help with C o G on .40 cap 232
10-22-2008, 11:38 AM
#1
Junior Member
Thread Starter
Join Date: Oct 2008
Location: Hamilton, OH
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
help with C o G on .40 cap 232
Alright I'm getting back into flying R/C again. I have mastered the trainer and have decided to move onto a .40 size cap. Yeah I know, big leap. I have flown before a few years ago, and feal like I'm ready for the cap.
I picked up the cap from a swap meet, it is a used ARF, in excelent condition. The PO told me that it was a thunder tiger Cap 232. I cannot find a manual for a thunder tiger cap.
The problem I have ran into is that I do not know where to set the center of gravity. From my past expierences I remember that it is ~ 25% of the wing cord from the leading edge, which puts it about 3 1/8" back from the LE which is also the tallest part of the wing. This seams to me like it would be about right, do you agree?
The next problem is that in order to balance at that point, I will need to add about .4 pounds (6.4 oz) to the nose. That seams like a whole lot to me. Is it really a lot, or no? I have already moved the battery as far forward as possible.
The motor on it is a evolution .40. I have an OS MAX .58, but just weighed it and it is actually less weight so swapping wont help.
There is two lead weights on the firewall from the PO and it looks like there used to be 3 more.
Please help me out, I'm eager to get this thing into the air.
I picked up the cap from a swap meet, it is a used ARF, in excelent condition. The PO told me that it was a thunder tiger Cap 232. I cannot find a manual for a thunder tiger cap.
The problem I have ran into is that I do not know where to set the center of gravity. From my past expierences I remember that it is ~ 25% of the wing cord from the leading edge, which puts it about 3 1/8" back from the LE which is also the tallest part of the wing. This seams to me like it would be about right, do you agree?
The next problem is that in order to balance at that point, I will need to add about .4 pounds (6.4 oz) to the nose. That seams like a whole lot to me. Is it really a lot, or no? I have already moved the battery as far forward as possible.
The motor on it is a evolution .40. I have an OS MAX .58, but just weighed it and it is actually less weight so swapping wont help.
There is two lead weights on the firewall from the PO and it looks like there used to be 3 more.
Please help me out, I'm eager to get this thing into the air.
10-23-2008, 12:08 AM
#2
RE: help with C o G on .40 cap 232
I believe that the Cap has a straight tapered wing, if so then the MAC will be the chord line half way from the fuselage to the tip. If you put the balance point somewhere between 1/4 and 1/3 the way back from the leading edge on this chord, then transfer these points back to the fuselage side you will be safe enough for the first flight. The weight required to balance the model matters not a jot. If you need it, you need it. The Cap has a reputation as a bit of a tip staller (snaps easily), so do not be persuaded to move the balance much further back, and closer to the 1/4 chord than the 1/3 for that first flight.
Evan.
Evan.
10-23-2008, 06:16 AM
#3
Junior Member
Thread Starter
Join Date: Oct 2008
Location: Hamilton, OH
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
RE: help with C o G on .40 cap 232
Thanks for the help. I couldn't remember how to find the MAC so I looked it up. I'll post it here for anyone else's future reference.
Here is a link,
http://www.airfieldmodels.com/inform...amic_chord.htm
http://en.wikipedia.org/wiki/Mean_aerodynamic_chord
To Locate the Mean Aerodynamic Chord on a Tapered or Delta Wing
Measure the root and tip chord. Then draw the following lines on the plans:
At the root of the wing, draw a line parallel to the centerline of the fuselage extending forward from the leading edge and rearward from the trailing edge. Both lines should be the length of the tip chord.
Do the same thing at the tip but drawing the lines the length of the root chord.
Connect the ends of the lines so that they create an "X" over the wing panel. Where the two lines intersect is the spanwise location of the Mean Aerodynamic Chord.
If the plan indicates that the CG should be located at some percentage of the MAC, then measure the MAC and put the CG the given percentage back from the leading edge along the MAC. For example, if the MAC is 10" and the plan indicates the CG should be 25% back from the leading edge, then the CG is 2-1/2" back from the leading edge at the MAC.
This drawing should help you visualize what you need to do:
Note: The lines cross at the spanwise location of the MAC. It is not the fore/aft CG location (unless the CG happens to be located at 50% MAC).
The following formula will give the measurement (chord) of the MAC. It does not give the span wise location of the MAC.
rc = Root Chord
t = Taper Ratio = (Tip Chord ÷ Root Chord)
MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t ))
Using the drawing above, let's say the root chord is 11" and the tip chord is 6"
t = 6 ÷ 11 = .5455
Now plug t into the formula to find the MAC. Note that the wingspan and sweep do not matter. No matter what the span or how much the wing is swept, the MAC will always be the same length.
