Test for CG
05-05-2011 | 12:21 PM
#1
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From: Indio, CA
Test for CG
I have read to test the CG on a plane you fly it at a 45 degree up line and roll it inverted and see how it responds. Can someone explain why a 45 degree up line is necessary and why inverted is necessary. I can't seem to figure that out. Does the procedure apply to both high wing and low wing planes. Thanks
05-05-2011 | 12:35 PM
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From: BouldercombeQueensland, AUSTRALIA
RE: Test for CG
I have read of using a down line to test CofG where it pulls under for nose heavy and pulls up for tail heavy ( I think thats right) but I have always used the plain old straight and level inverted test with good success.<div>
</div><div>Just fly straight and level then roll inverted and see how much down you need to hold it straight and level.</div><div>The amount of down will indicate Just HOW nose heavy it is. I have even guestimated the amount of weight needed from the amount of stick required.</div><div>
</div><div>Works for me and is more K.I.S.S. than any other test I have seen.</div>
</div><div>Just fly straight and level then roll inverted and see how much down you need to hold it straight and level.</div><div>The amount of down will indicate Just HOW nose heavy it is. I have even guestimated the amount of weight needed from the amount of stick required.</div><div>
</div><div>Works for me and is more K.I.S.S. than any other test I have seen.</div>
05-05-2011 | 01:06 PM
#3
RE: Test for CG
There are various methods which are suggested/ claimed / bragged up on n on -plus a bunch of trim proofs etc..
Too many variables to make any one method best
The most simple is the best for me
Upright, hands off at usual speed used , then roll to inverted and need a touch of correction- close enough for me
The 45 degree line works too- but once you get a zero input correction- you also get more sensitivity.
No way around it
mother nature includes gravity in ANY position- so there will always be a compromise -
Just find the one which is easiest for you to accept. Don't go crazy trying for something which adds misery.
Too many variables to make any one method best
The most simple is the best for me
Upright, hands off at usual speed used , then roll to inverted and need a touch of correction- close enough for me
The 45 degree line works too- but once you get a zero input correction- you also get more sensitivity.
No way around it
mother nature includes gravity in ANY position- so there will always be a compromise -
Just find the one which is easiest for you to accept. Don't go crazy trying for something which adds misery.
05-05-2011 | 03:29 PM
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From: Indio, CA
RE: Test for CG
I don't think any one method of testing CG is necessarily better than another. I was just trying to understand why an airplane needs to be inverted to check CG rather than upright and why the airplane needs to be at about 45 degrees on an up line as opposed to straight and level or down line or any other attitude. There must be a specific reason for those attitudes. They must serve some particular purpose. I'm just trying to get an explanation as I don't see it.
05-05-2011 | 04:29 PM
#5
RE: Test for CG
Depends on the airfoil, symmetric sections and you are trying to get similar responses upright and inverted. Won't ever be the same, the wing has to fly at the same AOA up and down, so there will be some trim diff either way, you are looking to minimise the difference without getting into pitch divergence. Cambered sections need the '30deg down' trim thing, ie trim for 'S&L' then without changing the elevator trim, push the nose down, allow the speed to build, then release the controls and watch the model's response. You are looking for a gentle pull out from the dive. Sharp pull up = nose heavy, tuck down = tail heavy.
Evan, WB #12.
Evan, WB #12.
05-05-2011 | 05:26 PM
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From: Southern England, UNITED KINGDOM
RE: Test for CG
this is how i check the cg for a fully aerobatic model. trim the elevator for level flight at cruising speed (say 1/2 - 2/3 throttle). Roll inverted and see if it climbs or dives. If it dives then it has too much up elevator (longitudinal dihedral) which was balanced out by a forward cg when upright. move the cg back a bit and try again with the elevator re-trimmed for level flight as before - it will need less up elevator when upright due to the more rearward cg and when inverted the tendancy to dive will be reduced - rinse and repeat until the cg position is established that requires an elevator trim (i.e. incidence) which allows hands off level fight when upright and inverted, or close to it.
