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CG on Swing Wing Model

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Cliff Bastow24/01/2015 19:42:17
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824 forum posts
426 photos

I have purchased this from a member of my club who is giving up due to old age.

It is nicely built and on getting it home I discovered that the swing wing works from a servo and is not just ground adjustable as I had thought.

My question now is how do I work out the cg for both wing positions and how does it change as the wings move. Obviously the cg will move backwards as the wings sweep so do I set it up as a delta with the wings swept back?

Any help gratefully received.

Thanks

p1242044.jpg

Phil Cooke24/01/2015 19:50:05
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2245 forum posts
1532 photos

Hey Cliff

I wouldn't expect any major issues - yes the balance point moves rearwards a little as the wings sweep back but so does the centre of lift - so everything should stay more or less harmonious - you may find you need to alter the elevator trim a little with the wings fully back - but this will be model dependant so I cannot recommend how much to add and when.

Best bet is to balance it and trim it for flight with the wings as shown, then incrementally feed the wings back with the model high enough to recover if it all goes nasty, but close enough so you can see and get a feel for any elevator compensation you may need to apply.

Nice array of Humbrols you got going on there by the way - more than my LMS!

Mark Kettle 124/01/2015 20:54:20
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2343 forum posts
1364 photos

Yes I'd agree with Phil Cliff, I did some measurements on the plan of the Tornado which is currently being mass built by Phil and others in the Pssa section on here. I've took some photo's with the balance point marked on.

The Tornado is similar to your Tomcat, look at the pictures and see what you think. I think the balance point stays the same and the pressure moves around abit, and I think it's not beyond the control of a trimming elevator.

wings out tornado.jpg

wings in tornado.jpg

wings in tornado model and drawing .jpg

wings in tornado the maths.jpg

Cliff any chance of showing us the mechanism of the swing on the Tomcat?

Edited By Mark Kettle 1 on 24/01/2015 21:14:14

Cliff Bastow25/01/2015 06:47:53
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824 forum posts
426 photos

Thanks guys that was basically what I thought.

Mark I will upload photos of swing wing mechanism when I get home from work tonight.

Basically it works off one servo with an extended output arm. Actually i am not totally happy with it as the servo buzzes quite a bit at the end of its travel both ways. I am considering chainging it to a servo for each wing but am cautious about adding more weight.

Thanks guys.

Cliff Bastow25/01/2015 09:19:06
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824 forum posts
426 photos

Just a thought, would it be better to swap the servo for the wing sweep to a retract type servo, as I understand they work differently at the end points?

Is the servo buzzing at the end points because it is under too much load, or because it is being held there by a signal, wheras retract ones are not? having said that I dont have that problem on throttle servos or standard servos on flaps so maybe it is the load?

I can see that this model is not going to be as straightforward as I first thought!

The large collection of humbrols are mostly left over from my plastic Aircraft modelling days and have been collected over many years.

Edited By Cliff Bastow on 25/01/2015 09:22:11

Dane Crosby25/01/2015 10:19:41
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239 forum posts
23 photos

Hi Cliff, The centre of pressure will move rearwards as the wings are swept . At low speed however, the CP will move forward again.

The CG won't move greatly due to the relatively light moveable part of the wing. If you are planning ailerons then they will be less effective on the swept wing than the straight wing but I guess that you will be using spoilers and tail elevons as the full size.

I suggest that you only use one tough servo working a robust sweep mechanism. two servos may not operate at the same rate and if one fails you could be left with a very interesting control problem!

The sweep hinges will need to be strong to resist friction caused by the wing flexing up whilst producing lift in flight. Test the mechanism on the ground with the model only supported near the wing tips to show up any binding and friction snags.

I'm looking forward to hear the flying reports from you. Good luck with it.

Edited By Dane Crosby on 25/01/2015 10:20:18

Cliff Bastow25/01/2015 11:16:50
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824 forum posts
426 photos

Hi Dane,

Control is by elevons only, I presume to keep things simple. I had not thought about the implications of one wing sweep servo failing but as you say could be interesting! I will keep it at one servo and just upgrade it and make sure all the linkages are free moving.

I have also had to alter the rudder as it had one Central servo driving twin snakes which did not seem very free moving and caused centering problems. I have removed this and fitted individual rudder servos in the rear fuselage with short push rods.

The elevon servos were giving poor centering also but I discovered that this was because the servos were fitted in depron boxes but were not glued or screwed into the boxes and so were free to move up and down!

