PHIL GREENO

A fellow Jet flyer and club member has a big in flight trim change problem with his PST ReVision sports Jet.

The model is powered by a JetCat P120SX and has been only flown by experenced jet pilots including some well known names from this forum.

The Models takes off as normal and is trimmed on elevator to fly straight and level at cruise speed (just above half throttle).

In level flight if the power is cut to idle the nose drops violently at about 30 -40 deg. As soon as power is restored the nose comes up and level flight is restored.

When flying with normal cruise power and above all the usual aerobatics can be performed with no problem (rolls, inverted, derry turns etc.). Backing the power off after a reversal

also causes the nose to tuck under. Landings are no problem on full flap so what is the problem???????

My own ReVision model does not seem to have this problem and seems completely nomal in all flight aspects.

My own opinion is that the CG is too far forward and the model is nose heavy but others have a different opinion as to the cause.

Help is needed SOON as the owner is so fed up with his model that he is about to jump on it at any moment and throw it in a skip(his words).

Opinions needed soon to save this model from a size 10 boot.

Phil.

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GrayUK

Phil
I know the plane in question and as I have commented on before, I believe it is a decalage issue.
It would be interesting to measure the tail plane incidence (relative to the wing) on your plane and compare it with the model in question.


Paul G

trioval00

My Feedback: (17)

one of two reasons come to mind.

1) nose too heavy, check balance.

but I am leaning to the second reason, but with not seeing it fly it's just a guess....

2) thrust line of the turbine is off. maybe the turbine needs to be shimmed so it is on an angle, almost like those of the Kingcats.....

just my guess..

Mark

luv2flyrc

My Feedback: (6)

You mention it drops its nose when power is cut, what does it do at full power? Does it maintain level flight as in the "cruise" speed or does it climb? If the latter, it's likely a thrust angle issue ie: too much up thrust.

Mike

GrayUK

The model is fine in level flight with the throttle open.
When you throttle back it decelerates and sticks its nose down, this dive also tightens, ie: it 'tucks under'. You feel that if you left it alone it would complete a bunt!
On the face of it would seem to be thrust line related but, the thrust line is fixed by internal formers etc and has been checked many times.
Paul G

KC36330

IIRC the PST ReVision has a narrow cord on the horizontal tail, that makes CG very critical, shift the CG aft a slight bit and try it.

JP-1

I am sure you will have checked already, however is the thrust line of the turbine same as yours? Paul said the thrust line is dictated by pre fitted formers, have they been glued in correctly at the factory?

Could it be a combination of minor differences to the thrust line, tail plane incidence, and wing seat incidence?

Good luck.

rcpete347

My Feedback: (251)

CG. nose heavy.
Don"t be afraid to keep moving the CG. back, until it becomes uncomfortable to fly. Then you will then know your plane , better than anyone else.
Nose heavy= too much elevator in the turns, nose drops inverted, tracts good.
Tail heavy= too sensitive on controls, falls out on maneuvers
3/4'', is usually max., from manufacture setting
Move 1/4'' at a time
This is my procedure, maybe not for others.
Rcpete

basimpsn

My Feedback: (5)

If your model use a elevator push rod, It could be flexing at full power.

HarryC

My Feedback: (1)

Phil, you don't mention the time delay between closing the throttle, and the pitch down. It will provide a big clue.
In a prop driven plane, the cut in throttle causes an instantaneous change in the airspeed of the flow over the tail, thus for a prop plane a sudden nose down pitch can be either the thrust line or a too far forward CG. But for a jet model, where the backdraft from the engine is not directed over the tailplane, a cut in throttle does not cause the instantaneous change in airspeed of the flow over the tailplane. For a jet, the tailplane's airspeed is pretty much the whole plane's airspeed so the pitch down will occur but only gently and over some time as the plane slows down. If the onset of the pitch down is very soon after closing the throttle, such as within 2 or 3 seconds allowing for the time the engine takes to spool down then it won't be a CG issue because the airflow over the tailplane has not changed, it will be a thrust angle issue.
Harry

PHIL GREENO

To answer a few questions/opinions.

1. The Revision does not have a narrow chord tailplane but the opposite, a massive tailplane with a huge chord (320mm x 820mm span)

2. It is not a decalage problem as this in effect is always pilot adjusted in flight to suit a models set up by trimming the elevator. On a Hawk like my Carf model

with an all flying tailplane the decalge will end up where the pilot sets his elevator trim to suit the set up (cg position etc.) while trimming in flight. It is in effect a totally adjustable

decalage and will end up where it needs to be to suit all aero forces on the airframe. The effect is the same with an elevator tailplane.The tailplane assembly of any model acts

as an aerodynamic actuator and controls the AOA of the main plane. While flying at a reasonable speed the weather cocking effect of the tailplane keeps the main plane AOA

at a point to maintain lift but when you back off on the power and airspeed is reduced airflow over the tailplane also reduces and the tailplane becomes less effective and other forces

come in to play such as CG position.If the CG is way too far forward the nose will drop as speed is reduced. Nose down tail up power off butas power is increased so airflow over the

tailplane increases pulling the tail back down and nose back up again. So in my opinion it has to be a forward CG postion.


