Tiger Moth C of G

[Leslie Crane 1]

Hi all, has anyone built the 56" span Tiggie by a company called ELERC and sold by Ali Machinchi (hope spelt right) and flown it. To balance it as per instructions mine needs 1lb 8oz of lead in addition to the 4s battery which seems an awful lot - or am I just not used to building models this size. The fuselage is all light ply so the rear is heavier than a frame construction which may account for this. Any thoughts would be appreciated. Les

[Manish Chandrayan]

If you have done all that could be done to shift weight as forward as possible there is not much you can do but add all the required ballast.

Generally all the Tiger Moth's I have built had the CG at the rear cabane. Is it the same for this particular model as well?

[Jimbo565]

Les

How about this,this is 66" moth but I reckon the c/g will be in the same place.

[Leslie Crane 1]

Hi guys thanks for the advice. I have a smaller tiggie and the C of G is at the rear cabane where it meets the upper wing - as above. This model shows it on a vertical line from the front edge of the lower wing touching the front of the rear cabane strut where it exits the fuselage and is about 25-30% from the rear of the top wing centre section. Funnily enough the model balances at the rear cabane as you both say and in line with my smaller one. However am worried about going with that because that's not what the instructions say and if it is wrong then it will be very tail heavy!! The difference is a good 1.5 ins so it is significant. Les

[Leslie Crane 1]

I should have said balances without the extra weight at the rear cabane strut.L

[cymaz]

 

Try having a go with this calculator

Edited By cymaz on 01/12/2015 17:55:16

[PeterF]

Here is a snippet from a 1/4 scale plan (89" wing span) which shows the CofG for that model at the rear cabane support at the back of the fuel tank. Not exactly what you want but more support for something in that general area.

[Manish Chandrayan]

Leslie could you post a pic from your plan showing the CG? What make is the model? Some online research would help you in finding what others did with the same model.
I have built a Mercury FF kit, a 48 inch from Gordon Whitehead plans another one from DB kit and currently on to a 1/4 scale. All had the CG at the rear cabane struts

[Geoff S]

I think that's the one Tim Hooper is building. It's the same as the one sold by Banggood and HobbyKing. There's some discussion here

The 58" ws DB Tiger Moth CoG is given as 100mm back from the L/E of the top wing. Where have you mounted your battery? IIRC Tim has placed his in the fuel tank bay.

Geoff

[Geoff S]

Posted by PeterF on 01/12/2015 18:36:57:

Here is a snippet from a 1/4 scale plan (89" wing span) which shows the CofG for that model at the rear cabane support at the back of the fuel tank. Not exactly what you want but more support for something in that general area.

That seems a long way back, Peter. On the DB sixth scale (58" ws) Tiggie that would be 50mm further back than shown on the drawing. ie 150mm from the top wing L/E as opposed to 100mm.

Geoff

[PeterF]

Posted by Geoff Sleath on 01/12/2015 20:15:02:

Posted by PeterF on 01/12/2015 18:36:57:

Here is a snippet from a 1/4 scale plan (89" wing span) which shows the CofG for that model at the rear cabane support at the back of the fuel tank. Not exactly what you want but more support for something in that general area.

That seems a long way back, Peter. On the DB sixth scale (58" ws) Tiggie that would be 50mm further back than shown on the drawing. ie 150mm from the top wing L/E as opposed to 100mm.

Geoff

Geoff,

Yes that is correct, 230mm for the model I am building from the Falcon kit = 150mm at 1/6 scale. The flair 1/4 scale is 190-210mm and the plan for the Duncan Hutson 1/4 scale appears to be around the 200mm mark as well. Both of these correspond quite well to the CofG of the full size plane and would give 133mm at 1/6 scale. The DB plan appears to be well in front of the CofG of the full size and the model I am building is therefore somewhat behind the location for the full size. The designer of the Falcon Kit states that this is OK and elevator response is OK so I will follow that. The Falcon kit is much closer to scale in terms of thing like wing section than most other kits, perhaps that has something to do with it.

Something to keep in mind.

[Tim Hooper]

Leslie,

As Geoff says, I'm building the same model, but not as far advanced as you.

I was also concerned about the stated CG position, as it seemed too far forwards, whereas I'd heard that most Tiggies balance on the rear cabane strut.

Using my CMEI software, I worked out the balance point on each wing using 28% MAC as that's my usual starting position, and then drew the wing sections and stagger full size.

A line was drawn between the two points, and then it was bisected to give an average position, before projecting this point up (to the rear cabane) and down (to a position 40mm behind the lower LE).

That's some 40mm behind the specified position, and the one I'll be using!