During the testing, checking for pitch stability is advisable - this means putting the model into a steepish dive (30 degrees or so) with no elevator input held in. If the plane gradually pulls out, back to level flight, you can move that cg back even more, it the plane steepens its dive or 'tucks' into the dive, then the cg is a little too far back - any further back and the plane could become divergently unstable - which means it goes home in a plastic sack.
A nose heavy plane that requires excessive longitudinal dihedral to fly level will also exhibit the characteristic of climbing steeply under high power, whilst often diving steeply when power off - the steep climbing due to over-elevation can be countered by adding loads of downthrust but its not ideal and won't help the power off diving condition. This is true for all planes. In the caet of a fully aerobatic plane it will make it a bit of a pig to fly, rolls became barrelly and she'll dig hard into tight turns like chronic understeer on a car. So, getting the cg right is vital for a sweetly flying plane of any type.
<span class="info"> </span>
During the testing, checking for pitch stability is advisable - this means putting the model into a steepish dive (30 degrees or so) with no elevator input held in. If the plane gradually pulls out, back to level flight, you can move that cg back even more, it the plane steepens its dive or 'tucks' into the dive, then the cg is a little too far back - any further back and the plane could become divergently unstable - which means it goes home in a plastic sack.
A nose heavy plane that requires excessive longitudinal dihedral to fly level will also exhibit the characteristic of climbing steeply under high power, whilst often diving steeply when power off - the steep climbing due to over-elevation can be countered by adding loads of downthrust but its not ideal and won't help the power off diving condition. This is true for all planes. In the caet of a fully aerobatic plane it will make it a bit of a pig to fly, rolls became barrelly and she'll dig hard into tight turns like chronic understeer on a car. So, getting the cg right is vital for a sweetly flying plane of any type.
<span class="info"> </span>
05-05-2011 | 05:48 PM
#7
RE: Test for CG
ORIGINAL: Indiomike
I was just trying to understand why an airplane needs to be inverted to check CG rather than upright......
I was just trying to understand why an airplane needs to be inverted to check CG rather than upright......
ORIGINAL: Indiomike
........and why the airplane needs to be at about 45 degrees on an up line as opposed to straight and level or down line or any other attitude.
........and why the airplane needs to be at about 45 degrees on an up line as opposed to straight and level or down line or any other attitude.
Same principle applies for a 45-degree down line, but it is less forgiving of mistakes.
Degree of decalage depends directly on stability margin.
During horizontal flight, the first compensates the second perfectly.
The lift force and the weight hanging from the wing gradually result reduced for any climb or dive, reaching both zero value for perfectly vertical flight (up or down).
Because of that, the down nose pitch decreases for any climb or dive; however, the up nose pitch produced by the decalage remains constant and even increases for increased speeds (diving especially).
The strength of that up nose pitch during climb or dive is directly related to the strength of the down nose pitch for level flight (which is determined by the separation distance between the points of application of the weight and lift=stability margin).
Check this related article:
http://www.charlesriverrc.org/articl...GMarkDrela.htm
05-05-2011 | 08:41 PM
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From: The Villages, Florida NJ
RE: Test for CG
I was told to do it 45* invert so I did. I liked it because it was going away from the hard stuff. With sailplanes we dove them, you haven't seen pucker until you've had one tuck and you can't pull it out.
05-06-2011 | 05:38 AM
#10
RE: Test for CG
ORIGINAL: Lnewqban
The reason is the decalage, as explained in the attached schematic.
The stability margin respect to the horizontal is reduced this way, making it less ''pitch unstable'' during inverted flight.
Same principle applies for a 45-degree down line, but it is less forgiving of mistakes.
Degree of decalage depends directly on stability margin.
During horizontal flight, the first compensates the second perfectly.
The lift force and the weight hanging from the wing gradually result reduced for any climb or dive, reaching both zero value for perfectly vertical flight (up or down).
