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CofG

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Peter Goldsmith14/06/2019 12:43:51
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14 forum posts

Hello people, just a question to help with my sebart sebach 342. Having purchased the model second hand and done a couple of minor repairs, actually got to fly it for the first time last weekend, (still in one piece so obviously did something right) After checking the CofG with ballancer and a vannessa rig I had two different CofG points so decided to set up somewhere between the two, only talking about a centimeter difference. Anyway my question is, the plane flies ok, at about half throttle flies nice and level, give gas and it climbs, so far so good, now when I fly straight and level and cut the throttle to zero, the plane begins to dip, down, picks up speed and then climbs, slows again and drops and so on, its not wild just like a mild roller coaster. Would this indicate that it is still a little nose heavy, tail heavy or just about right. Many thanks for any advice . Peter

J D 814/06/2019 13:41:21
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1198 forum posts
74 photos

To far forward I would say.

Your elevator response may be sluggish and inverted flight can only be held with a lot of down elevator. Getting into a proper is spin difficult or impossible and will also pitch excessively with speed changes.

  A Sebart 342 and similar types should have a neutral handling at most speeds, in other words it keeps going in whatever direction you last pointed it, at least until the energy is gone.

Edited By J D 8 on 14/06/2019 13:51:44

Mike T14/06/2019 13:46:12
405 forum posts
28 photos

Sounds a tad nose-heavy to me, but clearly nothing to worry about. I'd much prefer that to a 'galloping gertie' rearward CG. You could try moving the batt. back a bit. I'd be looking for a steady non-rollercoaster descent.

IMO you can only live with a marginal CG if your elevator set-up, from the servo gears to the hinges, is completely free from play or backlash. This ensures precise return to the trimmed setting when the control is centralised (accurate gimbals on the tx are vital too). And most setups aren't like this...

Colin Carpenter14/06/2019 14:02:43
548 forum posts
35 photos

My Sebbie SBach 30 e flies absolutely neutral with the CG close to the manual indication ! No nasties whatsoever as do my Angel , Mytho and recently sold , Shark ! Colin

Robert Welford14/06/2019 14:10:44
158 forum posts
4 photos

Yes I agree it sounds like the CofG is nose heavy - too far forward

To assess the CofG position for powered aircraft: trim for level flight; pull into 45 degree upline; roll to inverted and observe the trajectory. Aircraft should gently descend whilst inverted.

If the (inverted) aircraft pulls down => CofG too far forward

If the (inverted) aircraft pulls up => CofG too far back

If the aircraft trajectory stays constant at 45 degree upline => CofG maybe ok for 3D, but still too far back for general aerobatic flying.

For gliders dive (in trim state) in a 45 degree downline and see what happens.

Peter Goldsmith14/06/2019 15:26:08
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14 forum posts

OK thanks again to for all the replies, will try out at the next flying oppertunity

Peter Goldsmith14/06/2019 15:26:59
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14 forum posts

opportunity

Peter Jenkins15/06/2019 00:01:03
1217 forum posts
132 photos

Peter, once you have flown and aircraft successfully, ignore measuring where the CG is and just follow the advice that Robert Welford has given. Measuring CG accurately is not especially helpful as the optimum CG position will only be found by flying the aircraft and adjusting the CG to meet the criteria set out. Once you have established the optimum CG by experimentation then by all means measure it accurately to allow you to return to it if you ever make any major repairs or change equipment.

The single most powerful trimming tool we have it adjusting the CG position. Any CG position given by a designer will only be there as the starting point as each aircraft will be different.

Peter Goldsmith04/07/2019 17:07:36
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14 forum posts

Hello all again, seems I have the CofG cracked with my S Bach, seems to fly now with no problems. The seller of the model said he had flown with a 4s 2600 mah batterie, I bought a couple of robbe EVO lipos but seem to have a short flying time of around 5 minutes even though I am only using about half throttle until I get used to the model, I was thinking that I need a larger mah batterie, Any suggestions for good lipos ,

Thanks Peter.

Colin Carpenter04/07/2019 17:17:46
548 forum posts
35 photos

Peter. I take it it's a 30 E if you are using 4S ? I have 4 Sebbie 30 planes and usually get 5 minutes whatever I fly like. My SBach uses 3000 4S and don't push it much beyond 5 ! Not a Hacker motor though ! Have just purchased a mint secondhand Angel flown with a 13/6-5 prop and expected a longer flighttime ! Nope ! Just enjoy a superb plane and land safely !!😍😍😍 Colin

Martin McIntosh04/07/2019 21:45:14
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2875 forum posts
1062 photos

Interesting. I have recently built a much lighter version of my old (still flying) TN 72" Spitfire. It climbs like hell on full power and has to be down trimmed a lot in this mode. I have progressively moved the cg further forward which helps but it is now 12mm more than the original which has none of these tendencies. On the last flight I mixed down with throttle which made it flyable but played hell with it when I had to do a flaps down overshoot. Looks like the thrust line is out to me but I am always very careful to set this up during the build. Very difficult to change it now. Any ideas please?