MAC = 11 x 2/3 x (( 1 + .5455 + .54552 ) ÷ ( 1 + .5455 ))
MAC = 22/3 x ( 1.8431 ÷ 1.5455 )
MAC = 7.3333 x ( 1.8431 ÷ 1.5455)
MAC = 7.3333 x 1.19254
MAC = 8.7453"
Here is a link,
http://www.airfieldmodels.com/inform...amic_chord.htm
http://en.wikipedia.org/wiki/Mean_aerodynamic_chord
To Locate the Mean Aerodynamic Chord on a Tapered or Delta Wing
Measure the root and tip chord. Then draw the following lines on the plans:
At the root of the wing, draw a line parallel to the centerline of the fuselage extending forward from the leading edge and rearward from the trailing edge. Both lines should be the length of the tip chord.
Do the same thing at the tip but drawing the lines the length of the root chord.
Connect the ends of the lines so that they create an "X" over the wing panel. Where the two lines intersect is the spanwise location of the Mean Aerodynamic Chord.
If the plan indicates that the CG should be located at some percentage of the MAC, then measure the MAC and put the CG the given percentage back from the leading edge along the MAC. For example, if the MAC is 10" and the plan indicates the CG should be 25% back from the leading edge, then the CG is 2-1/2" back from the leading edge at the MAC.
This drawing should help you visualize what you need to do:
Note: The lines cross at the spanwise location of the MAC. It is not the fore/aft CG location (unless the CG happens to be located at 50% MAC).
The following formula will give the measurement (chord) of the MAC. It does not give the span wise location of the MAC.
rc = Root Chord
t = Taper Ratio = (Tip Chord ÷ Root Chord)
MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t ))
Using the drawing above, let's say the root chord is 11" and the tip chord is 6"
t = 6 ÷ 11 = .5455
Now plug t into the formula to find the MAC. Note that the wingspan and sweep do not matter. No matter what the span or how much the wing is swept, the MAC will always be the same length.
MAC = 11 x 2/3 x (( 1 + .5455 + .54552 ) ÷ ( 1 + .5455 ))
MAC = 22/3 x ( 1.8431 ÷ 1.5455 )
MAC = 7.3333 x ( 1.8431 ÷ 1.5455)
MAC = 7.3333 x 1.19254
MAC = 8.7453"
10-23-2008, 06:29 AM
#4
Senior Member
RE: help with C o G on .40 cap 232
ORIGINAL: 4x4fab
Thanks for the help. I couldn't remember how to find the MAC so I looked it up. I'll post it here for anyone else's future reference.
Here is a link,
http://www.airfieldmodels.com/inform...amic_chord.htm
http://en.wikipedia.org/wiki/Mean_aerodynamic_chord
To Locate the Mean Aerodynamic Chord on a Tapered or Delta Wing
Measure the root and tip chord. Then draw the following lines on the plans:
At the root of the wing, draw a line parallel to the centerline of the fuselage extending forward from the leading edge and rearward from the trailing edge. Both lines should be the length of the tip chord.
Do the same thing at the tip but drawing the lines the length of the root chord.
Connect the ends of the lines so that they create an "X" over the wing panel. Where the two lines intersect is the spanwise location of the Mean Aerodynamic Chord.
If the plan indicates that the CG should be located at some percentage of the MAC, then measure the MAC and put the CG the given percentage back from the leading edge along the MAC. For example, if the MAC is 10" and the plan indicates the CG should be 25% back from the leading edge, then the CG is 2-1/2" back from the leading edge at the MAC.
This drawing should help you visualize what you need to do:
Note: The lines cross at the spanwise location of the MAC. It is not the fore/aft CG location (unless the CG happens to be located at 50% MAC).
The following formula will give the measurement (chord) of the MAC. It does not give the span wise location of the MAC.
rc = Root Chord
t = Taper Ratio = (Tip Chord ÷ Root Chord)
MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t ))
Using the drawing above, let's say the root chord is 11" and the tip chord is 6"
t = 6 ÷ 11 = .5455
Now plug t into the formula to find the MAC. Note that the wingspan and sweep do not matter. No matter what the span or how much the wing is swept, the MAC will always be the same length.
MAC = 11 x 2/3 x (( 1 + .5455 + .54552 ) ÷ ( 1 + .5455 ))
MAC = 22/3 x ( 1.8431 ÷ 1.5455 )
MAC = 7.3333 x ( 1.8431 ÷ 1.5455)
MAC = 7.3333 x 1.19254
MAC = 8.7453"
Thanks for the help. I couldn't remember how to find the MAC so I looked it up. I'll post it here for anyone else's future reference.
