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Old Jan 20, 2005, 11:45 AM
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Joined Dec 2004
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Tail heavy vs. nose heavy

How does a tail heavy balance show itself in flight? What about nose heavy?

I'm finishing up a TM400 and want to understand how to diagnose any strange flight behavior.
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Old Jan 20, 2005, 12:06 PM
Mark 12:30
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United States, GA, Alpharetta
Joined Jan 2004
676 Posts
There is an old saying...a nose heavy plane flies poorly and a tail heavy plane flies once.

A nose heavy plane will tend to exhibit some or all of these symptoms...
Feels heavy to the sticks...slow to react.
Lands very "hot"....fast that is..
Requires some "up" elevator to fly level...
Power off glides tend to be very steep....
Will tend to "porpose" in flight..up then down, up then down...

A tail heavy plane tends to feel overly sensitive on the sticks.
Falls off to one side.
Flies with its nose high.
Glide is almost non-existant..tends to fall off or "tail dive"..
does not fly vey long..pilots get very nervous with this condition.

I've only been flying for a little more than a year on mostly electric ARF's and there are probably more...but this should help get you started.
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Old Jan 20, 2005, 12:10 PM
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Southern MA. USA
Joined Jan 2003
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Quote:
Originally Posted by mstrcntrlprgrm
How does a tail heavy balance show itself in flight? What about nose heavy?

I'm finishing up a TM400 and want to understand how to diagnose any strange flight behavior.

A slightly nose heavy plane will fly, some fly better if they balance just a little heavy in the nose, most will fly but will have trouble gaining altitude.

A slightly tail heavy plane will climb at a fast rate, stall, and the nose will drop untill it picks up a little speed then climb again only to stall again, they call this porpoising, if a plane is more than slightly tail heavy it will not fly long before crashing.

A rule of thumb saying in R/C flying is " A nose heavy plane will fly poorly, A tail heavy plane will only fly once ".


Hope this answers your question, MM
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Old Jan 20, 2005, 12:19 PM
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Tail heavy airplanes will be very twitchy *especially* in pitch, like they're hunting for the attitude they want. Nose heavy airplanes are slow to respond in pitch. You'll see a difference in they way they recover from a dive, too (don't do this if you suspect it is tail-heavy):

Go to a low throttle setting, put the airplane in a dive, let it speed up, and let go. A nose heavy airplane will immediately and sharply pull itself out of the dive, stall, and repeat. With a neutral CG the airplane will pull out very slowly, and could keep the exact attitude you gave it until you pull back on the stick yourself. Tail heavy airplanes will get very squirrely, very quickly, and may even try to tuck under instead of pulling out.

Airplanes fly a lot more efficiently with the CG close to neutral (so it's great for soaring) but I've found I want the CG much, *much* further forward with my warbirds than I ever kept it with my sailplanes to dampen the pitch response.

Speaking of power: Can you talk about the relationship between CG and power on/off behavior? I had a Slow Stick that would climb steeply under power but dive sharply off power, almost like it needed lots more down thrust-- but I built it stock and it's hard to screw up the thrust angle on that airplane?
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Old Jan 20, 2005, 12:27 PM
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Perth, Australia
Joined Mar 2004
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Depending on what type of plane it is. If you hand launch it, it will try to climb steeply and turn off to one side. Once its flying, It will feel very light on the joysticks and unstable, will be reluctant to penetrate into the breeze and will try to turn downwind . It will be twitchy and difficult to keep heading in a straight course wanting to climb then stall and fall to one side repeatedly.

If its balanced nose heavy. on launching It will be reluctant to climb without a fair bit of elevator, it will keep wanting to lose altitude rapidly gaining speed, it won't want to turn readily, will feel heavy and require more joystick input.

I'm sure other members will have experienced other symptoms as well. It depends on the type of plane and how out of balance and out of trim it is.

Ron...
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Old Jan 20, 2005, 12:30 PM
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Quote:
Originally Posted by mstrcntrlprgrm
I'm finishing up a TM400 and want to understand how to diagnose any strange flight behavior.
The manual will give a reccomended CG range. Look around in the GWS or Parkis forums to make sure the range in the manual is correct, then set the airplane up so it balances near the front of the range. It should fly fine there and be relatively docile.

If it does, trim it to fly straight and level, the try the dive test, then land. Based on your flight, shift things around to move the CG. Most likely you'll want to move it back a little, between 1/8 and 1/16 inch. (never move it back much at a time- overshooting can make the airplane unflyable!)

Then go fly again. If you moved it back a little the airplane should feel lighter and peppier, like it's happier to be in the sky than it was before. It should still pull itself out of a dive, but will do so more slowly.

Land, shift things again, and repeat untill you're happy with the way it responds. If it starts to get twitchy or hard to control, move the CG back forward to the previous location.

Dive tests will show you when you're getting close- the airplane may not stop self-recovering before it gets sensitive enough to bother you, but it will stop self recovering (or do it slowly enough to scare you) before it becomes unstable.
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Old Jan 20, 2005, 12:31 PM
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On comparing notes it sounds like we've experinced much the same symptoms.

Ron...
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Old Jan 20, 2005, 12:41 PM
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Southern MA. USA
Joined Jan 2003
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Quote:
Originally Posted by rdeis
Tail heavy airplanes will be very twitchy *especially* in pitch, like they're hunting for the attitude they want. Nose heavy airplanes are slow to respond in pitch. You'll see a difference in they way they recover from a dive, too (don't do this if you suspect it is tail-heavy):

Go to a low throttle setting, put the airplane in a dive, let it speed up, and let go. A nose heavy airplane will immediately and sharply pull itself out of the dive, stall, and repeat. With a neutral CG the airplane will pull out very slowly, and could keep the exact attitude you gave it until you pull back on the stick yourself. Tail heavy airplanes will get very squirrely, very quickly, and may even try to tuck under instead of pulling out.

