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Trimming and CG

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Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Trimming and CG

Old 08-12-2007, 07:48 PM
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KurtS
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Default Trimming and CG

I downloaded an excellent 2-page "trimming chart" a few weeks ago and have been trying to apply what it says not only to my actual RC aircraft but to an aircraft in my FS One simulator. So far the simulator is responding to CG changes as expected but my airplanes are not. Here's the situation.

The chart says one of the ways to check CG is to roll inverted and see if the pitch increases or decreases. It says if down elevator is required to hold level flight, move the CG aft.

This has worked perfectly on the simulator and on a 33% Edge 540 that used to drop like a stone when rolled inverted, it now flies hands off level flight either rightside up or upside down. Cool.

One of the airplanes I've been trying this out on is an old CG Staudacher S-300 GS. It too drops like a stone when rolled inverted and when I really checked the CG I found that it was actually about 1/2" too far fwd according to the plans. I moved the battery rearward, checked the CG again and found that it was now balancing right at the aft point of the plan's CG range. I cautiously flew it this way and found two things. While it still stalled straight ahead with no real wing dropping tendancy, it still required a fair amount of down elevator to hold level inverted flight. The weird discovery was that when pitched upwards sharply it would start to porpoise up and down. The less elevator I used the less the up and down movement. Switching to low rate on the elevator made it easier to fly but it was a trait I'd never seen before.

Overall the airplane flies better, does perfect single snap rolls, spins upright and inverted coming out the instant you release the sticks, and lands easily as long as the speed doesn't get too slow.

So I'm wondering, if I've seen the result of a more rearward CG having the result shown on the trim chart when tried on the simulator model, why didn't I see it on a real one? Could it be the CG range on the plans is on the conservative side and I need to keep experimenting with moving it farther aft? This is more difficult because of course if I go to far, well I saw on the simulator what happens. And it would be expensive.

What about the up and down pitching when pulling up elevator that wasn't there with a fwd CG?

Is it possible to make any aerobatic airplane with a symmetrical airfoil, that is built straight, with proper wing and tail incidence, to fly hands off inverted or am I chasing shadows or just over simplifying things?

Inquiring minds want to know.

Kurt

Old 08-12-2007, 10:08 PM
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Default RE: Trimming and CG

You're on the right track. The kit plans are likely as not just conservative.

The other way is to trim the model for level flight at about 1/3 throttle. Then tip the model over into a 45* dive and watch it for a moment. A forward CG will make the nose lift back up. A CG that is behind the neutral point will make the model tend to tuck to a steeper dive. When it's right on or very close to the neutral point then it'll hold the same dive angle until you pull it out.

Remember that the CG position and elevator trim required to hold that level flight point are both related. The more aft the CG the more down(or less up) the elevator.
Old 08-13-2007, 02:30 AM
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Default RE: Trimming and CG

G'day Mate,
When you move your CG back, the elevators are more efficient, ie, less weight in front of the CG, so when you move your CG back, you must also reduce the elevator throws accordingly.
I bet your trim chart mentioned, that the test for CG, is when inverted, a much smaller amount of down elevator is required to hold the nose up, that means about 1/8" down stick travel, then you will be close to the Sweet Spot. You will find that when you have found, or are close to the Sweet Spot, if you fly at 3/4 power, approx, & bank about 20 to 25 degrees, you won't need any, or very little elevator, to keep the nose up in the turn.
Also forget what happens in the Simulator, real life is not copied on a computer.

PS, a forward CG won't make the nose rise in a 45degree dive, the fact that you have trimmed at 1/3 throttle & added up trim to keep it flying, will make the nose rise, as the speed increases, in the dive.
Old 08-13-2007, 05:44 AM
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Default RE: Trimming and CG


ORIGINAL: KurtS

I downloaded an excellent 2-page "trimming chart" a few weeks ago and have been trying to apply what it says not only to my actual RC aircraft but to an aircraft in my FS One simulator. So far the simulator is responding to CG changes as expected but my airplanes are not. Here's the situation.

The chart says one of the ways to check CG is to roll inverted and see if the pitch increases or decreases. It says if down elevator is required to hold level flight, move the CG aft.
A couple that I have say, "If a lot of down is required....."

