how to check Center of Gravity
03-11-2007 | 02:06 PM
#1
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From: Eindhoven, NETHERLANDS
how to check Center of Gravity
This might look like a stupid question, but I still have to ask. Basicly in the past months I have been working on getting my CG right by adding some lead everytime to the tail of my plane. It never seems to end.
The method I use is in level flight to bank the airplane at 60 degrees. The "book" says if the nose goes down, it is nose heavy, if it goes up, it is tail heavy. I check this at the beginning and end of the flight (since the tank is not at CG).
So my question to the pattern fliers is: is this the right method? I would think it is natural for the nose to go down when you bank, since you loose the lift of the wing.
The method I use is in level flight to bank the airplane at 60 degrees. The "book" says if the nose goes down, it is nose heavy, if it goes up, it is tail heavy. I check this at the beginning and end of the flight (since the tank is not at CG).
So my question to the pattern fliers is: is this the right method? I would think it is natural for the nose to go down when you bank, since you loose the lift of the wing.
03-11-2007 | 02:12 PM
#2
My Feedback: (45)
RE: how to check Center of Gravity
The easiest way that I've found is put the airplane in a 45 degree climb, roll to inverted and see what it does. If it maintains perfect flight or climbs, then it is tail heavy. You want just a slight drop of the nose from inverted.
Arch Stafford
Arch Stafford
03-11-2007 | 02:31 PM
#3
RE: how to check Center of Gravity
Just small suggestion, before adding lead consider moving batteries, servos, engine or other pieces of equipment. I don't think I have been using lead in many years.
I agree with Arch suggestion.
Vicente "Vince" Bortone
I agree with Arch suggestion.
Vicente "Vince" Bortone
ORIGINAL: wagen017
This might look like a stupid question, but I still have to ask. Basicly in the past months I have been working on getting my CG right by adding some lead everytime to the tail of my plane. It never seems to end.
The method I use is in level flight to bank the airplane at 60 degrees. The "book" says if the nose goes down, it is nose heavy, if it goes up, it is tail heavy. I check this at the beginning and end of the flight (since the tank is not at CG).
So my question to the pattern fliers is: is this the right method? I would think it is natural for the nose to go down when you bank, since you loose the lift of the wing.
This might look like a stupid question, but I still have to ask. Basicly in the past months I have been working on getting my CG right by adding some lead everytime to the tail of my plane. It never seems to end.
The method I use is in level flight to bank the airplane at 60 degrees. The "book" says if the nose goes down, it is nose heavy, if it goes up, it is tail heavy. I check this at the beginning and end of the flight (since the tank is not at CG).
So my question to the pattern fliers is: is this the right method? I would think it is natural for the nose to go down when you bank, since you loose the lift of the wing.
03-11-2007 | 02:57 PM
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From: Eindhoven, NETHERLANDS
RE: how to check Center of Gravity
Arch, thanks for the info, will try this. I am still curious though if the "60 degree bank" method is valid or not.
Vince: basicly everything I could move, is already moved backwards. Before doing more drastic things, I decided first to use lead to find the correct dynamic CG. Once that is accomplished I want to take of the lead and move the battery to the final correct location. You are right, I normally never use any lead as well.
Vince: basicly everything I could move, is already moved backwards. Before doing more drastic things, I decided first to use lead to find the correct dynamic CG. Once that is accomplished I want to take of the lead and move the battery to the final correct location. You are right, I normally never use any lead as well.
03-11-2007 | 04:29 PM
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From: Austin,
TX
RE: how to check Center of Gravity
When I setup CG I must first prove that the wing incidences and engine thrust angle are correct. Without these being set you cannot test for good CG.
To test for wing incidences; climb to a very high altitude and push the plane over to a vertical nose dive and cut the throttle to idle. If the plane pitches to the top or bottom you must trim it out. Try to trim with elevator until you get a perfect down run without pitch.
Now, fly straight up at full throttle. Does the plane pitch to the top or bottom? If so, your engine thrust angle is not set right. Add down/up thrust until you can get a perfect up run without pitch (do not touch your elevator trims).
Now, fly straight and level. Does the plane pitch up or down? If so, it is probably a CG problem. Try moving the weight forward or aft to cure the pitching problem without touching the elevator trim. Once you can get straight and level without elevator trim AND get a straight down run (with the engine cut) AND get a perfect up run, you are pretty close to a perfect CG.
Now, roll inverted and fly straight and level.. Does it require a bunch of elevator correction? If so, you have a wing incidence problem. Adjust wing incidence and start the process all over again.
Ultimately, you should be able to fly staight up at full throttle without pitch problems, fly straight down with the throttle cut without pitch problems, fly straight and level without pitch problems, fly inverted at a 45 degree up angle without pitch problems and fly inverted straight and level with only a slight down pitch.
