Unusual Trim Change Problem
 

I have run into a problem with my JMP T-33/RAM-1000 setup that is baffling. The fuselage gets really warm with the engine running on the ground. My test pilot noticed that the trim on the elevator had changed. Instead of being at neutral with no stick input, it had a fair amount of up elevator without any stick input. After cooling down, the elevator went back to it's normal trim position.

I cannot pinpoint why this is happening. The obvious answer would be that the pushrod is expanding but that is definitely not it since the servo pulls on the rod for up elevator. Expansion of the rod would cause down elevator trim.

Could it possibly be the servo losing its center due to the relatively high heat? The servo is a JR3411. The top of the fuselage was pretty hot but not hot enough to soften or damage the fiberglass.

Has anyone ever experienced this phenomenon? How was the problem solved?

Unusual Trim Change Problem
 

Could be the fuselage expanding as it gets warm.Also, did you mean an 8411 servo, not a 3411?

Frankly, I think you need to find out why the fuse is getting so warm. I would think a proper set-up would not be letting things get that hot.

Unusual Trim Change Problem
 

sounds like either the fus or the sleave to the push rod is expanding.

jason

Unusual Trim Change Problem
 

We thought there was a remote chance that the fuselage could be expanding but we really don't know to what degree fiberglass expands. The new installation will have the ceramic blanket from BVM installed to try to insulate the fiberglass some and avoid an excessive amount of heat.

Unusual Trim Change Problem
 

I don't know the thermal expansion of fibreglass, but it is basically a plastic, so I would think it would have to be more then a steel pushrod. BTW, what is the pushrod composition?

Unusual Trim Change Problem
 

Polyester resin will expand more than Epoxy with heat. Both Will get soft at the higher temps, tho. Look for about 30 K per foot of fuse length. definitely enough to affect the trim of the elevator push rod/rods.

Cooks by pass and exhast pipe??
 

Tom Cooks inlet design and tail pipe won't let happen what you are experiencing, I would call Tom and ask him, but something sounds very strange -------------
Lee-----

trim problem
 

C'mon guy's let's put our thinking caps on. Do we really think heat is causing the problem? A steel rod three feet long gets longer by .040" with a change in temp of 200 degres F. , very small indeed. also if heat lengthens the pushrod the same heat is there to lengthen the fuselage, no change there. and fiberglass isn't mostly plastic it is mostly glass and glass has about the same coefficient of thermal expansion as steel.

Expansion is not the problem there, I would suspect the heat may be changing component values in the servo amp.

JMP Tail Pipe?
 

If this is the same T-33 that was discussed in an earlier thread, it did NOT have a JMP by-pass or tail pipe. All of the JMP T-33 examples that I have seen, and there have been several, had very cool aft fuselages during ground running operations. All also had JMP (Tom Cook) by-pass and tail pipes.
Roger

Heat/Trim Changes
 

Roger,

Yes I believe I did have another post much earlier about my T-33 and you are correct, it did not have the JMP bypass or tailpipe. It had a BVM bypass shroud and a Tamjets double-walled pipe and RAM-1000. This was the setup when we noticed the trim change. I have not used the JMP bypass because I purchased the plane mostly built with BVM turbine T-33 intakes installed and came complete with the shroud.

The BVM intakes are still in the airplane but the rest has been removed. I have ordered a Jetcat P-120 engine to put in it and a Jet-Tech(Gary Mueller) bypass and pipe for the P-120. I previously did not have any insulation in the rear of the fuselage but when the re-installation takes place, I will be using the ceramic insulation material that BVM sells.

It is still a mystery to me why the elevator trim changes up from center so noticeably since an expanding pushrod(carbon fiber) would give down elevator. At the same time, we did not notice any change in the rudder trim. The rudder is actuated via pull-pull cables.

Maybe the guys are right when they say fuselage expansion but I am getting a lot of up elevator trim change . I was hoping that someone would know for sure what the the cause is since I consider myself still on the steep side of the learning curve when it comes to the care and feeding of turbines.

