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Old Jul 13, 2011, 06:21 AM
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Originally Posted by vtdiy View Post
Start with just straight thrust. Usually, depending on how you mount the engine, you can later adjust that if needed. On my combat version with a BP-21 I looped out of my hand, first flight, so I rebuilt the nose with some downthrust. Next launceh she dove into the ground ten feet in front of me. My best flights towards the end were with zero downthrust.

Yours is much smaller lighter and less powerful, but I'd still start with a straight motor.

For CG, masking tape your equipment on and try to get a cg at about 25-30% of the wing chord aft of the leading edge before making holes.

A little harder to figure on a tapered wing like this one, but with your 60% size plane, I'm going to say it's at about 1" aft of the leading edge.

Or if you can find a CG recommendation by Scotta earlier in this thread for the P-40, take 60% of that. I'm too tired to look for it now.
Thanks a lot! I think I will also try a CG calculator too. I am thinking that my last build failed because I gave too much thrust angle down and right. Let's see how this one goes.

Thanks again for all your help, will continue on it today.

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Originally Posted by mrexcel View Post
In about 40 seconds, I managed to tame that beast. lol
Hey that's pretty cool! Can you link me to the plans? Thanks!
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Old Jul 13, 2011, 06:23 AM
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Yes. This model is calle BYOB http://www.rcgroups.com/forums/showt...highlight=byob
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Old Jul 13, 2011, 08:42 AM
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Generally a very light slow flying model like mrexcel's is not going to suffer much damage (or any, often) if the CG is incorrect. Tail heavy is going to look like slow porpoising. A heavier or faster model will however act uncontrollable and stall or tip stall in with a good chance of destruction.

I've had a few heavier models break in glide testing, so I tend not to do that for them these days. Lightweight models I still do.

On a heavier model I'd rather err on the side of a fwd CG than too far aft. At least you can fly the model and trim out of a diving tendency. But if CG is too far aft, your model is basically uncontrollable.

Here's an video example of what an aft CG looks like in a first test flight, coincidentally in a P-40 model:

http://www.rcgroups.com/forums/showa...6&d=1233689683

The really important things, I think, for model test flight survivability is:

1.) check and double check CG to be as specified
2.) ditto thrust line
3.) ditto all control surfaces neutral
4.) no warps -- or correct them with hair dryer
5.) check that control throws are not excessive -- on the combat version, I am using extremely limited throws to start with, and still have plenty of authority. First tries had way too much throw.
6.) test fly in a field of high grass - hayfield if available (really makes a difference!)

Also checkout the incidence of your cambered wing. By putting a ruler under it LE to TE. The flat wing was at 0 - 0 degrees incidence -- in other words parallel with the tail and fuselage level line. This means the plane will need to fly at a slight positive angle, and a slight up elevator trim. VERY slight, at the speed this model flies per weight.

But yours may be different with the added camber, so check it out. Make sure it isn't off by a lot. If 0-0, as per original design, you may find you need a little up trim when flying.

If light enough to glide test,and you have a field of tall grass to doit in, this will show up there. Adjust until you have a good glide, not too shallow or too steep.
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Old Jul 13, 2011, 10:31 AM
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Thanks for link excel!

---

Awesome tips, you are really knowledgeable. I have learned a lot from you, it's unbelievable.

That video really made me understand what you mean. I'm going to check the CG 35mm aft from the LE. I'll use my battery as a ballast to balance my plane as perfectly as possible, but will give it slight nose heaviness.

I will do some test gliding too.

BTW, when you mean ditto thrust lines/surfaces, you mean everything straight? So no angling at all for the thrust line according to you?

One more thing. This is my gearbox (http://www.amazon.com/Gear-Box-with-.../dp/B003DYUTB0). Now, I am unsure if it is okay to mount it upside down - i.e. the pegs facing upwards. Is this not a good idea? I am unsure as to where the axle should be aligned on the plane. Should it be exactly where the two pieces of foam intersect at the front, or is it alright to have it slightly above that intersection point too.

Many many thanks!
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Old Jul 13, 2011, 11:50 AM
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How did you arrive at 35 mm?

I'm guessing you were considering the width of the wing to be the root width (or chord). But because this is a tapered wing with a 90 degree LE, the average chord is an average of the wing and tip chord.

So calculate your average chord (average the tip and root chord). Then take 25-30% of that and that is probably a good place for the CG if you want to be conservative in a test flight.

Or -- if you found a recommendation by Scotta for the CG distance on the full size plane, just multiply that by 60% for your version.

Actual experience always over-rules calculation.
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Old Jul 13, 2011, 12:00 PM
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Of course, but I have read Scotta mentioning 60mm as the CG for his plane in several posts in this thread. So I multiplied that by 60% to arrive a at ~35mm. Is that alright?

And can you comment on the motor/gearbox please? Thanks!
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Old Jul 13, 2011, 12:02 PM
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Quote:
Originally Posted by astronaut View Post
BTW, when you mean ditto thrust lines/surfaces, you mean everything straight? So no angling at all for the thrust line according to you?

One more thing. This is my gearbox (http://www.amazon.com/Gear-Box-with-.../dp/B003DYUTB0). Now, I am unsure if it is okay to mount it upside down - i.e. the pegs facing upwards. Is this not a good idea? I am unsure as to where the axle should be aligned on the plane. Should it be exactly where the two pieces of foam intersect at the front, or is it alright to have it slightly above that intersection point too.