Because of that, the down nose pitch decreases for any climb or dive; however, the up nose pitch produced by the decalage remains constant and even increases for increased speeds (diving especially).
The strength of that up nose pitch during climb or dive is directly related to the strength of the down nose pitch for level flight (which is determined by the separation distance between the points of application of the weight and lift=stability margin).
Check this related article:
http://www.charlesriverrc.org/articl...GMarkDrela.htm
ORIGINAL: Indiomike
I was just trying to understand why an airplane needs to be inverted to check CG rather than upright......
I was just trying to understand why an airplane needs to be inverted to check CG rather than upright......
ORIGINAL: Indiomike
........and why the airplane needs to be at about 45 degrees on an up line as opposed to straight and level or down line or any other attitude.
........and why the airplane needs to be at about 45 degrees on an up line as opposed to straight and level or down line or any other attitude.
Same principle applies for a 45-degree down line, but it is less forgiving of mistakes.
Degree of decalage depends directly on stability margin.
During horizontal flight, the first compensates the second perfectly.
The lift force and the weight hanging from the wing gradually result reduced for any climb or dive, reaching both zero value for perfectly vertical flight (up or down).
Because of that, the down nose pitch decreases for any climb or dive; however, the up nose pitch produced by the decalage remains constant and even increases for increased speeds (diving especially).
The strength of that up nose pitch during climb or dive is directly related to the strength of the down nose pitch for level flight (which is determined by the separation distance between the points of application of the weight and lift=stability margin).
Check this related article:
http://www.charlesriverrc.org/articl...GMarkDrela.htm
the correct combo of speed/decalage and balance will produce a nice undulation over a long time period or a a dive - or a climb or level flight.
On a pattern model which feels good and is predictable in all maneuvers - there are other considerations
the vertical center of drag and the vertical center of weight and the distribrution of TOTAL side areas also affect the total feel .
The response time from TX input to model movement is another factor .
So what may seem right on paper - can end up being a poor performer.
The correct combo is like a good meal - The flavors all compliment. You can't calculate a really good aerobatic model - -just the various parts of it
(may not be textbook info but it is a very real part of the picture)
05-06-2011 | 02:44 PM
#12
RE: Test for CG
Yeh the cg setup is a basic starting point- the stuff which makes the model really shine takes a bit of time and experience - Just like a heli setup - getting the feel right, is a case by case thing.
05-06-2011 | 04:42 PM
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From: Southern England, UNITED KINGDOM
RE: Test for CG
agreed totally - if there was one perfect model with one perfect setup, that always handled crisply and predictably like a Ferrari of the skies we'd all be flying that one and there would be no need for differnt models of the same type. Its the different feels of various models that keeps expanding our hanger collections 
Vive la differance...........
Vive la differance...........
05-17-2011 | 11:45 AM
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From: Mohave Valley,
AZ
RE: Test for CG
I start at the forward end of the recommended range, then gradually move it back until I like the stall characteristics. It all depends on your preference. Farther back gives better elevator response at low speeds and better stall type manuveres ( spins and snap rolls). Farther forward is more stable.
05-17-2011 | 12:29 PM
#15
RE: Test for CG
ORIGINAL: DwightMann
I start at the forward end of the recommended range, then gradually move it back until I like the stall characteristics. It all depends on your preference. Farther back gives better elevator response at low speeds and better stall type manuveres ( spins and snap rolls). Farther forward is more stable.
I start at the forward end of the recommended range, then gradually move it back until I like the stall characteristics. It all depends on your preference. Farther back gives better elevator response at low speeds and better stall type manuveres ( spins and snap rolls). Farther forward is more stable.
A little observant flying is worth a book full of formula. Of course -one has to be able to fly a model-
05-18-2011 | 12:43 PM
#16
RE: Test for CG
Once I learned how to harrier land one, setting up CG became very easy. Also, I haven't found one single major manufacturer that gives an aft CG point in the manual that is far enough back for my tastes.