Here is a link,
http://www.airfieldmodels.com/inform...amic_chord.htm
http://en.wikipedia.org/wiki/Mean_aerodynamic_chord
To Locate the Mean Aerodynamic Chord on a Tapered or Delta Wing
Measure the root and tip chord. Then draw the following lines on the plans:
At the root of the wing, draw a line parallel to the centerline of the fuselage extending forward from the leading edge and rearward from the trailing edge. Both lines should be the length of the tip chord.
Do the same thing at the tip but drawing the lines the length of the root chord.
Connect the ends of the lines so that they create an "X" over the wing panel. Where the two lines intersect is the spanwise location of the Mean Aerodynamic Chord.
If the plan indicates that the CG should be located at some percentage of the MAC, then measure the MAC and put the CG the given percentage back from the leading edge along the MAC. For example, if the MAC is 10" and the plan indicates the CG should be 25% back from the leading edge, then the CG is 2-1/2" back from the leading edge at the MAC.
This drawing should help you visualize what you need to do:
Note: The lines cross at the spanwise location of the MAC. It is not the fore/aft CG location (unless the CG happens to be located at 50% MAC).
The following formula will give the measurement (chord) of the MAC. It does not give the span wise location of the MAC.
rc = Root Chord
t = Taper Ratio = (Tip Chord ÷ Root Chord)
MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t ))
Using the drawing above, let's say the root chord is 11" and the tip chord is 6"
t = 6 ÷ 11 = .5455
Now plug t into the formula to find the MAC. Note that the wingspan and sweep do not matter. No matter what the span or how much the wing is swept, the MAC will always be the same length.
MAC = 11 x 2/3 x (( 1 + .5455 + .54552 ) ÷ ( 1 + .5455 ))
MAC = 22/3 x ( 1.8431 ÷ 1.5455 )
MAC = 7.3333 x ( 1.8431 ÷ 1.5455)
MAC = 7.3333 x 1.19254
MAC = 8.7453"
If you're going to go to that much work to find the MAC, then you're a believer in formulas.
Since the pitch control of a wing is a function of the tail, why not consider it too. And do almost no real work and let another guy do all the figuring. How do you do that?
Take 5 minutes and measure your model with a yardstick. Plug the measurements into http://www.geistware.com/rcmodeling/cg_super_calc.htm and click.
10-23-2008, 06:41 AM
#5
Senior Member
RE: help with C o G on .40 cap 232
Bottom line on the CG is that you set it to begin with by any means you know to be safe. The mfg's CG is usually pretty safe, but with today's ARFs might be a sad mistake. (his mistake, your sadness) Next safest are the deals that're supposed to work for everything, but are really only safe for a model that has everything average about it. Safest is to consider what things give an airplane pitch stability and control. Easiest way to do that is to use somebody's application that does the figuring for you and uses all the important details.
That's what http://www.geistware.com/rcmodeling/cg_super_calc.htm does for you.
All you need is a yardstick and the time to make 9 measurements on your airplane.
But the real bottom line is that the numbers you get from ANY of these methods is just a safe starting point. There ain't no magic CG. And if there was, you wouldn't get it from "conventional wisdom" or from less than a humongous formula and maybe some additional tests (like in a windtunnel). The airplane knows where it wants the CG, and all these methods give us a good start.
So bottom line on the CG is a safe starting place for the maiden, and you need to have the elevator throws to match. After/during the maiden, you then start to tune your elevator throw and CG location to where your airplane tells you it wants them. Don't get hung up thinking any of our applications, formulas, and simple methods are going to give anything more than a start.
That's what http://www.geistware.com/rcmodeling/cg_super_calc.htm does for you.
All you need is a yardstick and the time to make 9 measurements on your airplane.
But the real bottom line is that the numbers you get from ANY of these methods is just a safe starting point. There ain't no magic CG. And if there was, you wouldn't get it from "conventional wisdom" or from less than a humongous formula and maybe some additional tests (like in a windtunnel). The airplane knows where it wants the CG, and all these methods give us a good start.
So bottom line on the CG is a safe starting place for the maiden, and you need to have the elevator throws to match. After/during the maiden, you then start to tune your elevator throw and CG location to where your airplane tells you it wants them. Don't get hung up thinking any of our applications, formulas, and simple methods are going to give anything more than a start.
10-23-2008, 07:00 AM
#6
Senior Member
RE: help with C o G on .40 cap 232
BTW, one reason the CAP 232s have a bad rep has to do with their wing. (Sort of the same reason most tapered wing models have bad reps with newbies.)
It is a straight taper, but it's also a forward swept wing. If you notice in the attached picture, there isn't much LE sweep. But there's a lot of TE sweep. That actually means that aerodynamically, that wing is swept forward.
A CG is actually an "average" value. And we place it according to where the average of the wing is along the fore/aft line on the fuselage. Where is the average of the wing along the fuselage? Depends on the sweep of the wing. With a CAP, the root chord isn't a good indicator of where the wing works. And on tapered wings, you really have to consider the tip chord's size and location to even begin to guess where the MAC might be along the span.