Many many thanks!
The motor can be positioned any way but keep the prop shaft as close to the centerline as you can. side to side is more important that vertical position. Yes the best location is in line with the intersection of the plane fuselage parts, but not too critical vertically.

More important is to make the motor mount adjustable in some way, if possible. Even if only to add shims or washers.

re thrust angle:

For first tests, straight ahead has a good chance of flying the plane. Down and side thrust are refinements that you can add once you've got it flying with transmitter trim, if needed.

Downthrust is added if the plane tends to dive, motor off. Upthrust is added if it tends to balloon, motor off. Right thrust is added if it tends to turn right, motor off. etc.

But generally, they are refinements, not absolute requirements on a first flight.
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Old Jul 13, 2011, 12:09 PM
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Looking at that motor, maybe cut away a slot in the top fuselage web some, and mount the motor on centerline with pegs down into the side plates of the fuselage -- straddling the center line.

The thrust line will be a bit high, I think it will be okay.

Are those pegs tapped in the ends for screws, or do they glue into the Champ fuselage? Probably the latter.

It would be easy to hot glue this whole thing in, but then hard to adjust it later, if needed.

Any ideas?
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Old Jul 13, 2011, 01:36 PM
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Maybe cut off those pegs, and drill 4 mounting holes for 2-56 screws washers and nuts in the motor plate near the corners.

Glue a small piece of thin plywood to the bottom of the side webs about the same size as the motor plate. Screws go through motor plate, then through foam side members, then through plywood to make a sandwich.

You can then adjust the motor if ever needed.

Hard to tell the scale of things. But maybe a piece of popsicle stick or tongue depressor could be substituted for the plywood if you don't have any.
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Old Jul 13, 2011, 01:41 PM
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Actually, with a piece of tongue depressor, maybe tiny wood screws would work -- 9 gm servos tend to come with screws about the right size.

Another possibility for running the wood is run two lengths of popsicle stick on top of the foam fusealge side webs either side of the vertical center web. These would become your engine bearers. Screw the motor down to that.

Edit. okay final try:

two popsicle sticks like the above suggestion, but this time run them underneath the side web, not on top. Then cut away the side webs to just clear your engine plate, so it sits down below the top of the foam side plates, and on top of your popsicle engine bearers. This will bring the prop shaft down closer to the plane's designed thrust line.

You can adjust down thrust (if and when needed) by putting washers under the motor plate at the rear screws. Side thrust is adjustable if the motor plate holes are slightly oversize, with washers under the screw heads and on top of the motor plate, giving you a little wiggle room for the motor.

Popsicle stick bearers might be 2" long or so, run fore n' aft to give good attachment gluing surface to the fuselage side webs.
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Old Jul 13, 2011, 01:45 PM
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Here is how I have place it. I cut precisely, so the gearbox fits very snugly inside the foam. I thought about it a lot, and found this to be the best way. Without any glue or anything, the gearbox is already quite secure. This doesn't mean I won't need any adhesive, but it means I won't need too much, which will allow me to modify it's positioning if need be relatively easily.

The pegs don't have screws, they are just meant to be glued into the foam. Right now there are no angles, but they can be easily implemented. I will secure the peg parts of the gearbox by just dropping some hot glue where they poke out of the foam. The gearbox, as you can see, will be installed upside down (motor above shaft).

Let me know what you think! Thanks!
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Old Jul 13, 2011, 02:04 PM
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Okay, I like the glue drop idea on the pegs.

What about going with what you have now, but gluing popsicle sticks on top of the foam, cutting the foam away and moving the motor assembly upwards 5mm?

Or you could even put another piece of foam on top instead of the popsicle stick.

Anyway, looks good now, and certainly lighter than anything else, so stick with what you have if you like it.
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Old Jul 13, 2011, 02:34 PM
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Ya I was thinking of ideas to push it up, those are quite plausible ones. Thanks!

However, I think I will stick with this for now though, unless you think it will make a difference in performance if the gearbox was moved upwards.

What kind of impact will the current positioning have on its flight, if any?

Will glue everything together tonight, and if possible, install the receiver. Looks like only about 1 - 1.5 hrs more work.
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Old Jul 13, 2011, 02:35 PM
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Originally Posted by astronaut View Post
Of course, but I have read Scotta mentioning 60mm as the CG for his plane in several posts in this thread. So I multiplied that by 60% to arrive a at ~35mm. Is that alright?

And can you comment on the motor/gearbox please? Thanks!
Here's what I read:

"CoG correction
Don't know what I was thinking when I put up the CoG position for the P51D.

To have the CoG at 30% MAC the measurement from the front cut edge of the wing panel where it is glued to the Fuse Front X section:

P51D = 63mm
P-40 = 44mm
Zero = 50mm"

I get

60% P-40 = 60% x 44mm = 26.4 mm

EDIT,

Or are you building the P-51?

Looks like it in the motor view actually. If so, ignore my comment above, and the former 1" suggestion.

Okay, back a few pages I see the full model - P-51 for sure. nevermind......
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Old Jul 13, 2011, 03:00 PM
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Originally Posted by astronaut View Post
However, I think I will stick with this for now though, unless you think it will make a difference in performance if the gearbox was moved upwards.

What kind of impact will the current positioning have on its flight, if any?
Not much. I think you'll be fine. If anything it will act like up thrust. So the effect will be to nose plane up on power, and if trimmed properly for powered flight, the nose will drop with power off.

You could try a little down thrust to counter it. But don't overdo it. A little down thrust goes a long way.
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