All moving tail plane setup

[Mel Jones]

I am in the process of putting together a RCRCM Sunbird which I will fly at the slope.

This model has a all moving tail elevator, I am wondering where the neutral position of the elevator should be and how is should be set?

Can anyone offer advise?

[Erfolg]

I do not know the Sunbird, although I have built (many of own design) and flown a number of gliders using all flying tail planes.

In my case assuming the section is pretty much flat bottomed, and I am mainly interested in flying at the lower end of the speed envelope, I would do the following.

Firstly I would use a spar type strip, at approx 48" lg by temporarily fastening the spar on edge(for rigidity) to the underside of the wing touching at the TE then along the underside. Secondly I would then set the TP so that the LE and TE were both parallel to the spruce spar.

If a slope model, where I expect the model to be operating pretty much in the mid speed range, then I would arrange the spar to be held, equi distance from the nose radi and the TE. Then set the tailplane as before.

For a symmetrical section, as above, but with the tail plane set, so that there is a negative angle to the spar, that is tailplane LE down, TE up, by an angle that is a few degrees, about 2 degrees.

One aspect that needs to be recognised is that the all flying tail plane moves through quite a angle, compared to a conventional elevator.

Using the above methods, I have never been a long way out.

For flat field models, I would always give the model a big chuck, to access how near to being initially acceptable the model set up is. Then fly the model to properly trim out.

[Frank Skilbeck]

I just followed Erfolgs procedure using a Mk1 eyeball rather than the spruce spar on a Multiplex Fiesta I've just finished, on the test flight it needed about 3 clicks of elevator.

[russ d]

Hi, I set the tail plane on my Sunbird by eye !

Inline with wing when assembled (was almost bang on the centre of the slot) but with loads of elevator movement available for the maiden flight with the c of g at 75mm (boy! was the elevator twitchy !)

After a few flights I have the c of g at 80 mm and reduced the tail plane travel to 11 mm each way at the trailing edge, I reckon I can go further back with the c of g and reduce the tail plane travel as well, At these settings it will pull out of a 45 degree dive and needs a bit of of down when inverted.

russ

[Erfolg]

For some reason there are people who want to move the CG as far back as possible with a glider, claiming that it reacts more positively to a thermal.

I will put forward the counter argument. What you are seeking is a model which has a degree of stability, that is it does not stall, when hit by a gust. You loose more height from a stall, than you can ever gain, from being very sensitive..

When the CG is correct and the model gliding straight in all winds strengths, by knowing your model, and recognising, that if the model moves of to the right, there could be a thermal to the left. If the model pitches slightly up, you could be entering or passing through a upward moving body of lift. More often and not it is knowing the model and recognising that it is sinking faster than usual, or that it is holding height which tells you about the general surrounding air.

I do not believe in the dive test in its entirety. I find that inverted flight and how much forward elevator is needed, gives a better understanding. Above all, it is that impression of being a consistent controllable flyer that that tells you if the CG is optimum for you.

It could on the other hand be me that is a poor flier, but I have found that a rearward CG (to avoid lead in the nose) is a misery to fly, for me having lower duration.

[Wiltshire Flyer]

I set the tail on my Bird Of Time by eye, made sure it was a little up more than down and it was fine.

[russ d]

The Sunbird is not much of a thermal glider, much more a shove it off a slope with a fair wind and tear around the sky type of glider, there is a very long thread on RCG about the Sunbird (worth a read).

russ

[Frank Skilbeck]

Erfolg the reason for tuning the c of g on a glider to make sure it's not too nose heavy isn't really to help in detecting thermals but to make sure the glider is flying most efficiently, if you have to have excessive down force at the rear for level flight it means that the main wing will be at a higher AoA and you'll have slightly more drag, might be OK if you've got a big whirly thing on the front beating the air into submission, but with a glider you want the best aerodynamic efficiency.

The dive test is a great way of tuning the glider so it's stable but efficient.

[Erfolg]

Frank

I contend that you loose more height from stalls than from being slightly more positive in pitch.

Having competed often in the Barcs league in the past for many years, it is noticeable that winning or even dong well has more to do efficient flying in all conditions and in the turn than being on the edge.

[PatMc]

As Russ said, the Sunbird not designed for thermaling. It's a damned good sloper & so should be set up accordingly.

As far as trimming a thermal model I've always found that getting the cg back as far as is comfortable makes the model more sensitive to any vertical air movement which is a great help spotting when it's entering an area of lift or sink. This cg trim also allows the greatest working speed range for the wing loading.
I don't see any reason why this trim should be susceptible to stalling unless the model's being flown too slowly in the first place or if the elevator movement is too coarse. Programing exponential elevator in the Tx can be a great aid to smooth thermal glider control.

