 1°) Down
NOTE: Neutral is rigged as follows: The angle between the airplane leveling lugs and the upper surface of the aileron, parallel with and next to the rib rivets, 6½ inches from the inboard end of the aileron is 12°.
Stabilator (T.E.) 8½° (+ 0°, -1°) Up 4½° (± 1°) Down
Rudder 27° (+ 1°, -0°) Right 25° (+ 1°, -0°) Left
NOTE: With rudder pedals aligned laterally rig rudder 1° right of the airplane centerline
line, then rig to the above limits in normal manner.
Stabilator Trim Tab 9° (± 1°) Up 9° (± 1°) Down
Flaps 27° (± 1°) Down

Interesting that the rudder has two degrees removed (and "center" is no longer center) as well.

FWIW, Piper Service Letter 558 is titled "Air Flow Modification Kit."

 

[Henning]

Flaps 27° (± 1°) Down

That's all it has?

 

[Henning]

Interesting - when I was flying the Twinkie, I found that I got my best landings by trimming slightly nose UP on final (to where I would hold a slight forward pressure on the yoke to maintain blue line on short final. Then pull power add slight back pressure on the yoke and I could consistently grease it on the mains.

Why? Why hold blue line on short final?

 

[flyingcheesehead]

That's all it has?

That kinda surprised me too.

 

[Aztec Driver]

That's all it has?

Yea, not those crazy barn door flaps, for sure.

 

[Aztec Driver]

Why? Why hold blue line on short final?

Short final, I am down to just above red line. At blue line, you have more control authority, enabling you to grease a landing on if you hold attitude really carefully, so as not to balloon back up, but you will burn up a whole lot of runway.

 

[Fearless Tower]

Why? Why hold blue line on short final?

Because I was doing my ME rating and they (school CFIs) wouldn't let me get below blue line until approaching the threshold unless it was the short field approach. FWIW, it was at PVD - Class C jet airport with plenty of runway. I haven't flown the Twinkie since my checkride unfortunately.

 

[Dr. O]

If holding it at blue line gives you better rudder authority over the fence, then cruise power will do even better than that, and logically we finally arrive at red line as our best over the fence speed... Obviously that is absurd...
And the insistence on blue line during all phases of the landing is the result of fuzzy thinking... It is also fuzzy thinking on take off as most light twins in takeoff will not stay on the wheels until blue line is reached without rapid tire wear and excessive thrust forces on the gear...

Let's go back to what blue line is - you all know it - the minimum controllable speed with one engine caged and one at FULL THRUST in level (or slightly climbing) flight... But, we are in a descent ( a 3 degree gradient IFR approach and steeper in VFR) and engines back to 15 inches... Even if we lunch an engine just then we don't need full power on the other engine to maintain the descent angle to the runway, so we don't need blue line... A lower speed than that will still be controllable at the amount of thrust needed...

denny-o

 

[Dr. O]

Ummm,l I mean't better elevator authority - durn typos...

 

[denverpilot]

Yea, not those crazy barn door flaps, for sure.

Heh. What's wrong with barn doors? At flaps 40, four white lights on the PAPI means I'm dead-on the power-off glideslope!!!

(Even though I'm kidding, it's not hard to start with four white and sink all the way down to four red with all 40 hanging out in the Skylane, especially with a good headwind. Haha.)

 

[Henning]

If holding it at blue line gives you better rudder authority over the fence, then cruise power will do even better than that, and logically we finally arrive at red line as our best over the fence speed... Obviously that is absurd...
And the insistence on blue line during all phases of the landing is the result of fuzzy thinking... It is also fuzzy thinking on take off as most light twins in takeoff will not stay on the wheels until blue line is reached without rapid tire wear and excessive thrust forces on the gear...

Let's go back to what blue line is - you all know it - the minimum controllable speed with one engine caged and one at FULL THRUST in level (or slightly climbing) flight... But, we are in a descent ( a 3 degree gradient IFR approach and steeper in VFR) and engines back to 15 inches... Even if we lunch an engine just then we don't need full power on the other engine to maintain the descent angle to the runway, so we don't need blue line... A lower speed than that will still be controllable at the amount of thrust needed...

denny-o

Uhhh... sorry, no it's not. That's the bottom Red Line, Vmc. Blue Line is Vyse, the best rate of climb on a single engine. Then there is yet another Red Line at the top of the scale for Vne.

