Balancing a plane model

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zbheli

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I'm mainly using RF to teach myself how to fly a heli... but could not resist trying to create a model for a plane, from scratch. I now have a (very basic) model in 3Ds Max and have been able to export it to RF, and to configure it. It sort of flies, so it's great fun, but...

1/ it tends to pitch up
2/ it tends to roll to its left

So I'd have to constantly apply down elevators and right ailerons.

As for the pitch I guess it has to do with the CG position? Is this adjusted in 3Ds Max or in RF? Any other plausible explaination? As for the roll... I have no idea. Engine torque?

Is it possible to help me at that point, or are these far too broad questions? Are there specific things I should pay attention to? Or should I just go back to reading more forums threads until I understand models better?
 
3D Studio is used to create a VISUAL representation of the plane/craft and bring that into Real Flight.

When you do so, Realflight creates a rudimentary physics definition for your plane.

The Physics definition ( aka Physics model ) has absolutely NO actual resemblence to how a plane of the type you've created in 3D Studio flies or behaves.

You have to DEFINE the physical characteristics of the plane in great detail in the PHYSICS EDITOR.

e.g.

Where is the thrust line at?

What is the incidence of the thrust line?

What is the incidence of the wings? Elevators?

What type of airfoil is used for each?

What is the weight of each individual wing without electronics & gear?

What does the motor power curve look like?

etc. etc. etc. etc. etc. etc.etc.


Exporting a plane from 3D studio into Realflight is only 1/3 of the battle.
 
OK thanks. You're right, I now come to understand that having fun designing a (albeit very basic) plane in 3Ds Max really only is the easy part ;-) I wasn't so sure as to whether RF used some of the parameters of the model for physics setup. What I understand is... you can design a teapot and have it fly like a plane, and the only thing that's important is to position the 3D model on the 3Ds Max grid (x/y/z wise) so it looks like it's at the right place.

At the physics level, I'm wondering what would cause a plane to want to go upwards (would be CG position or winds surface / position / incidence, and also prob. the thrust line)... and to roll to the right (have no idea yet).

But at the moment most of my weights are random, along with a few other parameters. So back to reading the forums and learning.
 
Tail heavy will pitch the nose up but will generally be more unstable. Rolling can be from torque. Check your back torque factor. If you are swinging an oversized prop it can do that too. This is all in relation to the model weight.

While the physics setup IS independent of the 3D model it IS important to match the physics to the 3D model. Keep in mind that RF sees cube dimensions. So, frontal drag on a radial tube-style aircraft will have reduced frontal drag (precent area of square minus percent area of round) and your airfoil selection for top and sides as well. This is something most folks gloss over and leave at the default NACA 0016. The issue with changing your fuselage airfoils is that is messes with your center of pressure. The default no longer behaves as it should. There is lots to learn and I have learned all of this through my years of modeling here and in consultation with real people in the field.
 
willsonman: thanks. I've started to set my weight to more plausible (non-random) values and things make more sense. Now have my wings and most appendices profiles, incidences, etc almost correct. But I totally overlooked the fuselage profile. Absolutely missed the fact that the fuselage has a profile too. Will look into it.

opjose: I have a good physics background so I sort of understand willsonman... the challenge seems to be able to link the theory to what happens in the air!
 
There are other "tricks" as well that you can do to fool the sim. A great example is for odd cases where the wing airfoil you want is right but upside down for what you want. Set your wings up as normal with that airfoil but set your dihedral to 180 degrees and BAM you have the inverted airfoil you want. In custom models you will have to play with your naming conventions and pivots to get it to look right but working around RF capabilities is a really fun part of the sim.
 
Reading and re-reading https://forums.realflight.com/showthread.php?t=17498 and... then re-read again. Learning a lot. But there's one thing I can't explain, it must be obvious but... So once I have sort-of configured my model, the CG is far aft. Somewhere at 60% of the wing cord, starting from the root. The plane is very pitch-unstable.

It is my understanding that the CG should be around 30%. So I tried adding a bit of weight in the nose, but a) I must add quite some weight to move the CG and b) even at full throttle, the plane wants to go down.

So I end up with: either it flies sort-of straight but is very unstable, or it is stable, but it wants to go down.

What's wrong, doc?
 
Adding weight is a "crutch" which at worst should only be used for fine tuning.

If the C.G. is too far aft, then you have not assigned the weights properly.

e.g.

Are you using the right engine and does it have the right weight?

Is the engine far enough forward.

Hint: The engine has one of the biggest effects on the C.G.

Adjust the weights of EACH component until the C.G. is where it should be.

Don't forget the wheels, landing gear, etc... help bring the C.G. forward as does the electronics, servos, etc...

---

As far as "going down"...

Again this depends upon the aerodynamics of the plane.

What is the incidence of your main wing? ( Most wings have a slight positive angle )
What is the incidence of your elevator? ( sometimes a slight negative angle is required )
What is the downthrust you've dialed in? ( 1-2 percent is what is normally used on RC planes.
How high is the engine over the wing line and elevator line?
What type of wing did you choose for your surfaces? ( this has a big effect and inter-relates with the wing incidence to determine glide )
What is the shape of your fuselage "wing"? ( Yup the fuselage is treated as a lifting surface, so choose a wing shape that resembles it in both dimensions )


FAI trimming charts / directions are very useful to learn what to adjust and the effects of the adjustments on the performance of the plane.

Since FAI routines require precision, the pilots REALLY know what they are doing in regards to RC aircraft. That knowledge translates directly to the sim.

---

The engine, engine thrust, etc. have a big effect on the aerodynamics, skewing the results.

That is why most testing is done with the plane heading straight down, or straight up... straight down towards the ground, gives the most accurate results.

For a pattern, 3D plane and most RC planes except trainers, cutting the engine and point the plane straight down, should result in absolutely NO pulling to the belly or canopy and NO axial ( aileron type ) rotation.

That is the first thing you must strive for.

After you achieve that, adjust the engine thust line parameters.

What plane are you trying to model?
 
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First, many, many thanks for your answers. All those points you mention help me move forward. I had a rather big incidence on the elevator (don't know where it comes from) and playing with that incidence does help. Though it does not move the CG. I understand that lead is the wrong solution. Will check the engine weight, battery weight... again.

I'm trying to model a cardboard plane we've built with my 6yo son. Not joking. The one we've built is smaller but the RF model is about 1.5m wide. It really is for fun and learning, and understanding how it all works. See if we can "do it and get it to fly in RF". I know it's going to take time, not an issue.

So it can very well be that our geometry is wrong, although it is remotely inspired from http://www.topmodel.fr/en/product-detail-19053-ipanema-ii-ep-148m-arf
 
If you've modeled the wing shape on the plane example you gave, then that is a FLAT BOTTOM wing as used in trainers.

It requires some significant postive incidence 3+ degrees and the elevator requires an good bit of negative incidence.

In a "nose straight down with power off" test the plane SHOULD pull to the canopy not the belly, as almost all trainer designs do.
 
Also check that the stock CG has not been previously adjusted. Sometimes your base model has has that done and I have forgotten to take all adjustments out. So in the editor click on airframe and verify that the CG adjustments all are at zero.
 
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