I can see that I am going to have to give this bargain! a very good look over and some careful thought!

Unfortunately I do not know the original owner to ask advice as it was being sold through a third party at our club. The owner was not a member of our club. As I said earlier it is well made and finished but I have been left wondering about some of the engineering, and indeed if it ever flew.

Cliff Bastow26/01/2015 19:58:20
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824 forum posts
426 photos

p1262050.jpgp1262049.jpgp1262048.jpgp1262047.jpgp1262046.jpgMark,

Photos of wingsweep mechanism as promised. Sorry they are later than I promised but I did not get time last night.

I apologize for poor quality but I think you can see what is going on.

As I said earlier I am not really happy with it as I think its putting too much load on the servo, but it might help give you some ideas.

Cheers Cliff

Mark Kettle 126/01/2015 20:30:39
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2343 forum posts
1364 photos

Cheers for the pictures Cliff.

Cliff Bastow10/02/2015 10:07:58
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824 forum posts
426 photos

All alterations have been done now except the wingsweep. I still think it needs a bigger/stronger servo but this will entail quite a bit of alteration.

For the moment I have decided to disconnect the servo and lock the wings in each position on the ground. I will then test fly it with the wings in each position to sort out cg issues before I start altering anything in the wingsweep servo mounting area.

I just have not had a combination of free time and decent weather to test fly it yet but I am itching to try it.

Simon Chaddock10/02/2015 10:43:45
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5419 forum posts
2835 photos

Cliff

I would be expect the servo buzz to be caused by excess load usually by the mechanism fouling somewhere.

The best way to test it would be to remove the servo arm but leaving the linkage attached and by hand see just how much force is required to get the arm to the full out/in positions checking if it rises significantly at the extremes of travel.

If the mechanism works freely I certainly would not expect there to be any load with the arm in the second picture as it is 'at centre' so the servo could handle an extreme load. The complete reverse at the other end of the travel as the arm is virtually at 90 degrees.

I assume the 'at centre' position is with the wings in the fully forward.

Cliff Bastow10/02/2015 10:53:03
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824 forum posts
426 photos

Simon,

No the servo is at centre position at about half wingsweep. It is at full travel either way with the wings fully forward or fully swept.

I suspect that the rod ends are moving up and down as the mechanism moves and this is putting a twisting moment on the servo arm at each end. Ie one rod is pushing its end of the servo arm down and the other is trying to lift the servo arm up.

Cliff Bastow10/02/2015 11:23:18
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824 forum posts
426 photos

I have done some research online and the servo that is fitted is actually a metal geared one with a high tourque rating so it should be ok. I will have to have a look at the rods and where they move to see if I can find any binding but this will mean removing the wings if possible!

Martin Harris10/02/2015 13:02:58
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8741 forum posts
214 photos

Getting back to the original question, if in any doubt at all, I would always set the C of G of any type of model in the takeoff/landing configuration - an unsuitable setting at any other configuration is an option not a necessity!

Cliff Bastow10/02/2015 13:28:02
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824 forum posts
426 photos

That is what I have done Martin, I have set the cg with the wings forward, I dont think things will alter much with the wings swept as above posts.

Cliff Bastow12/02/2015 11:51:05
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824 forum posts
426 photos

p1242045.jpgp1242044.jpgFor some reason my initial post seems to have lost its photos so I have added them again.p1242043.jpg

Andy Meade12/02/2015 11:56:12
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2597 forum posts
679 photos

Very nice, will make a lovely refurb project. Is she a pusher or a lovely PSS'er?

Cliff Bastow12/02/2015 12:49:38
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824 forum posts
426 photos

She is a pusher. In the photos I had removed the origional motor as it had a snapped shaft. I have fitted a replacement now and just wiating for nice weather to test fly.

Andy Meade12/02/2015 13:04:07
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2597 forum posts
679 photos

Good luck and keep us updated Cliff, there are swing-wings in my future I am sure of it

Cliff Bastow16/02/2015 15:50:27
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824 forum posts
426 photos

p2162071.jpgp2162070.jpgp2162069.jpgwell I maidened it today and as you can see from the attached photos it did not go well! On launch it rolled immediately right very fast and dove into the ground inverted before I could stop it. there was a decent breeze so I don't really know what was wrong. It all happened so fast I did not really get time to draw any conclusions.

Luckily the break is a clean one so should go back together easily.

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