[Erfolg]

Rather than being super sensitive in pitch what is very desirable is for a model that to essential purposes takes care of itself. If we were to consider a model where we had managed to get to a great height, where it appeared to be all but a small silhouette. In these circumstances, the model needs to take care of itself, as we observe it. We need to be confident that it will not suddenly stall, as we will not know with any certainty, how to recover the situation.

Most of the time when flying a thermal glider, we are observing, what it is doing. When we interfere with it, it is because we have decided there is some benefit to gain. It could be because we belive that there is good air, or we need to move somewhere else, always for some benefit.

There is no substitute in understanding for knowing your model and understanding the message it is giving.

As for slope models, others will know far better than me what is required, and what is not wanted.

[PatMc]

Why assume that the will be super sensitive in pitch when trimmed with the cg as I suggested ?

When fully trimmed I aim to have a glider able to fly hands off as much as possible whilst thermal hunting. It won't be trimmed to fly too close to stall so will certainly be able to take care of itself until I decide to change it's course. Trimmed like this any small changes in attitude not due to control inputs are easier to detect than in the same model with a forward cg.

[Mike Etheridge 1]

This old Micro Mold Sprite I bought in the 1980's has an all moving tailplane. My nephew and I set the tailplane to the neutral position such that it was parallel to the fuselage centre line but the plane has never been difficult to trim and still has old 27 mhz radio such that the trim needs adjusting each time the plane is taken out for flying.

[Frank Skilbeck]

I have two all moving tailplane gliders, a 4.2m Topaze and 3.2m Multiplex Fiesta, neither of them need re-trimming when they go out, so your adjustments are probably to compensate for a linkage or servo problem, unless the main wings don't go at the same incidence when refitted or the c of g changes between flights. One other thought, do you take the tail plane halves after after flying, if they are a slightly different section and you put them on the opposite way round you'd have to retrim to compensate also.

[Mike Etheridge 1]

The adjustments on the Sprite are only necessary because the old Futaba M4, 27 mhz radio has no memory and the trims get shifted accidentally or have been moved to suit other gliders: Flair Heron, Bluebottle, and Colt.

The tailplane is a flat section on the Sprite and is linked with two piano wire couplers that push into brass tubes. One of the couplers acts as a pivot which I guess is the same arrangement on other planes? I always leave the tailplane in place but of course remove the 8 foot span wings for transport. It could do with some ailerons and less dihedral. It does fly from a bungee and I think this causes the piano wire links to the wings to bend giving excessive dihedral. Last flight was some years ago at Castle Hill Kent.

[Frank Skilbeck]

Posted by Mike Etheridge 1 on 08/11/2015 11:15:25:

The adjustments on the Sprite are only necessary because the old Futaba M4, 27 mhz radio has no memory and the trims get shifted accidentally or have been moved to suit other gliders: Flair Heron, Bluebottle, and Colt.

Doh, I'd forgotted that an old 27mhz radio would probably have mechanical trims, how soon we forget eh

[Erfolg]

Patmac

It all depends on what you call a forward or rearward CG.

I would not advocate an extreme forward CG, as the model will exhibit greater longitudinal stability. What this means in the extreme, is that the model is reluctant to do anything in pitch, other than what it is doing. An input results in the same characteristic, initially it resists, then wants to continue doing what is the new norm. In control theory this would be described as over damped.

It is the same with an extreme rearward CG. Which is often tempting with gliders, due to their long tail moment arms, trying to minimise nose weight. In this case, the most minor disturbance, be it a gust, or a control input, results in a model that is very likely to stall, alternatively, does not settle , it does not self damp. You on the Tx can often chase the oscillations. Again in control theory an under damped system.

It is not the AoA that is the problem, as the model will stall at the very same AoA irrespective as to the position of the CG. The problem is one of system stability or in our case the relationship of the CG to the Aerodynamic Centre.

If some have the impression that I am advocating an extreme forward Cg, that is not so. Although a extreme rearward CG is not desirable.

The great success of the likes of the PZ Radian and the Multiplex et al versions is that the CG is positioned to provide predictable handling, they are all light weight, which has endowed all of them with pretty impressive performance. In being able to live with specialised models from no wind right up to moderate wind of 12-15mph. That is when tootling about, not required to go to extreme speeds, and the lack of braking makes for precision spot landing, a more skilled and slow business.

In my opinion you need a predicable model and just as important, you need to know your model, to read what it is doing.

Edited By Erfolg on 08/11/2015 12:43:32

[PatMc]

Erf, I've never mentioned nose or tail heavy extremes nor have I mentioned AoA. I can see there's no way we are going to reach any sort of agreement when we seem to be considering different things & the subject matter is being expanded with each of your posts.

I'm dropping out of this thread since the OP hasn't anything to do with thermal glider or their performance & the query seems to have been adequately answered by Russ d.

[Erfolg]

Patmac, I feel I have been deliberately misrepresented. I never advocated a forward CG. Strongly warning of the problems with a rearward CG.

[Mel Jones]

I would like to thanks all who contributed to answering my question.

You have been a great help.