Elevator Trim Change
 

Hi Aptor,
I would make some sort of neutral reference mark inside the fuselage at the servo arm with everything cold. Then run it up until the trim shift occurs, and quickly remove the hatch and see if the servo arm still lines up with the reference mark. This would narrow the problem down to servo drift or linkage shift.
Roger

changing trim
 

Helloooo guys,
read my post #8. fuselage and or pushrod expansion is REDICULOUS. The heat may be causing the servo to change center but "expansion of the fuselage" get real!!!

Unusual Trim Change Problem
 

Captainbob,

Is there any way you can be constructive WITHOUT being insulting? Just wondering.

insulting
 

Tony,
Just what was it i said that you thought was insulting? I thought I was beeing constructive by stopping people from barking up the wrong tree. but even after my first comment about expansion rates we still have people blaming this fellows problem on something common sense would would tell anyone with a few beans up stairs couldn't be. Or is it because you are the one who thought fiberglass was mainly plastic.

insulting
 

I have a tendency sometimes to be blunt but i would never intentionally insult anyone. so maybe we could start over.

This expansion theory that is being kicked around in this post is way off base. A steel pushrod 3ft long will grow by .040" with an increase of 200 deg F. I doubt the pushrod in question is anywhere near that length. For the sake of this discussion let's say it is 6" long. That pushrod would get longer by .007" if the temp rose by 200 degF, one turn on the clevis would make it change more than this. A rise of 200deg is doubtful because if we started with an ambient temp inside the fuselage of 100 deg F a final temp of 300F would be burning paint off. Another thing that shoots this theory full of holes is that glass has nearly the same coefficient of thermal expansion as steel so any rise in temperature that would lengthen the pushrod would also lengthen the fuse and everything inside it.

now back to a more plausable explanation for aptor's problem.
Some people have had problems with gyros in there jets drifting. Bob Violet recommends that you warm the gyro with a heat gun to note the drift. I would suspect that servos also have a temp range during certification and that temperature outside that range might affect the servo similar to the way it does a gyro.

Unusual Trim Change Problem
 

Only part of a lay-up is glass. The rest is resin, which is a plastic, and may have a higher rate of expansion then glass. An earlier post by Terry indicated a rate of .030"/ft. I don't know if that is correct, but let's assume it is. I'm not sure what the length is in a JMP T-33 between the servo and elevator horn, but I believe the servo is located in the front of the model, so it could be 3 feet. If Terry's #'s are correct, that would mean a change in length of .090". If the control horn on the elevator is short, that would mean a significant change in the trim. Again, I am not saying this is what was ocurring, just one possibility.

Another very plausible cause mentioned by jason is the outer sleeve of the pushrod. If it is the typical plastic sleeve, it will expand a large amount with heat and will cause a change in length of the pushrod. My bet is that this is the cause.

IMO it's a jump to think that because a gyro may have a trim change with heat that a servo will also. Servos do not have the piezo-electric crystal that is in a gyro. It's my understanding that it is this crystal that can cause the trim drift.

Aptar, just take a heat gun to the servo and see if the center changes. Should be an easy test to conduct, and may eliminate that as a cause. I still feel that the real solution is to reduce the heat build up. One thing to look at is if there is enough source of intake air into the fuselage to cool the tailpipe. Another would be the back of the pipe. Is there an augmentor nozzle at the back to induce flow over the inner tailpipe? The end of the inner pipe should be some distance in front of the end of the outer.

JMP T-33 turbine convertion
 

This is beginning to be a good lesson in doing things correctly! Heat buildup of any kind would not be a problem if aptar would have installed the inlet / bypass / engine mounting system, and tail pipe that Tom specifically designed for his T-33.

I did the turbine conversion a few years back, with Tom's complete conversion package, and had zero heat problem. The inlet / bypass / tailpipe that Tom sells is designed to utilize the extra size of his T-33.

I believe that without a doubt when the new higher power turbines are available i.e. the P-160 & P-200 Tom's standard flow through system including tailpipe will need no redesign and will work perfectly! The only thing I would recommend when installing 35lb thrust turbine would be to use a speed limiter, set to a maximum of around 180mph -------

Hey, when building a pre-designed, engineered, and manufactured model aircraft of the nature and caliber we are talking about why try to re-envent the wheel?
Lee --------

Unusual Trim Change Problem
 

Aptar-
You seem to be buying additional parts from other manufacturers to solve your problem. You need to go directly to JMP and get the items you need. It would be money well spent. You are just going to keep spending good money after bad and get marginal results if any.
I agree with several others on this post in that I've never seen a JMP T-33 with any heat problem in the aft end of the aircraft. Trying to use a ceramic insulator is just going to add weight with minimal results when the proper designed components will make all your problems go away.

