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Does CG affect an aircrtaft's pitch at slow speed?

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Does CG affect an aircrtaft's pitch at slow speed?

Old 08-06-2008, 02:43 AM
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RappyPilot
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Default Does CG affect an aircrtaft's pitch at slow speed?

Hi guys

Busy setting up CG on Pulse XT 40 and was wondering if the CG affects the pitch of the plane when it's flying at just above stall speed. I am flying from a very very short runway and want to set her up that I will be able to land very slowly and as level as possible without the tailweel making contact with the runway first ... Apart from good piloting skills , was wonderting if CG affects it.

Any comments on this?

Marcel
Old 08-06-2008, 02:56 AM
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Default RE: Does CG affect an aircrtaft's pitch at slow speed?

Yes. It makes it more sensitive and responsive over the entire speed range. But the attitude you hold the model at is still up to you and your flying skills on the sticks.
Old 08-06-2008, 02:59 AM
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Default RE: Does CG affect an aircrtaft's pitch at slow speed?

Thanks
Old 08-06-2008, 06:02 AM
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Default RE: Does CG affect an aircrtaft's pitch at slow speed?

To address the "pitch at slow speed" question specifically............

Basically the AOA of the wing is resolved by the speed of the airplane and the airfoil's ability to generate lift. The airfoil winds up at whatever angle of attack needed to fly at whatever speed the pilot causes. That AOA is slightly modified by the drag of the airplane at that speed, so if the airplane is cleaner (no flaps etc) there will be less drag and more speed capability and possibly less lift needed.

So where would the CG come into play in this mess? A CG can artificially load an airplane. If it's too far forward, it creates extra load that the horizontal tail has to "carry" over and beyond the pitch stability function of that tail. So the tail has to create downforce to balance the downforce being created by the too-far-forward CG. And the wing has to carry all that extra load. It has to create more lift and the only way it can do that is with more AOA. And AOA is pitch.

And when a CG is located farther aft, it creates artificial load too. Good thing is that as a CG goes aft it catches the attention of even the least experienced pilot. So most are not apt to have the CGs back there.

So yes, the CG can affect the pitch you see at low speed. But will you see it? Or more correctly, will any of us be able to tell the difference in pitch from one landing to another? Probably not.
Old 08-06-2008, 06:11 AM
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Default RE: Does CG affect an aircrtaft's pitch at slow speed?


ORIGINAL: RappyPilot

Hi guys

Busy setting up CG on Pulse XT 40 and was wondering if the CG affects the pitch of the plane when it's flying at just above stall speed. I am flying from a very very short runway and want to set her up that I will be able to land very slowly and as level as possible without the tailweel making contact with the runway first ... Apart from good piloting skills , was wonderting if CG affects it.

Any comments on this?

Marcel

Bottom line is that we can make our models more efficient and we can make them less efficient. But most times things like the absolute slowest landing speed possible aren't of real value to us. We can only affect that speed a little. And most of us won't recognize the difference in speed that results.

On the other hand, we can see and feel the difference CG that's tuned for best flight performance. We see it in the quality of the maneuvers and the feel of the model.

If you're wanting the best landing model, use some exponential so you can more accurately control the elevator on approach and flare. Or land on low rates. Getting the stick's sensitivity down a bit for landing makes far more difference than finding a couple less mph or a degree or two of AOA.
Old 08-06-2008, 06:16 AM
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Default RE: Does CG affect an aircrtaft's pitch at slow speed?

That makes sence.

Took off once, and probably from a previous "not so good" landing the battery pack shifted forward. Took off and almost had to give full up elevator just to stay level!!! turned her around and landed safely in a very high nose-up attitude.

So I was guessing if the CG was further back (Not too far back that the planes is dangreous to fly) that one might be able to come in with a more nose down attidude flying very slowly. More so than if the CG was further forward.
Old 08-06-2008, 06:25 AM
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Default RE: Does CG affect an aircrtaft's pitch at slow speed?

Thanks for all the info. I am fairly new to planes. Been flying heli for as long as I remember.

But have come to a point where I can confidently fly aeroplanes and start experimenting with such matters. Have yet to play with expo on my TX. But will give that a try as well. Seeing the difference that CH makes with regards to the feel of the plane an doing aerobatic manuevers is very visible. How it affects slow flight is very interesting though.
Old 08-06-2008, 09:27 AM
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Default RE: Does CG affect an aircrtaft's pitch at slow speed?

Keep in mind that the affect of the CG location is not linear. So lots of effects you observe don't always line up neatly on a straight line chart.

And also keep in mind that what we feel as a result of the change to a CG is greatly affected by whether or not we've adjusted the elevator throws to keep pace with the CG's affects.

Good thing about our hobby is that we can easily test something and follow it immediately with another test.

The other day a guy was flying a "perfectly trimmed" plane he'd just bought from a buddy. Turns out the CG was almost at the leading edge of the wing. We taped some lead on the sucker and flew it around the circuit a couple of times. Added some more lead and went right back up. The airplane was flying much better by then. But the stick had gotten pretty sensitive, so I landed and we changed the elevator rigging. Took less than a minute and up again for another circuit. We'd lost no stability, but the stick was way less twitchy, and the airplane was lots more stable FEELING. We did all that on half a tank of gas and maybe 5-10 minutes time. And you know what we'd done?

The airplane wound up capable of landing with a lot less speed. And I could bring it in nose-up, way nose-up, on the prop. Because the sucker was lots more efficient and wasn't wasting all it's effort trying to keep the bloody nose from being pushed down. What we noticed was that the original owner had moved his choice of engine (oversized for the model) as far back as it would go, and that wasn't enough. Some designs can deal with more or less weight up front. Some can't. This one has a low aspect ratio tail that's not very far back. CG location is a very specific detail that is often affected by LOTS of other details.

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