delta c of g

[john melia 1]

DELTA PUSHER CONFIGURATION

sorry guys i'm back to bug everyone again the plan of my force 1 delta stipulates the c of g to be 13 inches from the trailing edge , but when balancing at this point the model is extremely nose heavy ,

on the plan it shows the balance point to be approx half way along the fuel tank , but on mine this point equates to just the very front edge of the tank ,there is no room to move the tank in the bay , if i move the c of g 1/14 inches forward the model balances just slightly nose heavy , and the balance point is more towards the centre of the fuel tank .

what do i do ? i cant move the tank , and i cant move the battery which is right in the nose (its a 2s lipo)

[Gary Binnie]

Looking at the Balsa USA site it says the C of G is 13" forward of the control hinge line (not the trailing edge)?

In another photo of your model there seems to be no centre control surface (presumably the elevator), is that right?!

Couldn't help noticing that your wing leading edges look very blunt, almost square, is that per the instructions?

Cheers

Gary

[john melia 1]

hi gary yes no elevators , I decided to go the elevon route , I dont understand what the difference is between 13 inches from the trailing edge (not including control surfaces) and control hingle line

[Gary Binnie]

I would have taken 'trailing edge' to mean the trailing edge of the control surfaces without seeing their note, so they mean the trailing edge of the wing structure. Would have been clearer if they gave the distance aft of the root leading edge which is more conventional!

If the plan is full-size then perhaps just measure directly.

Gordon Whitehead gives the CofG of a delta as 1/6th of the average chord in his RC scale aircraft book.

You could input your model dimensions into this calculator and see what it comes out with (I would take the root measurement as the centre of the wing and not the side of the fuselage and include the control surfaces).

Cheers

[Erfolg]

You have to be providing good advice in the use of the CG calculator.

In essence they are a more sophisticated method of finding the 30% area point (ignoring reflex) with a additional bit of calculation which takes account that it is the mean aerodynamic chord that is wanted. Which is forward of the 30% point.

None of which matters of what and how it is done, it is the value you want. More importantly the value calculated provides a safe position.

I am sure that once the figure is calculated, it will be obvious from where it was intended to measure.

I do suspect they have chosen the hinge line, as this is the point of where the reflex starts. The trailing edge is just as covenant though.

Edited By Erfolg on 05/12/2013 21:05:07

[john melia 1]

gary I'm not sure what you mean by " I would take the root measurement as the centre of the wing and not the side of the fuselage and include the control surfaces"

sorry for the dumbness but this is my first delta wing , would it still fly even though it is pretty nose heavy ?

[Matt Jones]

John, if you extend the line where the wing meets the aileron to the centreline (where you joined the wings) this should be your reference. The hinge/aileron join is the 'hinge line', measure from here if that's what the instructions say.

Also, very concerned on the bluntness of your leading edges, that can't be right and needs changing if you want any hope of a) predictable flight characteristics and b) a landing speed less than warp 4.

[john melia 1]

thanks matt , heres a photo off balsa usa , the leading edge is quite flat too .

[john melia 1]

matt I'm still confused , the wings were never joined , it was built as one wing over the plan , no join .

[Erfolg]

I am not convinced that the LE is blunt at the wing root, in the picture.

A blunt LE will almost certainly be the cause of a lot of turbulence, generated in a probably unpredictable way.

Well rounded LE, allow the stagnation point to move in a smooth manner as the AoA changes. This allow an effective change in camber. Sharp edges, be it a blunt edge or a knife edge, encourage sudden changes in flow patterns, rather than smooth transitions.

I do believe Matts advice has a lot of merit. The very minimum is to study the drawing and or the instructions on the build.

I would imagine you have quite a bit of experience John, in flying models. It certainly looks that it could be a very fast machine and I guess you will have considered the colour scheme to aid orientation.

[Gary Binnie]

If you extend the line of both leading edges inboard they would meet at the tip of the delta (or triangle), this point measured back to the trailing edge is the root chord, the distance from that line to the tip would be the half span.

Nose heavy is better than tail heavy. There is a saying 'nose heavy flies badly, tail heavy flies once'!

Are you using high volt servos with the 2S lipos? 7.4 volts unregulated would burn standard servos out quite quickly.

Hope you get there with this, I'm looking at building an F-7U Vought Cutlass which has its own aerodynamic puzzles!

[john melia 1]

yeas gary I'm using align ds 610's , the lipo is through a ubec giving 6v , I'm basically in a quandry now with this delta , I think I'll have to take it to someone with far more knowledge of deltas than me , I may know a guy who just might be able to help.