Now go do the right thing.

Marty O

expansion
 

I would try this:

1. UNDO the elevator push-rod from the servo horn.

2. mount a new horn on a small piece of balsa or ply using a
wood screw to where it rotates as it normally would hooked
to the servo.

3. Hook your pushrod to this assembly and tack glue it in place
to where it duplicates being connected to the servo.

4. Center the elevator and run the engine. If its heat expansion
the surface should move as stated.
If it doesn't then it could be the servo.

Just a thought.

W.G. Hunter

expansion rates
 

Tony,
Respectfully, you and terry are still missing my point. if heat lengthens the pushrod by .030" per foot then it also lengthens fiberglass by about the same amount. If the pushrod grows and the fuselage grows also therefore you have no net change in the linkage geometry.

heat on servo
 

How about reversing the servo in question and running the turbine. If the heat is affecting the internal parts of the servo, might we expect the control surface would move in the opposite direction. Quick test without tearing stuff apart.

You can also paint the sleeve on the pushrod with BVM Heat Shield or wrap in aluminum foil and it will reduce the heat gain. For a while.

Steve Flaten

Trim Change Problem
 

Quote:

---

Aptar-
You seem to be buying additional parts from other manufacturers to solve your problem. You need to go directly to JMP and get the items you need. It would be money well spent.

---

Quote:

---

Hey, when building a pre-designed, engineered, and manufactured model aircraft of the nature and caliber we are talking about why try to re-envent the wheel?
Lee --------

---

As I said in an earlier post, this plane was mostly built and had BVM T-33 intakes installed already. Tom's products are undoubtedly of high quality but the primary reason I have not used his by-pass and pipe is, for example, the pipe is more than double the price of other high quality pipes which are double walled with the "accumulator" at the rear of the pipe. Another example is why I did not use his fiberglass fuel tanks is that the set of three costs $600. Knowing the prices for these two items, I never even inquired as to the pricing of his intakes and bypass. Now I know first hand how expensive this end of the hobby is but that type of pricing is just over-the-top in my opinion. A guy has to draw the line somewhere. It just doesn't make sense to me to spend more for a few overpriced components than the cost of some pretty high quality complete jet kits. If I could just win the lottery!

Unusual Trim Change Problem
 

The heat will not be affecting the fuselage structure the same around it's circumference. It is quite possible that the fuseslage is softening/extending/deforming on one side only which could cause the observed problem.

I have a glass-fuse Tucano that will deform under pressure of the tie-down bungee if I leave it in my trailer out in the Florida heat.

I vote with the others that say - Just go buy the correct parts.

Unusual Trim Change Problem
 

[QUOTE]Originally posted by Marty O
[B]Aptar-
You seem to be buying additional parts from other manufacturers to solve your problem. You need to go directly to JMP and get the items you need. It would be money well spent. You are just going to keep spending good money after bad and get marginal results if any.

Uhmm,
So my product cause the problem is that what you saying?

Brg,
Tam

TOO HOT?
 

O.K. so your putting a P120 and new bypass into the T33, maybe you should just try this new set-up first before pulling anymore hair out over this problem. It would be surprising to me if the fuz is stretching, (paint bubbling etc. would be suggestive of this) its way more likely a linkage/servo??? IMHO. Tams pipe would not be the cause either! I presume it is of the correct diameter/size/length for the RAM 1000 and JMP T33? For your model to be getting so hot at idle only? would indicate to me a air flow concern more than likely caused by the incorrect inlet/bypass installation or incorrect placement of your turbine within this set-up, restricting air cooling flow through bypass and pipe....

Aptar if you plan to continue running this set-up again, why not try posting some pics here of your T-33 by-pass and pipe installation, that way we can maybe offer somewhat more accurate verdict on this problem.... : )

Cheers