05-18-2011 | 12:58 PM
#17
RE: Test for CG
The school book approaches are for stable flight -
Basically on a modern really decent ,flyable IMAC/3D setup -about 30-33% of MAC is the limit
for just flogging about -further back makes for a very agile but not stable setup
A 33-40 % aerobatic setup can be flown slowly in these setups - pretty easily- IF you understand vectored flight and know how to do it .
On foamies - 50% cg is flyable - slowly
As you noted - your own tastes are not the same as suggested by stable flight guys.
Basically on a modern really decent ,flyable IMAC/3D setup -about 30-33% of MAC is the limit
for just flogging about -further back makes for a very agile but not stable setup
A 33-40 % aerobatic setup can be flown slowly in these setups - pretty easily- IF you understand vectored flight and know how to do it .
On foamies - 50% cg is flyable - slowly
As you noted - your own tastes are not the same as suggested by stable flight guys.
05-18-2011 | 11:48 PM
#18
RE: Test for CG
ORIGINAL: Rick.
this is how i check the cg for a fully aerobatic model. trim the elevator for level flight at cruising speed (say 1/2 - 2/3 throttle). Roll inverted and see if it climbs or dives. If it dives then it has too much up elevator (longitudinal dihedral) which was balanced out by a forward cg when upright. move the cg back a bit and try again with the elevator re-trimmed for level flight as before - it will need less up elevator when upright due to the more rearward cg and when inverted the tendancy to dive will be reduced - rinse and repeat until the cg position is established that requires an elevator trim (i.e. incidence) which allows hands off level fight when upright and inverted, or close to it.
During the testing, checking for pitch stability is advisable - this means putting the model into a steepish dive (30 degrees or so) with no elevator input held in. If the plane gradually pulls out, back to level flight, you can move that cg back even more, it the plane steepens its dive or 'tucks' into the dive, then the cg is a little too far back - any further back and the plane could become divergently unstable - which means it goes home in a plastic sack.
A nose heavy plane that requires excessive longitudinal dihedral to fly level will also exhibit the characteristic of climbing steeply under high power, whilst often diving steeply when power off - the steep climbing due to over-elevation can be countered by adding loads of downthrust but its not ideal and won't help the power off diving condition. This is true for all planes. In the caet of a fully aerobatic plane it will make it a bit of a pig to fly, rolls became barrelly and she'll dig hard into tight turns like chronic understeer on a car. So, getting the cg right is vital for a sweetly flying plane of any type.
<span class=''info''> </span>
this is how i check the cg for a fully aerobatic model. trim the elevator for level flight at cruising speed (say 1/2 - 2/3 throttle). Roll inverted and see if it climbs or dives. If it dives then it has too much up elevator (longitudinal dihedral) which was balanced out by a forward cg when upright. move the cg back a bit and try again with the elevator re-trimmed for level flight as before - it will need less up elevator when upright due to the more rearward cg and when inverted the tendancy to dive will be reduced - rinse and repeat until the cg position is established that requires an elevator trim (i.e. incidence) which allows hands off level fight when upright and inverted, or close to it.
During the testing, checking for pitch stability is advisable - this means putting the model into a steepish dive (30 degrees or so) with no elevator input held in. If the plane gradually pulls out, back to level flight, you can move that cg back even more, it the plane steepens its dive or 'tucks' into the dive, then the cg is a little too far back - any further back and the plane could become divergently unstable - which means it goes home in a plastic sack.
A nose heavy plane that requires excessive longitudinal dihedral to fly level will also exhibit the characteristic of climbing steeply under high power, whilst often diving steeply when power off - the steep climbing due to over-elevation can be countered by adding loads of downthrust but its not ideal and won't help the power off diving condition. This is true for all planes. In the caet of a fully aerobatic plane it will make it a bit of a pig to fly, rolls became barrelly and she'll dig hard into tight turns like chronic understeer on a car. So, getting the cg right is vital for a sweetly flying plane of any type.
<span class=''info''> </span>