Phil.

HarryC

My Feedback: (1)

Yes but it won't be anything like as pronounced as it is with a prop driven plane. For a prop driven plane, the flow over the tail is dominated by the propwash, in other words it is controlled by the throttle. That causes extremely rapid, and massive changes in the airspeed over the tail, speed changes which are many times the model's speed range. The jet model does not have that and so the flow over its tail is simply the plane's airspeed. That means that changes are slow to happen and are restricted to the range of the model's speed. With a jet you have to wait for considerable changes in the whole plane's airspeed to let the tail have any effect, so there is some time delay between cutting the throttle and the nose going down. If the nose goes down strongly within a couple of seconds of cutting the throttle, the plane has not had time to slow down much unless it is a barn door, so the tail has not changed its downforce by any substantial amount. Therefore if the pitch down is very soon after closing the throttle it won't be a CG issue.

Harry

HarryC

My Feedback: (1)

You can do the good ol' CG test. From trimmed level flight, don't close the throttle and put it into a dive. If it is nose heavy it will pull up out of the dive as it gains speed. If it is a thrustline problem, it will push the nose further down.
H

PHIL GREENO

Harry,

You make a very good point and the more I think about it you could beright and itcould well be a thrust line problem. My ReVision does not have this problem at all and on sunday I noticed that the model in question had its turbine mounted much higher off the factory fitted mounting rails than mine in fact it had at least 6mm packers under the jetcat mounts.Also the jetpipe entry cone is mounted about 6mm higher off the mounts than mine although the tailpipe exit is the same on both models.With the trust higher on entry and lower in effect on exit this pushesthe back end of the model down atmiddle to high thrust levels and the nose up. The model is then trimmed to accept this incorrect thrust line and fly straight but when the power is cut the trim is all wrong for the glide and the nose dives. Dave dont jump on it yet.

Phil.

readyturn

My Feedback: (9)

Bingo !

Rick

David Gladwin

I built a PST Revision with a PST 1300 for RCJI review and did everything (control throws, C of G etc., ) exactly as per the instructions. The result is a model which flies perfectly and exhibits NO problems at all.

I have looked at Dave's and just can't see any obvious problem, and as for alignment of wings and stab., they all come out of the same mould so that seems an unlikely problem. However I am convinced it is a thrust line problem. However, if the engine is mounted too high and the exit is lower that would try to raise the tail (it creates a downward component of thrust at the tail ) requiring the use of up stab. trim to compensate. Remove the thrust and that should cause the nose to rise because of the up trim. However that process is complicated by the relationship of the thrust line to the center of drag of the model. That said, as the centre of drag (mainly from the wing) of the model is below the thrust line it should mean that the model would pitch up with power off !

Very confusing to say the least.

So, one step at a time:

Make sure the engine (height and angle) and tailpipe are mounted in David's model exactly as yours Phil, and that the centerline of the engine AND tailpipe are exactly continuous. .

Ensure the C of G is exactly as per manual.

Test fly and evaluate further.

We also need to determine whether this pitch change is a result of airspeed or thrust value.

(Take it high, fly and trim it at high speed with engine at idle, then reapply thrust and see what happens, may give us another clue.

It might be interesting to compare its problems with different fuel states too, it just might be caused by fuel sloshing forward in the tank moving the C of g forward as a result of decelleration. .

I use the supplied PST tank which is quite short.

I have heard of rogue aircraft in the RAF (mainly Javelins and Canberras) which had to be scrapped because they just couldn't be made to fly correctly even after extensive work but I think we will eventually solve this Revision's problems.

It would also help me to try and get to the root cause if you could advise exactly the elevator position at medium speed and thrust.

Regards,

David.

bigplumbs

I dont want to seem too stupid and I am sure you will have checked this but I assume there are no odd elevator to throttle mixes that might have ended up on the radio.

I am sure that this will not be the case but worth a mention I thought

Dennis

PHIL GREENO

Yes Dennis that also crossed my mind and it is not something you usually check or might see at the flying feild as the only time you move the throttle when in a static position is when you start the turbine.

I will have to ask DB to check that ifhe hasnot read this thread.

Phil.

skywarrior

Hi,
There are no odd mixes that relate to throttle or elevator and I have been through this a couple of time now. The height of the turbine mounts compared to Phils are different because of the fact he is using a Jet Munt 140 with a central mounting frame and I am using the rather confusing off centre Jet Cat 120 stock mounting.
However if you lay a piece of stiff wire across the the top of the mounts and view this from the rear end of the thrust tube it is bang on central with the turbine cone of the engine so it should be correct ( if that is the original mounts are correctly glued to the airframe).