I'm now also worried about the leading edge , do I get some stock and make a rounded leading edge ? or go with whats there , the force 1 in the photo above , does seem to have a flat leading edge ( to my eyes anyway) ,

although the instructions say its a fast model , they also say it flies very slowly as well , I have a flying friend who spends his life searching for faster and faster models , he has a rad-jet which he now has running on 6s and is clocking around 180mph , so maybe if I get this sorted , I'd ask him to initially fly it .

there seems that theres nothing I can do to make the model less nose heavy , as theres nothing aft of the c of g other than the engine , even the tank is forward of the cg .

maybe take everything out , and relegate it to hangar queen

[john melia 1]

Posted by Gary Binnie on 05/12/2013 22:15:11:

If you extend the line of both leading edges inboard they would meet at the tip of the delta (or triangle), this point measured back to the trailing edge is the root chord, the distance from that line to the tip would be the half span.

Nose heavy is better than tail heavy. There is a saying 'nose heavy flies badly, tail heavy flies once'!

Are you using high volt servos with the 2S lipos? 7.4 volts unregulated would burn standard servos out quite quickly.

Hope you get there with this, I'm looking at building an F-7U Vought Cutlass which has its own aerodynamic puzzles!

gary would it be possible to draw a small diagram , I still cant work out how to draw a line from the leading edge and end up at the tip , the leading edges terminate at the tip anyway !!

please feel free to direct as many expletives as you think necessary at my lack of understanding at this point

[john melia 1]

One down

one to go

[Matt Jones]

Is that second LE on the right way round? Looks like the chamfer you've put on there shoud butt up to the side of the fus?

[Matt Jones]

Oh, and looking at the plan in the background, measure the CG from where the rearmost point of the elevator would have been if fitted. Do this along the centreline of the fuselage when viewed from above

[john melia 1]

thats where I originally measured the 13 inches from matt , very nose heavy , and yes the chamfer is the wrong way but hey ho , at least its got a leading edge now , cant see the chamfer being the wrong making any difference to its flight characteristics , in fact I'm not holding out much hope full stop with this one .

probably find out at the weekend if the weather improves a bit up here .

[Peter Jenkins]

John, in the picture you posted above, I can see the plan showing the wing leading edge being a nicely shaped aerodynamic section. You need to get your leading edge looking the same.

Re the CG position, can you get the battery under/over the fuel tank?  I take it you can get the battery out for charging?  Charging in-situ is rather risky since if it were to catch fire that's your whole model done for.

Edited By Peter Jenkins on 06/12/2013 17:34:47

Edited By Peter Jenkins on 06/12/2013 17:38:16

[Bob Cotsford]

John, as Peter says you have a drawing hanging on the wall, take a look at the wing section - it shows a rounded leading edge, not the sharp edge you appear to have in the first photo. As a rough guide aim for a radius somewhere between 3 to 5mm, flowed back into the wing section. Try to keep a constant radius all the way along the wing and the same radius on both wings.

[crispin church]

you need to get cog as on plan what engine are you using should be 46 ish

are you doing cog with a full fuel tank ?

if you have to have servos at back and rx and battery over tank if not further back

deltas can fly slowly you dont have to fly flat all the time and keeps your rates low

it worth doing right a good delta is great no fixing wings on no screws to loose or leave behind

[Gary Binnie]

No worries John!!

I just noticed that the balance point on the plan is exactly in line with the fin leading edges where they meet the wing skin, a spanwise line between the two fins would be the position to balance it on.

This is just about where your '1 3/4' mark is in the first photo, you say it is very slightly nose heavy there so fingers crossed your C of G is pretty much as it should be!!

Good news about the servos and UBEC, I had to ask!!

I'm still puzzled about the lack of elevator, is that a design option to leave it off? Your elevons look a bit on the small side to do the job and the central elevator would be working in the propwash which would make it more effective.

I think the leading edge mod was the only way forward, well done for persisting.

Cheers

Gary

[Erfolg]

I personally use elevons, partly because they are simpler to hook up, during the build. Another plus is that the reflex is optimal.

I have used elevons on two fliyng wings, one a Me 163 and my Delta, I cannot see a down side, with Tx mixing.

I am nor sure that deltas can be flown slowly without a fair bit of care. I have found that at high AoA a fair amount of power can be required, and still be sinking. Yet put the nose down, and the thing leaps forward, on the same power setting.

[john melia 1]

Well it was now or never today for the delta , we videoed it , but please be advised i am not responsible for the text comments on the vid

After the flight we found it was very lazy to control input , so my next question is .. could I make the elevons bigger , ie , make them the full length of the trailing edge . Other than that it flew well , wasnt particularly that fast with the sc 52 , but we were running it rich as its a brand new engine .

All in all very satisfactory day , considering I didnt have much hope for it before today , oh and we had to take out all reflex on the elevons it just didnt need it

Edited By Pete B - Moderator on 07/12/2013 21:25:21

[Pete B]

Sorry John, as you probably realised that video is a clear breach of the C of C. Perhaps whoever edited it would like to do it again and exercise some self-restraint, please?

There are plenty of folk who would like to see it, I'm sure....

Pete

[john melia 1]

Thats what I thought pete , thats why I just linked it , it was unfortunately totally beyond my control , I'll ask and see if he'll change it