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May 23, 2019, 11:02 AM
Keith
kknox's Avatar
Thread OP
Quote:
Originally Posted by Twangmaster
for over 50 years, i've just used my fingers (or mine and a helpers) to check CG. never had a problem, probably never will. over thinking makes my head hurt
I understand but trying to balance a 27 lb plane on your fingertips is a tad difficult.............LOL

Keith
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May 23, 2019, 11:08 AM
Keith
kknox's Avatar
Thread OP
Guys, I am having issues and not sure what the problem is. Since my scales don't go high enough for my P-40 I decided to give my FMS 1450 P-51 a shot on the scales......

I normally use the Great Planes balance unit and the book says 110mm for CG. I put the plane on the Great Planes balancer and it is very close to 110mm. When I put the plane on the 3 scales I get 74.76mm for CG. Right wheel weight is 1209 grams, Left wheel weight is 1185 grams and tail weight is 311 grams. Plane is propped up on the tail for level flight - best I can get it. The length Hub to Hub is 25.5 inches - measured from center of main gear to center of tail wheel............

Question - do you measure from center of the actual wheel or center of the main gear shaft as it goes into the airframe? I am guessing it would be center of the actual main wheel to center of the tail wheel but I want to confirm as my readings are way off compared to my Balancer setup. I have flown this plane before and it has been a little touchy but nothing crazy crazy.

Any thoughts?? I plan on throwing my Piper Cub on the scales as I know it is dialed in perfectly for CG and flys like a champ.

I am using the Excel spreadsheet that Andy sent to me on page 1.

Thanks much

Keith
May 23, 2019, 03:19 PM
Keith
kknox's Avatar
Thread OP
Okay I received my new scales and put the P-40 on all three. The plane is in flying mode - level and all scales were set to zero before putting the wheels on them.

Readings are:
Distance Main Hub to Tail Wheel Hub - center to center = 37.75"

Manual says CG = 3.75" / 95.25mm

Main Right Wheel Weight: 6270 grams
Main Left Wheel Weight: 6517 grams
Tail Wheel Weight: 800 grams

Calculations =

Total Weight: 29.95 lbs - very heavy for this plane - average is 22 lbs. I added a large amount of weight to the nose from the Vanessa Rig results.

Calculated CG = 2.22 inches / 56.46 mm

Am I correct is evaluting the results that the nose is very heavy??

I will remove my weight from the nose and start over and see what i get.........

Your suggestions and comments are much appreciated........

Keith
May 23, 2019, 03:27 PM
Registered User
ace4rc's Avatar
I don't know what spreadsheet you are using but I am going to assume it is correct. Measure the mains and nose/tail from the specified given point, my spread sheet is the leading tip of the spinner. From the specified given point (leading edge of spinner) measure and mark the kit or ARF's specified cg. Weight the plane and add or remove the weight per the spreadsheet. The cg point should not be moving around but rather the amount of nose/tail weight to get it's weight balanced at the given cg.

What ever your specified point is the measurement for the cg should be in there somewhere as well as that is what you are trying to balance to. If your spreadsheet is of a type where you have to keep playing with the weight until it gets to a measurement that matches your cg, pm me, I have a nice spreadsheet that tells you how much weight to add or remove to meet the cg point, much easier.
Last edited by ace4rc; May 23, 2019 at 03:35 PM.
May 23, 2019, 04:25 PM
Pro Hoarder
turbonut's Avatar
I dont see where you are converting the numbers form the scales to relate to the wing leading edge. You need to find the distance form the main gear to the leading edge of the wing with that calculation method.
I do all my calc as Ace4rc does using the nose of the plane as a datum
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May 23, 2019, 06:00 PM
Keith
kknox's Avatar
Thread OP
I understand what you are saying - I think there are different calculations out there. Please send me your formulas so I can double check mine........

My formula tells you where the CG is from the leading edge of the wing......... you then need to either add or remove weight to move the CG to a specific point.