Yes I am fed up with it as each time we go flying with this it seems an R&D exercise and I so much enjoyed flying the Reaction. This plane flies beautifully when trimmed for half throttle or full throttle and lands like a stroked cat going for the cream. However the trick is to blend the two flight modes which as yet remains a mystery to most.

I am intending to give the plane to Phil this winter to dismantle and re programme ( we have Futaba TX's so it should be easy) and then leave him to fly it. The hope is that with a total re start the plane will fly like his in the spring and we shall all be wiser folk as to the reasons why.

Thank you all for your help with this frustrating problem meanwhile I am off to build my C.A.R.F Hawk

Regards

David

alasdair

If the tail is abnormally large, I would ecpect the CG to be abnormally far aft to get the usual Stability Margin. If the Stability Margin is abnormally big (forward CG) then the reaction to a reducing airspeed would be a nose down pitch. I am not convinced that this is the case here. From the description I would be looking for a structural anomally something structural, or maybe a servo,is moving (bending, twisting, whatever)causing a trim change.

How about another flight check. This time trim straight and level at full throttle and close the throttle to check reaction. Nose down pitch, right?

Now fly inverted and trim for straight and level how much down trim does it need? If it is a lot (more than available), that indicates a forward CG.
Now close the throttle. If the problem is fwd CG or other general aerodynamic issue I would expect the model to drop towards the earth - ie that is nose up pitch to the aeroplane but 'down' relative to the lift.

If the model pitches towards the sky when you close the throttle while trimmed for inverted flight then the fault is within the structure, not the aerodynamics. Because the fault has followed the structural layout, not the aerodynamic layout. That is, we have turned all the structure (including the jet pipe and its angle) upside down, but the aerodynamic forces are much the same as in upright flight, relative to you (and the Earth). So does the fault (the pitch) follow the structure, or aerodynamics?

skywarrior

Hi Alisdair,

Thank you for your input here and I will consult my pilot JP as I am not sure we completed this manoeuvre.

I do know it pitched down from level flight when trimmed for this discipline as soon as the throttle was closed it also did the same when trimmed at full throttle.

When trimmed for level flight half throttle and the then set to full chat it climbed equally as when it dropped or tucked.
When we first flew it this all was apparent after the first reversal. Up to the top , flip over , shut the throttle ( now right side up ) for the glide down without throttle and the more she came down the more she tucked i.e. increase speed /equals more tuck

This is a good question to get the answer to and as they say "Thats why he gets the extra !"

Be in touch

Regards

David

PHIL GREENO

I had a chance to fly my ReVision from our local grass club feild two sundays ago and I must say it is a very nice model to fly.

Larger than my Flash and smaller than a Lighting it is a very smooth model to fly with good vision in the air.

I can now land it as slow as any Boomerang as its quite a light model with a very large wing/tailplane area.

I think the very large tail surfaces are one factor in making this model so stable at low speed.

As David Butt said he is going to drop his ReVision over to me soon so I will go over it with fine tooth comb and sort out his trim problem.

I will set his model up with all my settings and fly it in the new year.

Just received another Revision from PST in a custom yellow/red/black scheme so I will build this over the winter also.

Phil.

kumpol

Phil,
please note that theJETCATand PSTengine's strap center line are different. the Revision is sensitive on engine thrust line. for the CG if you feel the need of more elevator you can move the CG toward after about 10-15mm. I'm quite sure it is a thrust line problem.


Sky warier.
You might want to try your Revision with the big tip tank removed. As said before the Revision wing has quit some degree of wash out built in if the alignment not correct it will create even more problem and make it worst.
Kumpol
PSTJETS THAILAND

mavrick

My Feedback: (1)

HI Phil
Mine flys just great no nose tuck at any speed I have the C-G in the middle it sounds like it might be a tad nosey, I have my Kingtech 170 F in mine one thing we did notice was how quite it is
I have the 170 dialed down to 110,000. I do have the tip tanks on mine as you know not sure if they would make any difference, they all come out of the same mould so I find it hard to believe
that there would be any structual variation in the model, when you look down the thrust pipe is it dead centre with the back of the turbine not a tad high at the front. Hope you can get it sorted
mine is fantastic.
Regards
Mav

PHIL GREENO

Kumpol,

I have flown my ReVision with and without the Panther large tip tanks and I felt the low speed handling was better with the tanks fitted.

Might just be me but it sure does feel very stable on a long slow approach.

I also have the CG 20mm further back than factory settings as I have said before I always set the CG as if the model was my design.

I only use factory settings as a guide.

Phil.