To test for wing incidences; climb to a very high altitude and push the plane over to a vertical nose dive and cut the throttle to idle. If the plane pitches to the top or bottom you must trim it out. Try to trim with elevator until you get a perfect down run without pitch.
Now, fly straight up at full throttle. Does the plane pitch to the top or bottom? If so, your engine thrust angle is not set right. Add down/up thrust until you can get a perfect up run without pitch (do not touch your elevator trims).
Now, fly straight and level. Does the plane pitch up or down? If so, it is probably a CG problem. Try moving the weight forward or aft to cure the pitching problem without touching the elevator trim. Once you can get straight and level without elevator trim AND get a straight down run (with the engine cut) AND get a perfect up run, you are pretty close to a perfect CG.
Now, roll inverted and fly straight and level.. Does it require a bunch of elevator correction? If so, you have a wing incidence problem. Adjust wing incidence and start the process all over again.
Ultimately, you should be able to fly staight up at full throttle without pitch problems, fly straight down with the throttle cut without pitch problems, fly straight and level without pitch problems, fly inverted at a 45 degree up angle without pitch problems and fly inverted straight and level with only a slight down pitch.
03-11-2007 | 04:39 PM
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From: Tarzana, CA
RE: how to check Center of Gravity
Nice check list, thanks
Mike R
Mike R
ORIGINAL: Jeff_edge540
When I setup CG I must first prove that the wing incidences and engine thrust angle are correct. Without these being set you cannot test for good CG.
To test for wing incidences; climb to a very high altitude and push the plane over to a vertical nose dive and cut the throttle to idle. If the plane pitches to the top or bottom you must trim it out. Try to trim with elevator until you get a perfect down run without pitch.
Now, fly straight up at full throttle. Does the plane pitch to the top or bottom? If so, your engine thrust angle is not set right. Add down/up thrust until you can get a perfect up run without pitch (do not touch your elevator trims).
Now, fly straight and level. Does the plane pitch up or down? If so, it is probably a CG problem. Try moving the weight forward or aft to cure the pitching problem without touching the elevator trim. Once you can get straight and level without elevator trim AND get a straight down run (with the engine cut) AND get a perfect up run, you are pretty close to a perfect CG.
Now, roll inverted and fly straight and level.. Does it require a bunch of elevator correction? If so, you have a wing incidence problem. Adjust wing incidence and start the process all over again.
Ultimately, you should be able to fly staight up at full throttle without pitch problems, fly straight down with the throttle cut without pitch problems, fly straight and level without pitch problems, fly inverted at a 45 degree up angle without pitch problems and fly inverted straight and level with only a slight down pitch.
When I setup CG I must first prove that the wing incidences and engine thrust angle are correct. Without these being set you cannot test for good CG.
To test for wing incidences; climb to a very high altitude and push the plane over to a vertical nose dive and cut the throttle to idle. If the plane pitches to the top or bottom you must trim it out. Try to trim with elevator until you get a perfect down run without pitch.
Now, fly straight up at full throttle. Does the plane pitch to the top or bottom? If so, your engine thrust angle is not set right. Add down/up thrust until you can get a perfect up run without pitch (do not touch your elevator trims).
Now, fly straight and level. Does the plane pitch up or down? If so, it is probably a CG problem. Try moving the weight forward or aft to cure the pitching problem without touching the elevator trim. Once you can get straight and level without elevator trim AND get a straight down run (with the engine cut) AND get a perfect up run, you are pretty close to a perfect CG.
Now, roll inverted and fly straight and level.. Does it require a bunch of elevator correction? If so, you have a wing incidence problem. Adjust wing incidence and start the process all over again.
Ultimately, you should be able to fly staight up at full throttle without pitch problems, fly straight down with the throttle cut without pitch problems, fly straight and level without pitch problems, fly inverted at a 45 degree up angle without pitch problems and fly inverted straight and level with only a slight down pitch.
03-12-2007 | 03:14 PM
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From: , ITALY
RE: how to check Center of Gravity
ORIGINAL: rcpattern
The easiest way that I've found is put the airplane in a 45 degree climb, roll to inverted and see what it does. If it maintains perfect flight or climbs, then it is tail heavy. You want just a slight drop of the nose from inverted.
Arch Stafford
The easiest way that I've found is put the airplane in a 45 degree climb, roll to inverted and see what it does. If it maintains perfect flight or climbs, then it is tail heavy. You want just a slight drop of the nose from inverted.
Arch Stafford
I think the KE turn test is not good, since you are adding more and more weight to the tail. In pattern a "forward" CG is preferred, since it gives more pitch stability, the plane is more locked in. Actually how much forward is quite personal, but the mentioned test is surely a good starting point.
I suggest you to follow one of the trim charts. A really good (and simple) is that one by the great Don Szczur:
http://www.horizonhobby.com/Articles...ArticleID=1194
Also check Don's forum here on RCU, it has plenty of suggestions even on C.G.