There's more info on this generic thread on another forum;

http://www.rcgroups.com/forums/showthread.php?t=2450345

tim

 

Edited By Tim Hooper on 01/12/2015 21:43:25

[Former Member]

[This posting has been removed]

[Leslie Crane 1]

Thanks guys there is a lot of info to digest here but the consensus is that the C of G for this model is a bit far forward which is why I was concerned in the first place. I think the fact that the fuselage is all ply means that the rear will be heavier than usual so perhaps rear cabane strut is just too far back . I will go with Tim's position as a start and thanks for all your help. Tim, I know that as you are doing this you will be preparing an article so you wont want to know issues in advance but if you want to compare notes as you go along to see if things are consistent between kits please feel free to contact me on [email protected] Les

[Geoff S]

Doing a similar exercise to Tim's on the DB Tiggie drawing results in a CoG at about the same point as he calculates ie 125mm (5" ) back from the top wing L/E and 25mm further back than that shown.

I'd be delighted if that was a good position because I'm struggling to achieve the specified CoG without loading some lead into the front. There must be someone here who's built and flown a DB Tiggie who can enlighten us. After all the time I've spent tarting mine up I'm reluctant to test fly it with a potentially rearward CoG. Of course I could get the (in)famous Reg Shaw to do the test, he offered at the club AGM and perhaps I should take him up. He's less likely to stuff it in than I am.

Geoff

Edited By Geoff Sleath on 01/12/2015 22:55:32

Edited By Geoff Sleath on 01/12/2015 22:56:08

[Gary Binnie]

Good stuff!

The full-size loaded C of G range is 7 to 15 inches aft of datum, the datum is the leading edge of the lower wing at the root.

Our Tiger typically flies at 12 inches aft of datum. The C of G is calculated during weighing with the fuselage top longeron level.

If your model is reasonably scale in geometry then you could scale the dimension (12 inches) to be in the ball park.

My Flair Tiger has over two pounds of lead sitting behind the engine despite building the rear end as light as possible!

Cheers

Gary

 

 

Edited By Gary Binnie on 02/12/2015 00:08:17

Edited By Gary Binnie on 02/12/2015 00:08:43

[Leslie Crane 1]

Hi guys. I balanced the tiggie at 5" from leading edge of top wing as per your calc Tim and it only reqd 10oz of lead instead of the 24 I mentioned in my first post if I had balanced where the model diag says to do it. What a difference. Without the wings it balances at the same 5" back point which to my simple brain confirms your calc Tim unless my logic is wrong!! Incidentally I kept looking at the wings and thinking they don't seem quite right but couldn't put my finger on it. Wing tips wrong, so obvious, what a plonker I am. I haven't added rib caps or the riblets and just feel the end result is a little weak but the first bad landing will confirm or refute that. les

[Jimbo565]

Les

You think your wing tips are wrong,what about mine.

[Gordon Whitehead 1]

Hi Guys

I remember that the late great Peter Russell once wrote in his RCME S&L column that the CG on a full size Tiger Moth is at the position of the front cockpit compass. That might well be within the range mentioned above by Gary.

Gordon

Edited By Gordon Whitehead 1 on 03/12/2015 18:29:48

[Gary Binnie]

I like that Gordon, next time I'm in the front I'll look at the compass and marvel that it's all balancing around it!

There is a lovely 50" Tiger Moth plan on Outerzone by Harold Towner, he has put the CofG dot on the front compass. That is a lovely plan, looks like he's used the full-size blueprints, even shows a left-hand propeller.

GB

[Tim Hooper]

Posted by Jim Carss on 03/12/2015 17:09:27:

You think your wing tips are wrong,what about mine.

Jim,

Your upper tips are perfectly correct. It's just the rest of the wing that's wrong!

tim

[Jimbo565]

Cheers Tim

I knew there was something not right,sussed it now, the bottom wing is the wrong way round, it should fly ok now.

[Leslie Crane 1]

Tim thanks for your help last year with the CofG of the ELEC R/C model which you are serialising the build for RCME. I balanced at your recommended point and it took 9oz of lead. It was maidened a few weeks ago and flew beautifully on a 900kv motor, 12x6 prop and 4s 4000 battery with a 60amp ESC. If it is used a friend has sent a photo to RCME today

Les

[Tim Hooper]

Les,

I'd already received word of your success, via your mate Grahame. Well done!

tim

[Geoff S]

That's great news, Les.

I maidened my DB Tiggie in December on the only decent day all month and it flew well with the CoG on that specified on the plan ie 100 mm back from the L/E of the top wing. It took a lot of lead to get it there so I'm hoping to remove ballast progressively to bring further back - perhaps by as much as 30 mm.

Mine has an 870 kv motor with a 13x4 prop on a 4S 4000 mAH and 60 amp esc. My current telemetry told me it was drawing only 20 amps on level flight and up to 50 amps flat out.

Here's to some better weather

Geoff