LukeWarm Show full post »
whatmovesyou
Another way to check CG is to fly upside down. If you need to push your elevator forward, you are nose heavy.

As to flying your plane in high winds, the key control is to work and I mean work, throttle,rudder with ailerons and elevator, Scott brings up the other points that are very valid. Remember the higher the wind(usually gusty), your chances of crashing on landing go up exponential. Many RC planes land between 10 and 20mph. If the wind is 10-20 and gusty as you are trying to land, is it worth it?.
I like to design and fly unique planes.
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LukeWarm
This is the long version of post #1. When I did post #1, I carved this version up to make it more readable.

We answer a lot of questions in these forums about finding the correct CG. Here, you will find a collection of useful of information on that subject.

There are some good reasons why you may not want to fly balanced.
Let's look at some flight balance point adjustment effects.

• Nose heavy is safer, has sluggish maneuverability, and will increase stability in high winds. Favor nose heavy for maiden flights. If the center of balance is way nose heavy, you have to fight to keep the nose up.

• Tail heavy is good for high-alphas and radical maneuverability, but is inherently unstable and should only be tried by seasoned pilots. If the center of balance is way tail heavy, you're all over the sky, hoping you can land your plane without crashing.

• The higher the wing loading, the more sensitive and critical the ideal center of gravity adjustment becomes. Light, floaty jets are much more forgiving. High wing loading also decreases maneuverability.

• The ideal CG moves progressively forward a small amount as the aircrafts speed increases. The reference point in the end should be a compromise that favors your normal cruising speed. Fast aircraft should feel a little nose heavy at slow speeds.

• If you intend to fly in high winds, adjust the CG a little nose heavy to increase stability.

• Wing loading (weight to lift area ratio) decreases as speed increases. After the plane reaches a fast enough speed that it has a well developed lift, the differences become less noticeable. The higher the wing loading, the more difficult the plane will be to dial in and fly. Heavy planes have a sensitive CG, and when they crash, they hit the ground hard.

Because of the above listed factors, The designers of RC Powers aircraft (Dave and Scott), often tell people that the CG mark on their plans is a really good place to start. The wing loading (weight to lift area ratio), the speed you intend to fly, your style of piloting, the weather, and the condition of the plane; will all move that mark a little one way or the other.

NOTE: If this is for a kit with a CG mark. Go directly to “step 2”
STEP 1: To find the ideal center of gravity of a scratch build: Assemble the aircraft without any electronics. Gently glide test the aircraft and adjust its balance by taping coins to it. Do this several times until the plane is balanced enough to glide straight and flat. Pick the aircraft up by the fuselage loosely using your thump and a finger. Adjust your hold position until you find the spot that makes the aircraft hang level. Make a mark on the aircraft fuselage one inch forward of that spot, this mark is the reference point for the first flight test. Remove the coins and Install everything except for the battery. Hold the aircraft in the same way at the reference point, and use the battery size (mAh) and mount position to balance the aircraft. The aerodynamics of the airframe has the biggest influence on the ideal center of gravity, but the thrust location, (component and surface) friction, airfoils, and velocity, also have an effect. So after the powered flight test, move the battery fore or aft a little, as needed. Readjust and retest until you are happy with the flight performance. For more in-flight test information see “The best in-flight ideal CG test is” in the second paragraph

STEP 2: The best in-flight ideal CG test is: The CG mark on the kit is a really good place to start. Fly your aircraft at a normal cruising speed and trim your radio till the plane flies level. Increase the speed, then cut the throttle off and watch the plane glide as it slows down (hold the stick still). If the nose goes down, it's nose heavy (move the battery back). If the nose goes up, it's tail heavy (move the battery forward). Land the plane and check out how your trims look on the aircraft; they should be somewhat flat. If your trims are not somewhat flat, move the battery fore or aft accordingly to adjust for this also. Keep retesting and adjusting until the plane flies balanced while the trims are reasonable flat. Find the balance point and mark it.

Now that you have found the ideal center of gravity, mark that spot as the CG reference point so you can balance your plane before every flight.

Any future changes that you make (different size battery, different size motor, added servos, and airframe changes) can necessitate a reevaluation of the ideal CG. Now for the good news; The old and the new ideal CG will be pretty close to one another.
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bobdabilduh55
Great Info. Thanks Luke.
"Five Easy Pieces"
WATT FLYER Forum's 2012 Scratch-Builder Award Winner
(For Posting outstanding scratch building threads on Wattflyer)
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LukeWarm
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Great Info. Thanks Luke.

Your welcome, I enjoy writing stuff like this. The information in "To find the ideal center of gravity" is the way the RCP aircraft designer (Scott Lott) uses to find the Center of Gravity on our planes; I learned this method from him. The "33% back from the lead edge of the wing" method of finding the Center of Gravity does not consistently work. It will occasionally cost you a plane. Scott's method is a little harder, but it has never failed me.

I just made quite a few changes in post 1 to make it easier to understand.
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ASD
I will be using this method to find the COG on my B-45 build
Cuz tooling around is just not my style of flying. Full stick and full travel control surfaces.

http://www.youtube.com/user/DollarTreeFlyer/videos
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doug7571
Thank You For the info. You do a great job helping us.
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MoTheG
Luke wrote another one: http://www.rcpowers.com/community/threads/the-quest-for-the-ideal-center-of-gravity.9685/

What people usually forget: the right CG depends on the speed. If it glides well without the electronics the CG is way behind were it needs to be for stable powered flight.
If it can not fly with NiMH it is not a plane.
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LukeWarm
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What people usually forget: the right CG depends on the speed. If it glides well without the electronics the CG is way behind were it needs to be for stable powered flight.

All true, we found it needs to be moved about an inch forward for the powered test. Thanks for the great advice MoTheG. I have added the below extra to post one.

The location of the ideal center of gravity varies with wing loading (weight), velocity (speed), and any aerodynamic changes to the aircraft. It usually doesn't change much, how much it changes depends on how drastic the changes were. But that is why you need to fly the plane the way you normally would while doing the CG finding flight test. That way, you will find the CG that best suits your style of piloting.
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bobdabilduh55
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Luke wrote another one: http://www.rcpowers.com/community/threads/the-quest-for-the-ideal-center-of-gravity.9685/

What people usually forget: the right CG depends on the speed. If it glides well without the electronics the CG is way behind were it needs to be for stable powered flight.

Good point and why I learned to do another glide test with all the gear onboard to make adjustments before I commit to powered flight. You may bust a nose but you will save a plane.
"Five Easy Pieces"
WATT FLYER Forum's 2012 Scratch-Builder Award Winner
(For Posting outstanding scratch building threads on Wattflyer)
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