 

[Aztec Driver]

If holding it at blue line gives you better rudder authority over the fence, then cruise power will do even better than that, and logically we finally arrive at red line as our best over the fence speed... Obviously that is absurd...
And the insistence on blue line during all phases of the landing is the result of fuzzy thinking... It is also fuzzy thinking on take off as most light twins in takeoff will not stay on the wheels until blue line is reached without rapid tire wear and excessive thrust forces on the gear...

Let's go back to what blue line is - you all know it - the minimum controllable speed with one engine caged and one at FULL THRUST in level (or slightly climbing) flight... But, we are in a descent ( a 3 degree gradient IFR approach and steeper in VFR) and engines back to 15 inches... Even if we lunch an engine just then we don't need full power on the other engine to maintain the descent angle to the runway, so we don't need blue line... A lower speed than that will still be controllable at the amount of thrust needed...

denny-o

I am aware of that, which is why I am below blue line pretty much the entire way in a VFR pattern, and reducing on short final. I do not try to land at that speed, just saying that there is more control authority to prevent a nose wheel landing at a slightly higher speed. Lower speeds are much better on the equipment, and certainly less runway consuming. That is why I experimented at the different setting, and it works very well.

 

[Aztec Driver]

Heh. What's wrong with barn doors? At flaps 40, four white lights on the PAPI means I'm dead-on the power-off glideslope!!!

(Even though I'm kidding, it's not hard to start with four white and sink all the way down to four red with all 40 hanging out in the Skylane, especially with a good headwind. Haha.)

Nothing wrong at all with barn doors. makes for a much slower touchdown, I just don't have them. They make incredible speed brakes in the Aerostar.

 

[gismo]

I am aware of that, which is why I am below blue line pretty much the entire way in a VFR pattern, and reducing on short final. I do not try to land at that speed, just saying that there is more control authority to prevent a nose wheel landing at a slightly higher speed. Lower speeds are much better on the equipment, and certainly less runway consuming. That is why I experimented at the different setting, and it works very well.

I agree that below 500 AGL on final you should be dropping below Vyse (to something like 1.2 to 1.3 times Vso which might correspond to Vmc (redline) in your airplane but I see no reason to fly below Vyse (or even Vyse+10) during the rest of the pattern.

 

[Aztec Driver]

I agree that below 500 AGL on final you should be dropping below Vyse (to something like 1.2 to 1.3 times Vso which might correspond to Vmc (redline) in your airplane but I see no reason to fly below Vyse (or even Vyse+10) during the rest of the pattern.

I am at blue line until bout 500 agl, which is still the majority of the pattern, considering the pattern starts at 800 agl.

 

[gismo]

I am at blue line until bout 500 agl, which is still the majority of the pattern, considering the pattern starts at 800 agl.

Ah, I get it. You're considering the vertical extent and I was thinking horizontally.

 

[Aztec Driver]

So I am now re-revisiting this issue.

I am faced with one of two legal possibilities. I cannot decide which I would like to do.

1. Leave it the way it currently is, meaning leaving it in compliance with Service Letter 558 from Piper.

2. Remove all modifications and rerigging all control surfaces back to the original factory specifications.

I know from experience that the airplane lands much easier with the stabilator rigged to the original specs. I just don't know what removing and rerigging will do to the handling characteristics of the airplane and whether it will be a serious detriment to the resale value of the aircraft.

Service Letter 558 adds stall strips to the front of the wings, a rudder gap seal, and a rudder- aileron interconnect to the aircraft. It also rerigs the control surface limits to move the ailerons up 1*, the rudder to the right 1*, and limits the up travel of the stabilator by around 6*. These are all supposedly to "improve the low speed handling characteristics" of the aircraft. I attached a copy of the Service Letter.

I would be afraid that, if I sold the aircraft to someone who may use it as a trainer, there may be problems because of this.

From all of the information I can dig up on this, all of these modifications were done to fix a nonexistent problem brought on by the training environment of the time.

Can anyone enlighten me as to the purpose of each of the changes, especially the rerigging of the control surfaces?