I had an excellent flying Kyosho CAP and a buddy got a knockoff of it. I wound up assembling it for him and setting the CG where I'd worked out mine, but a little forward. I wanted to use the same throws, but his to work a bit slower for him. Some time later, one of his buddies was checking the plane out and balanced it on his fingers. The buddy suggested the CG needed to be moved back because the plane was nose heavy. Keep in mind that it flew like gangbusters and was anything but noseheavy. The buddy was using one of the simple tests. And it was too simple for the layout of a CAP wing. AND getting just one little detail about his test wrong.
It's kewl to simplify things, but it's then best to apply those things to just the simple models.
It is a straight taper, but it's also a forward swept wing. If you notice in the attached picture, there isn't much LE sweep. But there's a lot of TE sweep. That actually means that aerodynamically, that wing is swept forward.
A CG is actually an "average" value. And we place it according to where the average of the wing is along the fore/aft line on the fuselage. Where is the average of the wing along the fuselage? Depends on the sweep of the wing. With a CAP, the root chord isn't a good indicator of where the wing works. And on tapered wings, you really have to consider the tip chord's size and location to even begin to guess where the MAC might be along the span.
I had an excellent flying Kyosho CAP and a buddy got a knockoff of it. I wound up assembling it for him and setting the CG where I'd worked out mine, but a little forward. I wanted to use the same throws, but his to work a bit slower for him. Some time later, one of his buddies was checking the plane out and balanced it on his fingers. The buddy suggested the CG needed to be moved back because the plane was nose heavy. Keep in mind that it flew like gangbusters and was anything but noseheavy. The buddy was using one of the simple tests. And it was too simple for the layout of a CAP wing. AND getting just one little detail about his test wrong.
It's kewl to simplify things, but it's then best to apply those things to just the simple models.
10-23-2008, 11:10 AM
#7
Junior Member
Thread Starter
Join Date: Oct 2008
Location: Hamilton, OH
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
RE: help with C o G on .40 cap 232
I wish I could have just found a manual for this plane.
Thanks for the info and for the link. I will use the link you suggested to do my figuring and hopefully maiden flight will be tomorrow.
Thanks for the info and for the link. I will use the link you suggested to do my figuring and hopefully maiden flight will be tomorrow.
10-27-2008, 11:08 AM
#8
Junior Member
Thread Starter
Join Date: Oct 2008
Location: Hamilton, OH
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
RE: help with C o G on .40 cap 232
i got her in the air for about 4 flights. I was amazed at how easy flying this plane is. However, it comes in fast. I cannot slow this plane down enough for a decent landing. I removed 1/2 OZ from the nose and it didn't help, it may have made it even worse........I'll have to continue playing with the CG
10-27-2008, 11:23 AM
#9
RE: help with C o G on .40 cap 232
Landing speed has nothing to do with the balance point. You sound like you need more elevator movement and more practise at slow/circuit flying with this model.
Evan.
Evan.
10-27-2008, 12:04 PM
#10
Senior Member
RE: help with C o G on .40 cap 232
ORIGINAL: 4x4fab
i got her in the air for about 4 flights. I was amazed at how easy flying this plane is. However, it comes in fast. I cannot slow this plane down enough for a decent landing. I removed 1/2 OZ from the nose and it didn't help, it may have made it even worse........I'll have to continue playing with the CG
i got her in the air for about 4 flights. I was amazed at how easy flying this plane is. However, it comes in fast. I cannot slow this plane down enough for a decent landing. I removed 1/2 OZ from the nose and it didn't help, it may have made it even worse........I'll have to continue playing with the CG
Try taking the plane up high and slowing it until it stalls. All that I've seen would almost come to a standstill. It's often a function of how low you've gotten the engine to idle and whether or not the prop pitch at that rpm works. This will show you for sure how slow it can go with you on the sticks and the elevator thows where you've got 'em. And it'll go that slow on approach or anywhere else.
Moving the CG back really doesn't change much about approach speed other than how effective the elevator is going to be and how quickly it'll bite you when it stalls. If you're not worried about that tapered wing stalling on approach, you really don't have the engine idle low enough.
Which engine did you put into it? And what prop? Evolution engines will idle down reliably, especially after the beginner protection bits are removed.
10-27-2008, 03:53 PM
#11
Junior Member
Thread Starter
Join Date: Oct 2008
Location: Hamilton, OH
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
RE: help with C o G on .40 cap 232
ORIGINAL: pimmnz
Landing speed has nothing to do with the balance point. You sound like you need more elevator movement and more practise at slow/circuit flying with this model.
Evan.
Landing speed has nothing to do with the balance point. You sound like you need more elevator movement and more practise at slow/circuit flying with this model.
Evan.
Thanks
