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Jun 19, 2020, 01:09 AM
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Thread OP
Discussion

Someone have experience with cg calculator?


I did build a foamy airplane. I did use a cg calculator for get the cg.... now on the drawing stands a cg point, but when I calculate with cg calculator I get the cg about 5cm off from the one on the drawing.

Witch would you thrust? Is a cg calculator correct more then the cg given on the drawing?

Thx, greetings from ed
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Jun 19, 2020, 01:36 AM
Every gram is sacred
Extreme Sports's Avatar
Can you post a picture of the foam plans with CG location clearly marked, and the calculator results (most of the calculators output a diagram, so that would be ideal). With a 5cm difference we should be able to tell which is the wrong one just by looking. Most likely an error when using the calculator.

If no pictures, take a photo of your plane and make marks to show us the two competing locations.
Jun 19, 2020, 05:53 AM
Registered User
Thread OP
I will do this weekend, I did had this BMI flycat x-75 before, but now it's not available anymore, so I build one from foam myself.
The then buyed bobcat had a cg about 2/3 from the leading edge.
The calculator show about the same cg at also 2/3 from leading edge.

But the pussycat drawing I have say it's about 1/3 from leading edge...
Somehow I dont trust the cg from the drawing cos my experience before with the original flycat.
Jun 19, 2020, 06:02 AM
Every gram is sacred
Extreme Sports's Avatar
If the pussycat is this glider or similar: https://outerzone.co.uk/plan_details.asp?ID=7778, then the CG will be about 1/3 from the LE. Nothing else with that name comes up on google.

If the Bobcat and flycat are the swept wing planes that come up with a google search, then their CGs will be further back from the LE. The reason being that the swept leading edge means that most of the wing area is further back relative to an unswept one, so the wing neutral point also moves back. CG needs to be just ahead of the neutral point, so it too must move back. 2/3 back is about right for a swept wing (depends on the angle of sweep of course).
Jun 19, 2020, 08:07 AM
Registered User
sdbruder's Avatar
Can you post a picture with measurements of your wing?

I've wrote http://wingcgcalc.bruder.com.br/ and http://winglib.com/, so perhaps I can help you.
Jun 19, 2020, 09:44 AM
Registered User
Thread OP
So far thank you for the help. Its pitty to throw it and not be able to controll and crash. I hope I do good, this is the model in this topic.
https://www.rcgroups.com/forums/show...shy-Cat/page15

The screenshot got the data measured.
Last edited by denof; Jun 19, 2020 at 09:52 AM.
Jun 19, 2020, 11:38 AM
Flying a Falcon or a 3D model.
fauconnier's Avatar
It's not a flying wing , this plane has a tail.

Use this one: Ecalc CG calculator.

https://www.ecalc.ch/cgcalc.php
Jun 19, 2020, 01:21 PM
An itch?. Scratch build.
eflightray's Avatar
The model looks like one of the 'Cat' series of twin boom pushers, Pushy Cat, Bob Cat, Tough Cat etc.

24" HouseCat (Mini-Bobcat) plans -- if you click on the 'Thread Tools' and then the 'Show Attachments in this Thread' in the drop down, you should see all the pictures of the variations. Plus you could search the thread for 'CG'.

.
Jun 19, 2020, 01:58 PM
Registered User
Thread OP
Thx all, I go do some search.
Jun 21, 2020, 05:00 AM
Registered User
Thread OP
The pushy cat did fly, 4 lipos then go home.
Cg is as on the cg calculator. Just before 12cm.

Thanks you all.
Jun 22, 2020, 07:15 PM
flyin' fool
goldguy's Avatar
All you need is this ...................... https://fwcg.3dzone.dk/ , works for flying wings and conventional designs with a tail.

The calc is a good start, but fine tuning in flight is still required.
Jun 22, 2020, 11:21 PM
flyin' fool
goldguy's Avatar
The only difference is to set the CG at 30% for a conventional design.
Jun 23, 2020, 10:56 AM
Registered User
Thread OP
Oké, so, if I not mistake my plane is at 30% ?
Then after calculate finetune when fly.
Jun 23, 2020, 01:21 PM
An itch?. Scratch build.
eflightray's Avatar
But 30% of ??

Try this one -- https://rcplanes.online/cg_calc.htm

It's not as complicated as it may look, only 4 basic dimensions.

Also read the note regarding 'static margin', it gives an idea of what effect having a forward or rearward cg can have.

.
Jun 24, 2020, 07:25 AM
flyin' fool
goldguy's Avatar
It's clear on the calc what is represented by 30%.


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