Peter Jenkins07/07/2019 01:20:06
1217 forum posts
132 photos

Martin, as you will know from your F3A days, if thrust line is wrong, tinkering with a mix will only work at one speed. I would think that your best bet is to bite the bullet and fix the thrustline mechanically.

Martin McIntosh07/07/2019 22:18:20
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2875 forum posts
1062 photos

You are quite correct Peter. It is on the list of things to do asap. The big problem with a Spit. will be to keep the spinner central. At 4" dia. any deviation will stick out a mile. Will keep you posted.

Peter Jenkins08/07/2019 00:17:10
1217 forum posts
132 photos

And the real ones didn't have side thrust as far as I'm aware so double jeopardy! Alternatively, you could resort to using a gyro in pitch to look after the problem for you.

Martin McIntosh19/07/2019 14:11:58
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2875 forum posts
1062 photos

Hi Peter,

Having now repaired a broken u/c caused by trying to land 3ft above the runway, I have now turned my attention to the t/l. Checked against the old one w.r.t. the wing c/l and discovered that it is actually about 1/3rd further down already, (basically it be zero w.r.t. the tail c/l) so that is unlikely to be the problem. As stated above, I have progressively moved the cg forward which appeared to be helping but I am not now quite so sure. It is way in front of where it should be. Never had this problem with a Spit. before and as you said, I am supposed to know what I am doing! Any further suggestions would be welcome.

Rather than remove lead I am seriously thinking of swapping the Laser 180 with the 155, which is lighter, in my Stampe. This would suit both models rather better but would involve a lot of work on each.

Denis Watkins19/07/2019 14:39:39
3746 forum posts
179 photos

Is this the 72" TN 1.20 4stroke Spitfire Martin

If it is then we had an issue with forward C of G at my field on a similar build, and the model did not fly well

The builder resorted to a 1.20 and all was well

With forward C of G there was a tendency at cruising speed for the nose to tuck under

Have no idea why, but maybe all the scale stick out bits altered airflow at a certain speed

The model became flyable when fitted out within the design brief

Edited By Denis Watkins on 19/07/2019 14:40:46

Martin McIntosh19/07/2019 15:08:48
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2875 forum posts
1062 photos

Hi Denis,

It is indeed the TN 72" one. I now think that moving the cg back a bit would help. I may have the extended u/c a tad further forward on the new one because it does not tend to nose over on take off/landing, surprising since the cg is now so far forward of the old one which switchbacks on take off and difficult to control, but is otherwise impeccable.

Denis Watkins19/07/2019 16:38:31
3746 forum posts
179 photos

To add Martin, just about 10mm elevator is right

I know why they go 2cm on elevator to stop noseing over

But large movements can cause big problems in the air

And landing as usual, keep it flying until touchdown

You know all this but for the benefit of all

Peter Jenkins19/07/2019 18:26:37
1217 forum posts
132 photos

Hi Martin

If t/l is OK then the usual other suspect is wing incidence. I presume you have checked this but if not worth doing. I would not have thought that moving CG will have too much effect on the tendency to climb as it's primarily stability that you are affecting. Would the wing fairings be contributing to the problem? Not easy to fix if they were mind you!

Having said that, is it worth trying to engineer some downthrust into the setup to try out the effect it has on the problem? Appreciate that the spinner alignment will be ugly but worth trying. I cannot think of anything else that would solve the problem you have.

Good luck.

Peter

Martin McIntosh19/07/2019 21:23:50
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2875 forum posts
1062 photos

Incidence cannot be far off because at cruising speed it is OK ( compromise between high and low speed regarding elevator trim, if that makes sense). I have a laser incidence meter so shall try this on both of them next. The maiden generally required no trim changes at cruising speed.

Nosing over on landing, bar the u/c position, is caused by the use of flaps which tip up the T.E. of the wing just when you do not want it. Trouble is if they are retracted too soon then the model will fall out of the sky.

Denis,

I use lots of expo and have set the throws and everything else as per my original so this is not a problem. The only thing that I have noticed here is that the roll rate on this one is faster with the same set up but this is almost certainly due to the lighter wing.

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