Example: My last calculation told me the CG is at 3.26" from the leading edge. The factory suggestion is 3.75". If I either move the weight forward or aft or change the amount of weight then the formula will tell me where the new CG is. The formula only requires the weight from each wheel and measurement from center of main gear wheel to center of tail gear wheel.


Thanks!!!

Keith
May 23, 2019, 07:45 PM
Registered User
ace4rc's Avatar
On this spread sheet you enter the parameters, including cg point, and it will tell you what the tail or nose should weigh. And if you have three scales once you know what the weight should be you can adjust components until the desired weight is reached without removing from the scales or recalculating. First thing you should do though is remove any dead weight, start fresh.
May 23, 2019, 09:39 PM
Keith
kknox's Avatar
Thread OP
Quote:
Originally Posted by ace4rc
On this spread sheet you enter the parameters, including cg point, and it will tell you what the tail or nose should weigh. And if you have three scales once you know what the weight should be you can adjust components until the desired weight is reached without removing from the scales or recalculating. First thing you should do though is remove any dead weight, start fresh.
Sounds good...... when you get a minute please attach the spread sheet or you can message me and I will give you my email.

Thanks much

Keith
May 24, 2019, 07:42 AM
AndyKunz's Avatar
Quote:
Originally Posted by kknox
Calculated CG = 2.22 inches / 56.46 mm
That is 2.22" behind the center of the axle, which is the 0 reference point for the spreadsheet.

It would be easy enough to add another entry in the spreadsheet to convert that to the leading edge reference.

To get where it is relative to the leading edge, put a carpenter's square on the table at the LE. Measure the distance from the vertical edge of the square to the axle.

Very good question, btw, and thanks for pointing out the need. I have updated the spreadsheet (attached below) to include this information.

Andy
May 24, 2019, 08:36 AM
An itch?. Scratch build.
eflightray's Avatar
Here's a couple of CG calculator using model dimensions and areas ---

http://www.willingtons.com/aircraft_...vity_calcu.htm

https://rcplanes.online/cg_calc.htm

.
May 24, 2019, 09:16 AM
AndyKunz's Avatar
Thanks, Ray. Between having an online calculator to predict a good starting place, and a spreadsheet to measure the actual location, it looks like the bases are covered!

Andy
May 24, 2019, 09:27 AM
Registered User
ace4rc's Avatar
If one of you guys can tell me how to load an Excel spread sheet I'll post mine. The website I got it from no longer has it posted.
May 24, 2019, 10:00 AM
AndyKunz's Avatar
You have to put it into a .ZIP file first.

Then click on the "Go Advanced" button.

Drag and drop the zip file into the box that is shown beneath the comment area.

Enter your comments.

When the zip upload is complete, click on the submit button.

Sounds like your spreadsheet does basically the same thing, telling you how to adjust. On my Weights & Balances spreadsheet (elsewhere) it helps you by telling you how much weight to put where.

Andy
May 24, 2019, 11:17 AM
Registered User
ace4rc's Avatar
Quote:
Originally Posted by AndyKunz
Sounds like your spreadsheet does basically the same thing, telling you how to adjust. On my Weights & Balances spreadsheet (elsewhere) it helps you by telling you how much weight to put where.

Andy
Sounds like ours is real similar, mine tell's you what the tail or nose should weigh. As for that other stuff, to be quite honest, I have had it for years and don't want to do anything to risk losing it. I can add it in to an email easily and have done so for others numerous times but that is about as far as I am going with it. Besides, I have no idea how to take it from my desktop and zip it.
Last edited by ace4rc; May 24, 2019 at 11:23 AM.
May 24, 2019, 11:26 AM
Registered User
ace4rc's Avatar
Ok, that went a lot easier than I thought it would be. No more than I posted I would not, I copied the file to another folder, zipped the folder, and it loaded here.


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