I assume the airflow strips change the airflow in a stall to allow the use of the ailerons.
The interconnect is to keep the aircraft coordinated in a stall to prevent a spin.
The stabilator limit is to keep the pilot from driving it too deep into a stall too quickly, I assume.
The others I can only assume will also help in handling during a stall.

I don't really want to remove the airflow strips, as that would require refinishing the leading edge. I also assume the removal of the interconnect is not a quick and easy task.

Anyone have any other thoughts on this?

 

[docmirror]

I have no experience in the Twinkie, but it sounds like the issue you are trying to solve is the flat landing or nose wheel first landing config. So, the increase in the up elevator travel seems like it will fix that one issue without changing the rest of the service letter. This would render the plane outside of it's certificated(and modified) type cert, but would solve your problem. So - being that I know how to do it, this is what I would do.

Pretty sure the rest of the service letter changes wouldn't be that bad, but once again if you do away with the service letter completely, are you now also in uncertified territory(not that I really care)? The interconnect is going to be some kind of bungie under the rear seat, or around the rudder area, it shouldn't be that big a deal.

The one thing I absolutely wouldn't do is the rudder gap seal removal. This air gap was apparently the cause of a fair amount of adverse yaw issues in OEI ops, and contributed to some of the accidents in training.

YMMV

 

[Chip Sylverne]

Bryon,

You might think about posting your questions over at the Delphi Airworthy Comanche forum. Some very, very expert twinkie folks over there.

 

[timwinters]

Kristin is here too...occasionally. Few, if any, know more than her. But posting over on her forum will probably generate a quicker response.

 

[Chip Sylverne]

Kristen, and the other one I was thinking of was Hans Nuebert aka the Hansmeister. I don't believe anyone on the planet knows more about the airframe than he. Pat Barry is no slouch either. Lots of Comanche institutional knowledge over there.

http://www.hdneubert.com/engrsvcs.htm

 

[comanchepilot]

The Comanche and Twinkie have the same wing - and while they are sensitive to pitch and speed on arrival, I have not found them either 'hard' or challenging to land - except in the sense that they are high performance airplanes and not Skyhawks or Cherokees. They require trim - lots of nose up trim - so you trim the airframe in the single to 80mph on short final [in my airplane and many others is one whole index spot up] and when you go to idle over the fence just flare the airplane at 6" and leave it there - you'll slow down to 55 or so in the float before you actually land and done correctly you feel like you are leaning backward slightly. It is then fairly simple to keep the nose off.

Try that instead . ..

 

[Aztec Driver]

The Comanche and Twinkie have the same wing - and while they are sensitive to pitch and speed on arrival, I have not found them either 'hard' or challenging to land - except in the sense that they are high performance airplanes and not Skyhawks or Cherokees. They require trim - lots of nose up trim - so you trim the airframe in the single to 80mph on short final [in my airplane and many others is one whole index spot up] and when you go to idle over the fence just flare the airplane at 6" and leave it there - you'll slow down to 55 or so in the float before you actually land and done correctly you feel like you are leaning backward slightly. It is then fairly simple to keep the nose off.

Try that instead . ..

The airframe is pretty much the same, UNLESS it has been modified by this service letter. The singles have never been modified. Then you run out of elevator at about the same time as you flare at those speeds. If the airframe is unmodified, it lands just as you suggest, I know, because I have done it. Unfortunately, in the modified form, you run out of elevator authority and the nose sinks at low speeds.

 

[Ted]

Bryon, if you're going to sell it I would leave it in compliance with the letter, but let the buyer know the results of your findings. If it were me, the elevator might accidentally get rigged back to old style.

 

[philiplane]

The SB worked OK when Twinkies still had 20 pounds of radio gear on the shelf behind the baggage compartment. Modern panel mounted radios took away this weight, shifting the CG forward enough to make the difference you are describing. Most of the B models also had the 5th or 6th seats and belts, adding some more weight to the aft end. This was also back in the day when an average pilot really was 170 pounds.

Put a case of oil in the aft baggage and problem is solved. Or two cases if you have "above average" front seat occupants.

 

[docmirror]

I was thinking of a pilot emer pack that happened to weigh 35Lbs or so, left in the rear baggage bay too. It sure helps my old Bonanza for full stall landings.

 

[dell30rb]

Ballast. Been used to solve CG problems since boats were invented.