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Old Feb 05, 2013, 02:42 PM
Fueled by Arabica Beans
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Quote:
Originally Posted by JetPlaneFlyer View Post
Nope, 35% back does not necessarily make the plane unstable. I've had freeflight gliders that fly very stable with the CG at 60 or 70% of the wing chord.

The thing that's often missed is that the biggest influence on where the CG needs to be for stability is the tail. A plane with a very large tail and a long tail moment can balance way back and still be stable. In fact it's perfectly possible for the CG to be behind the trailing edge of the wing and the plane still be stable.

The fact that the tail is so important to CG position is why any rule of thumb based on wing chord alone and totally ignoring the tail is little more than a guess.

Steve
You're right, I simplify things too much... the calculator really is the best way to do it. What I say really only applies to your typical cookie cutter type airplane which has similar dimensions to a majority of the planes out there. The larger your tail is among other things, it will scoot the CG back way beyond what you might see with a "typical" aircraft... ie with tandem wing planes.
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Old Feb 05, 2013, 02:59 PM
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Back in the olden days, I used to think nothing of chasing a free-flight across the meadows... at 70+ now, I think even less of it!
100% m.a.c. c.g.s were not unknown 50 years ago... on free-flights..
From Frank Zaic's "1959-61 Model Aeronautic Year Book"..(nothing like it availiable today).. a treatise on tail volume and c.g...
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Old Feb 05, 2013, 04:03 PM
Grumpy old git.. Who me?
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As a case in point; attached is a plan I did a few years ago for a micro rocket power freeflight glider for a postal contest in the USA. I mailed my plane over to the US, I got lucky, it positioned first in the contest.

The plan is based on a 60 year old Aeromodeller magazine plan originally powered by a Jetex rocket motor. Note that the design is quite conventional, it has a large tail but is not a tandem wing. CG is approx 80% back in the wing. The model was very stable, see the video.
Snowflake Rapier L1 power (1 min 5 sec)
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Old Feb 05, 2013, 07:00 PM
ARFs make me BARF
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Yup, there are exceptions to every rule....there are also "general" guidelines.
Ive seen hot, highly loaded models with very far forward CG, and some FF types (mostly competition duration types) with your 80% CG.
A lot depends on the type of model, it's intended purpose, and some math.
I have found that the vast majority of my RC designs, perform well with CG somewhere from 20% to 30%, depending on the type and usage. Try flying that P51 with an 80% Cg and you are in for a wild ride....but the FF duration ship with a 30% would be interesting as well....
Getting back to the AT6.....probably 25% of the MAC or forward would be a decent starting point with what your intended flight envelope is...IMHO.


Mark
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Old Feb 05, 2013, 07:35 PM
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1st flight is in the books...

Didn't go so well. nice take off and gain some altitude, make the turn and all is good, then had to tighten the turn so a little more angle ( don't know the technical term ) and some more elevator... and spiral of death. I let go and stops the spiral but couldn't get it leveled off before it made it to the ground.

Pulled out one landing gear and broke off the motor mount plate. Outside of the slight damage tearing the wheel off, the wing and tail are in great shape. Going out of town for a few days so I can't put it back together until Tuesday of next week.

Everything felt good in the short flight I had, How do I keep the spiral of death out of it? Was it simply too much elevator travel?
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Old Feb 05, 2013, 09:49 PM
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Dcope, do you know the wing load?
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Old Feb 05, 2013, 10:08 PM
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Total weight was 21oz. Certainly not on the lighter side of the scale.
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Old Feb 05, 2013, 10:24 PM
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wing load was 15.48 oz per sq. ft.
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Old Feb 06, 2013, 12:47 AM
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Quote:
Originally Posted by Dcope17 View Post
How do I keep the spiral of death out of it?
Simple; Avoid stalling
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Old Feb 06, 2013, 10:09 AM
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That's all it was... too slow? Okay... next flight will be faster...
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Old Feb 06, 2013, 01:24 PM
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You can stall at any speed. If you tried to pull it round in too tight a turn it could snap on you even if your flying speed was high. A stall occurs when the wing's angle of attack gets too high. The elevator controls the angle of attack of the wing, too much up elevator = stall
Small quite heavily wing loaded 'warbird type' models can be tricky.
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Old Feb 06, 2013, 01:34 PM
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Quote:
Originally Posted by JetPlaneFlyer View Post
You can stall at any speed.
Yes. I am surprised at the number of folks that don't understand this. I used to fly with a guy who was a full scale business jet jockey. He would talk about the Challenger series jets and how he loved it. But at max altitude he would talk about how fine a line it was between cruise and stall only a difference of a few knots.

So an airplane can stall at over 500mph!

Mike
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Old Feb 06, 2013, 02:21 PM
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The highest flier had a "coffin corner" at altitude where flying too slowly resulted in a stall, and too fast resulted in Mach buffet and tuck.
The speed range (IAS) was only about 20 mph!
from "Aerograph 3, Lockheed U-2"... Jay Miller
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Old Feb 06, 2013, 03:59 PM
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So to explain and you can correct my noob thinking... I can stall at any speed. Meaning even though I. Am moving fast, I can hit a full elevator and angle the wing upwards but if I have enough weight... the wing won't climb but simply skid through the air and loose its lift.

So what is my solution... fly gently. I usually fly with grace and never hack the sticks. This one was that if I left it at its current turning radius, it was heading straight over to of me. This thing was fast for a little plane and didn't think it required that much room. Next time will be easy flying banked no more than 45 deg and plenty of room... until I get the hang of it.
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Old Feb 06, 2013, 04:31 PM
ARFs make me BARF
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Stall is basically seperation of airflow from the top surface of the wing OR stab. You can "stall" at any speed. Too much elevator throw can put you into a snap, then a spin.
I'd say keep up a little more speed, cut down on ele throw. Just my opinion, hard to tell without a vid...

I've heard the U2 , at altitude, had to make HUGE turns to avoid having one wing go supersonic, and the other stall at the same time :O
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