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Old Oct 06, 2014, 07:16 PM
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Extreme climb

I'm having an issue with a scratch built foamie, pusher delta I put together.

Basically the issue is related to the rate of climb the delta has when throttle stick is applied. What I'm seeing is as the throttle is increased the delta goes into an ever increasing steep climb. I have to throttle back to below half throttle to get the bird to level off or try to counter the climb by adding down elevator.

I end up with a lot of throttle management and a slow flying bird.

The bird is basically $ tree foam with a wing span of 30 inches and a length of 20 inches, with power being provided by a 1700kv motor.

Any help would be appreciated.
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Old Oct 07, 2014, 12:41 AM
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Try moving the CG forward in tiny increments (like 1/8 inch) and retest till you get it where it feels right for you.
Also: Make sure your thrust line passes through the vertical CG (this has been discussed in many posts - do a search). If it is passing below the vertical CG, that could be another cause of the symptom you're seeing.
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Old Oct 07, 2014, 12:37 PM
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Quote:
wing span of 30 inches and a length of 20 inches
Need to confirm planform before offering suggestions. Does your wing look like the one in the linked cg calculator? If not adjust the factors in the calculator and post a link in this thread. (Deep link - This specific wing at lower right of calculator)

http://fwcg.3dzone.dk/?wing_span=30&...ow_mac_lines=0
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Old Oct 07, 2014, 09:48 PM
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No Sir (DT56), my wing does not have this delta shape. The bird has the shape of a chevron (traditional flying wing). Additionally, the elevons are 2 inches wide and the 1700 kv motor is affixed to the fus with a stick mount. I've added a photo of the bird.

The following are its dimensions:

Wing span: 30 in. / Root cord: 13 in. / Tip cord: 11 in / Sweep: 10 in.

Tried to get the Deep Link to show as yours did but I'm not savey enough with the computer to get it work properly.

Using the numbers above I got a CG of 7.27 inches, which is close to the build suggested cg of 7.

If I've done things correctly with the cg calculator it appears to me the bird is about right on.

I appreciate your time....
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Last edited by HeliMinion; Oct 07, 2014 at 10:23 PM.
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Old Oct 08, 2014, 02:16 AM
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Quote:
Originally Posted by HeliMinion View Post
Using the numbers above I got a CG of 7.27 inches, which is close to the build suggested cg of 7.
Lots of times what the calculator and/or the plan recommendation suggest for CG will not perfectly suit your particular build. And as you pointed out there's a quarter inch difference between what the calculator and the plan suggests - a world of difference in the chevron wing situation.
Have you tried moving CG forward in small increments and re-testing?
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Old Oct 08, 2014, 07:39 AM
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United States, TN, Dresden
Joined Sep 2013
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Quote:
Originally Posted by HeliMinion View Post
I'm having an issue with a scratch built foamie, pusher delta I put together.

Basically the issue is related to the rate of climb the delta has when throttle stick is applied. What I'm seeing is as the throttle is increased the delta goes into an ever increasing steep climb. I have to throttle back to below half throttle to get the bird to level off or try to counter the climb by adding down elevator.

I end up with a lot of throttle management and a slow flying bird.

The bird is basically $ tree foam with a wing span of 30 inches and a length of 20 inches, with power being provided by a 1700kv motor.

Any help would be appreciated.
Your tail heavy. These wings need to be nose heavy. As for what the cg is ?? It'll fly with the battery on the nose.
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Old Oct 08, 2014, 07:31 PM
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nuteman / gabe221 -

Yes, I have attempted to change the CG by moving the batt forward. If there was a difference in the flying attitude it wasn't noticeable. I don't have much more room for forward movement but I'll use what I have up. I suppose I could at weight and see how that turns out.

Thanks for the replies gents...
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Old Oct 08, 2014, 07:43 PM
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Use a heavier battery --30" span 3s 2200-3300
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Old Oct 09, 2014, 07:49 PM
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Many models climb when the CG is too far forwards. I know this is not what some people have said but, if the CG is too far forwards then you need to fly using more up elevator to balance the nose heavy airframe in straight and level flight.
As the speed increases the up elevator becomes more effective and the aircraft climbs.
Many people make this mistake.
You can alter the motor thrust line which can help.
One way you can check the CG is to fly it inverted and see how much down is required to hold it level.
If you have to push lots of down then it is nose heavy. If it climbs when inverted you are in a dangerous situation because the CG is too far back. This is an easy test to do.
Flying wings tend to need a slightly more nose heavy CG than airframes with tailplanes.
If you cannot fly inverted get some help from someone.
Jim
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Old Oct 09, 2014, 08:35 PM
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Here is another data point for CG that has worked well for me (click on the diagram): http://www.charlesriverrc.org/articl...GMarkDrela.htm
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Old Oct 09, 2014, 09:10 PM
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Fox-c / 1fullbird-

In reading yours and the other posts i find myself asking what constitutes too far forward/too far back? It was and has been my understanding that each model has it's specific CG location. In my model the CG location is suggested to be 7 inches back from the nose. The CG calculator (using my model's dimensions) indicated the CG should be 7.27 inches back.

Are you saying I can/should be balancing the model to change the location of the CG?
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Old Oct 10, 2014, 01:08 AM
Thailand
Joined Aug 2010
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CG problem

Hello, what I am saying is that you can work it out on paper and get an approximate CG position that will allow you to fly the thing but,, the only real way to find the perfect CG is to fly it. Even full size get it wrong sometimes and have to shove a load of weights in the fuselage to get it correct.
A very good way to check if it is correct is to fly it inverted. If you are flying a forward CG then you will have a lot of UP elevator to fly level the right way up but when inverted you will to push a lot of down elevator to fly level.
The more down you have to push in then the more nose heavy it is.
You can get to the point where you have moved the CG back too far and the model will need no down elevator to fly level and will probably be unstable. Deltas can go into a flat spin in this condition.
Most people think you should move the CG forwards to correct your problem but this is not correct.
If the CG is correct then it could be the engine thrust line and you could raise the motor above the wing centre line which would push the nose down with power or angle the back of the motor down and try it.
Regards Jim
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Old Oct 10, 2014, 03:45 AM
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I'd say it's not a CoG problem as much as a thrust axis problem. If the thrust vector from the prop isn't passing through the CoG then it will create a pitching moment which will need to be counteracted by elevon deflection. More thrust = more pitching moment.

Hang the aircraft by a thread from the prop like a plumb-line and see if the thread and the prop axis line up somewhat.

Even so, any aerofoil will generate a pitching moment to some degree, so there's complicating factors, as always...
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Old Oct 10, 2014, 05:21 AM
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Looking at your motor mount in the pic, it's hard to tell if there's any appreciable thrust line issue (below vertical CG). First make sure that's sorted out and re-test.
You said you moved the CG forward and no discern-able change resulted, but by how much did you move it?
It is also normal to have at least some mild pitch up on power increase, so it's a matter of getting it to a level that you feel comfortable with.
Unless you have a lot of reflex trimmed in (up-trim), it almost certainly not nose heavy, but there's nothing like empirical evidence, so here's what I would do:
First try moving CG back a bit from where it originally was. Is it better or worse?
Move CG forward (you ran out of nose room, so use some dead weight or heavier battery). Is it better or worse?
You will eventually find what you like with this method.
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Old Oct 10, 2014, 06:51 PM
Thailand
Joined Aug 2010
519 Posts
If you really can't fix it with the motor thrust line or the CG you can mix in some down elevator with throttle. Not a great idea but it can work for some models.
What is the wing section like.?
Jim
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