Airplanes fly a lot more efficiently with the CG close to neutral (so it's great for soaring) but I've found I want the CG much, *much* further forward with my warbirds than I ever kept it with my sailplanes to dampen the pitch response.

Speaking of power: Can you talk about the relationship between CG and power on/off behavior? I had a Slow Stick that would climb steeply under power but dive sharply off power, almost like it needed lots more down thrust-- but I built it stock and it's hard to screw up the thrust angle on that airplane?

If you took flying lessons in a full scale (real) plane, one of the first things the instuctor would impress on you is that the yoke and controls are for flight ajustments and changing direction, in taking off, enough power to get the plane moving forward and break free of the ground is what is needed, once you are free from the ground the plane will continue to climb, to climb faster more throttle is added, if you were to depend on the elevator you could induce stall, once you gain the altitude you wish to cruise at, throttle is reduced enough to maintain that altitude, if you want to climb to a higher altitude give more throttle, if you want to go descend reduce trottle, that's how planes fly...grin..


MM
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Old Jan 20, 2005, 12:43 PM
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Southern MA. USA
Joined Jan 2003
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Quote:
Originally Posted by rdeis
Tail heavy airplanes will be very twitchy *especially* in pitch, like they're hunting for the attitude they want. Nose heavy airplanes are slow to respond in pitch. You'll see a difference in they way they recover from a dive, too (don't do this if you suspect it is tail-heavy):

Go to a low throttle setting, put the airplane in a dive, let it speed up, and let go. A nose heavy airplane will immediately and sharply pull itself out of the dive, stall, and repeat. With a neutral CG the airplane will pull out very slowly, and could keep the exact attitude you gave it until you pull back on the stick yourself. Tail heavy airplanes will get very squirrely, very quickly, and may even try to tuck under instead of pulling out.

Airplanes fly a lot more efficiently with the CG close to neutral (so it's great for soaring) but I've found I want the CG much, *much* further forward with my warbirds than I ever kept it with my sailplanes to dampen the pitch response.

Speaking of power: Can you talk about the relationship between CG and power on/off behavior? I had a Slow Stick that would climb steeply under power but dive sharply off power, almost like it needed lots more down thrust-- but I built it stock and it's hard to screw up the thrust angle on that airplane?

If you took flying lessons in a full scale (real) plane, one of the first things the instuctor would impress on you is that the yoke and controls are for flight ajustments and changing direction, in taking off, enough power to get the plane moving forward and break free of the ground is what is needed, once you are free from the ground the plane will continue to climb, to climb faster more throttle is added, if you were to depend on the elevator alone you could induce stall, once you gain the altitude you wish to cruise at, throttle is reduced enough to maintain that altitude, if you want to climb to a higher altitude give more throttle, if you want to descend reduce trottle, that's how planes fly...grin..


MM
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Old Jan 20, 2005, 01:32 PM
Rat
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Rats nest Grafton ND
Joined Aug 2004
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I like my models to fly level at just under full power. I do this by adjusting the CG and wing incidence. I hate it when I apply power and the model wants to go into a climb. This makes for a challenge on days when the winds suddenly pick up on you and you have to bring it back to you from being blown down the field.
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Old Jan 20, 2005, 02:47 PM
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I prefer that the plane climb at > half throttle and descend at < half throttle.

mw
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Old Jan 20, 2005, 05:43 PM
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Quote:
Originally Posted by Moe M.
to climb faster more throttle is added,
I should clarify- the bad behavior wasn't merely altitude gain, but pitch increase.

I would expect a (mostly) neutrally trimmed airplane to climb/descend with throttle changes, but the pitch attitude should stay constant. It "rises," but doesn't go nose up. Likewise it should descend without pitching over. I thought that was the purpose of downthrust, Right?
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Old Jan 20, 2005, 08:04 PM
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Nah..., my planes, (except for the aerobats, which is another matter) are set up to pitch up a bit with throttle-up and vice versa with throttle-down. The important thing is that they donīt pitch up into a stall, which is where downthrust (or incidence) comes in. I donīt really see how a plane could increase lift with speed, without also increasing the pitch-angle. If the pitch angle is constant as you power up, the plane will gain airspeed without gaining altitude (as a general rule of thumb anyway)..., right?!

Edit: I gave this a bit more thought, and at the risk of becoming too theoretical, Iīd have to assume that whether pitch-up really is neccessary to gain altitude depends on the nature of the plane. Most slowflyers are setup with a fair amount of AoA of the wings, which are also often undercambered, which gives climb with higher speed even if the pitch-angle was left the same. Most trainers are setup with some sort of high-lift airfoil (flatbottom, or possibly modified flatbottom such as Clark-Y), which also provides more lift with raised speed even with pitchangle unchanged. Most aerobats and warbirds have a symmetrical or semi-symmetrical airfoil which needs a change in pitch to gain lift, regardless of the speed.
Yet, planes like my Protech Butterfly or Sig Bristol Scout which are both slowflyers with undercambered airfoils and a noticeable amount of positive AoA on the wings (especcially the butterfly) still does pitch up with an increase in power.

So Iīd say that a reasonable amount of pitch-up is to be expected. The main thing is that the pitch-up is maintaneable with the thrust your plane has. Although..., that isnīt totally true either, since I donīt trim the planes I have which are capable of sustained vertical (more than 1:1 in thrust to wheight) to actually do this on their own.

Anders O
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