There is one I've seen that says if no down is required that the airplane is tail heavy, and that is to be avoided.
Old 08-13-2007, 08:36 AM
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Jim Thomerson
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Default RE: Trimming and CG

After reading an article im Model Airplane News many years ago, I did some exprimenting with a roll of solder and one of my free flights. As it was trimmed it climbed in a nice near vertical spiral. As I started adding solder around the tail, the climb got flatter and flatter. I added solder until I got it to fly level under power. I did not add enough solder to make it dive.
Old 08-13-2007, 09:23 AM
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Default RE: Trimming and CG

Good stuff, thanks. I will keep experimenting and see what happens.

Kurt

Old 08-17-2007, 08:29 AM
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Andrew Ditchfield
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Default RE: Trimming and CG

I have been looking for a good trimming guide. Where did you get it?
Old 08-17-2007, 10:59 AM
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Default RE: Trimming and CG

The best way to check the CG is to fly the plane into the wind and pull up into a 45 degree upline, then roll inverted and take your hands off the sticks. The nose should just barely begin to drop off. If the plane climbs, (remember you're inverted), then the CG is too far back. If the nose drops bad, the CG is too far forward. Once that is set, when you fly inverted straight and level, you should use very, very little down elevator to maintain level flight. All of this is assuming your up and down, and side thrust is correct, and all incidences are correct, otherwise you're trying to trim out bad tendencies being caused by an airframe that is not straight and true. Once you know that is all set, then you can start trimming for multiple rolls (5 or 6) to the left and right without losing heading. That will set your aileron differential. That is also done on a 45 upline into the wind. You need to do all of this at full throttle, because that takes into account the full engine torque being produced. This is how how we trim the big IMAC gassers, and when it's trimmed out, it will be a real pleasure to fly. Good Luck.

Ken
Old 08-17-2007, 12:51 PM
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Default RE: Trimming and CG


ORIGINAL: Andrew Ditchfield

I have been looking for a good trimming guide. Where did you get it?

G'day Andrew,
This is a good one, http://www.palosrc.com/instructors/trim-chrt.htm
Old 09-07-2007, 06:39 PM
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Steve Steinbring
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Default RE: Trimming and CG

CG is certainly a consideration, but not the only one. I think other considerations in this discussion maybe the designed angle of attack of the wing and airfoil design of the aircraft in the first place. If you have some degree of positive angle of attack in upright flight you will have to compensate for that factor during inverted flight. Add in other factors as the horizontial stabilizer incidence it adds interest as well.
Old 09-07-2007, 11:10 PM
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KurtS
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Default RE: Trimming and CG

Valid points. Now I read somewhere that a symmetrical airfoil should be set at +1 degree and the stab at 0 degrees. ??

Kurt
Old 09-08-2007, 08:58 AM
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da Rock
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Default RE: Trimming and CG


ORIGINAL: KurtS

Valid points. Now I read somewhere that a symmetrical airfoil should be set at +1 degree and the stab at 0 degrees. ??

Kurt
Probably read that on the internet.

It's way too "sound byte".
The AOI is a function of the expected speed and weight of the aircraft. You reckon all airplanes weigh the same and fly the same speed? And models have a wide performance range too.

The wing is set to the fuselage to give the appropriate lift at the design speed while holding the fuselage to the least drag AOA. And whatever the wing does with the air determines the downwash that hits the stab. And the stab is set accordingly.

NOW, all that said, the stab isn't going to have to deal with an induced pitching moment, so one would assume the stab wouldn't need to be rigged to stabilize that extra moment, but you'd still have to consider the other details.

Would it work? With our models, probably "good enough". But that doesn't make it any more or less true.
Old 06-26-2008, 02:00 PM
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Garthwood
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Default RE: Trimming and CG

Ken......................Best explanation regarding how to check for correct CG etc... I ever read on RCU. Too bad that the majority of RC Flying Model Clubs don't have experienced instructors or can't explain why some models, ARFs or kits fly like dogs. Some instructors resort to the guessing game, if it's not a dog it's a cat attitude. Thank you for taking the time to explain in simple English without the need of a Master's degreee in aerodynamics and the confusion of a prescription written language.

Charles.

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