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Help for a Non-destructive Center of Gravity for new designs?

FoamyDM

Building Fool-Flying Noob
#1
CG - Can anyone help?

I've looked at a bunch of things online but I wanted to understand how to determine the Center of Gravity (CG) of a completely new designs. I want to do this without destroying the plane in the process.

What I know.
- Pick a point, (1/3 of wing on traditional wings) balance it.
- Toss it and observe. Stall - CG point's to far back. Dumps - CG's too far forward.
- Adjust, rinse repeat.
- When it glides nice, Mark the balance point, Add equipment, re-balance and fly.

Specifically I want to maiden my Forward swept wing without destroying it. My AF-X1 maiding was unsuccessful due to bad guess on CG. (and weak build).

I throw myself upon this communities collective mercy and knowledge.
 

makattack

Winter is coming
Moderator
Mentor
#3
After all the calculations and software simulations, if you want to try a glide test without risk of damage, I would build a mock/model without any electronics, balance it out, and give it a toss into either a net or a soft bed or pad. Foamboard is great for that! :)

Welcome to the FT Forums!
 

Hai-Lee

Old and Bold RC PILOT
#4
A much more difficult approach in determining how your design will perform is to make a very cheap wind tunnel out of FB or similar. Plans are available on the net.

For my initial prototypes I fit an undercarriage and do high speed taxi runs and after I get it to taxi properly I then try to do very short hops. After the short hops I then progress to lifting off and maintaining a level flight at a very low height for a short run. With all that done I do a high speed ground run and a sharp climb to gain a decent height before I attempt to evaluate its flight performance and maneuverability. Whilst at height I trim the aircraft for level flight and practice my landing approaches to judge where I stall out and then how it behaves without motor thrust.

Having done all of those tests I then bring it in for a "Greased" landing and study all of the control surface settings etc and adjust accordingly. Please note I also adjust all other prototypes of the same model identically as well.

When designing and prototyping any model aircraft I always plan to make at least three. When testing the design I feel happy when the third one flies well. On the odd occasion that the design works straight out of the box I gift the other prototypes to fellow fliers who are willing to test fly and evaluate a design with me.

I am sure even FT has a big bin for bent, destroyed and even failed projects because it is just part of the process.
 

TexMechsRobot

Posted a thousand or more times
#6
How do people find these calculators, this one is really cool! We should have a sticky dedicated to these kinds of calculators. I am still trying to find a motor calc that doesnt want money...
eCalc is worth it in my book and it's pretty cheap IMHO.
 

FoamyDM

Building Fool-Flying Noob
#7
TechsMechsRobot: Thank you. I used this: http://wingcgcalc.bruder.com.br/en_US/ with the results here http://j.mp/2jryIAU
Do you balance at this location first then? Is that because this is where the centroid of wing's vertical lift load is?

Makattack: Mock Model. - Build V2 try the CG out. good idea. kills two birds with 1 stone. Thank you for the Forum Welcome. I'm curious, what do you fly/do in the winter, and I grew up in CT and know snow time is long in MA?

Hai-lee: Thank you so much for details on your method and field testing. This is super helpful. Do you test the Taxi tests to ensure it doesn't fail while trying to get it off the ground? (like not veering off-runway, or turning when landing.) I'm not sure what you mean by "For my initial prototypes I fit an undercarriage..." What do you mean by fitting and undercarriage? Do you mean put wheels on it? The short hop check is similar to what I had in my head for seeing if it'll get airborne, but wasn't thinking about a taxi check.

I knew I'd make more after some testing (at least for a build article), but I find it interesting that you plan on three. Are these three copies or three iterations?

Sorry for the questions all, I'm just really curious. I'm hoping to plan my way out of my maiden flight anxieties, so I just do it already. Thanks everyone for the replies so far.

These are a great help. Please continue.
 
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TexMechsRobot

Posted a thousand or more times
#9
TechsMechsRobot: Thank you. I used this: http://wingcgcalc.bruder.com.br/en_US/ with the results here http://j.mp/2j4nJRr
Do you balance at this location first then? Is that because this is where the centroid of wing's vertical lift load is?
I took a look at that calculator and I love it! I like how you can program different wing shapes.

Based on the field for "CG Position" and how it ranks the percentages, it seems like you should balance your plane at that point and expect decent results. I would still use this as a ballpark though. Only actually flight testing will tell you if it's balanced correctly. However, you'll be starting from a point that's close to the actual point.
 

Hai-Lee

Old and Bold RC PILOT
#10
TexMech is correct in that calculators only ever give you a ballpark for CG. The reason is simply that different wing profiles have different efficiency and lift centres. The differences are not massive but enough to require you to search for the sweet spot.
 

FoamyDM

Building Fool-Flying Noob
#11
Wing load

Cool. Do you think Flite Test designers uses these CG calculators?

Secondly/annoyingly the CG calculator I used gives a very low wing load 4.5 oz./sf. according to some sources,this puts it in a beginner and sailplane catagory. What does this mean really? Will this lift big-time? The plane is larger than I've modeled (missing the cockpit and canards) I've read up on it but still feel like a noob in this area.
 

TexMechsRobot

Posted a thousand or more times
#13
Thanks for the motor calc and how did you do that google thing? Its like magic !!! Actually you dont need to explain, but pretty cool!
He used "Let Me Google That For You". Here's another example.

It is kind of a passive aggressive way of saying, "why are you asking a question on a forum when you can just do some research yourself and figure it out". You won't find a lot of that in this community. We are pretty friendly to newcomers to the hobby and find it more productive to just answer the question with our knowledge and insights rather than answering every questions with "go google it".
 

nhk750

Aviation Enthusiast
#14
Sometimes I ask dumb questions, but I found the answer entertaining and I learned a new trick! LMGTFY
I think I'm supposed to be working now too...
 
#15
He used "Let Me Google That For You". Here's another example.

It is kind of a passive aggressive way of saying, "why are you asking a question on a forum when you can just do some research yourself and figure it out".
No I was not "passive aggressive" but answering the question with a joke. He asked on how we find these things and I answered that question and in the sentence below an extra explanation.

You can put the "passive aggressive" label on yourself here.

Cheers
 

Hai-Lee

Old and Bold RC PILOT
#18
Hai-lee: Thank you so much for details on your method and field testing. This is super helpful. Do you test the Taxi tests to ensure it doesn't fail while trying to get it off the ground? (like not veering off-runway, or turning when landing.) I'm not sure what you mean by "For my initial prototypes I fit an undercarriage..." What do you mean by fitting and undercarriage? Do you mean put wheels on it? The short hop check is similar to what I had in my head for seeing if it'll get airborne, but wasn't thinking about a taxi check.

I knew I'd make more after some testing (at least for a build article), but I find it interesting that you plan on three. Are these three copies or three iterations?

Sorry for the questions all, I'm just really curious. I'm hoping to plan my way out of my maiden flight anxieties, so I just do it already. Thanks everyone for the replies so far.

These are a great help. Please continue.
I do the ground runs so that I get an idea how it tracks and at what point it starts to lift off. Also if there is a lift imbalance in the wings I won't discover it in a postmortem. Another benefit is knowing that my thrust angle, (side thrust), is close to correct because it can taxi and do a ground run without veering madly. Final benefit is the elevator and rudder response. All too often the control surfaces are either too sluggish or far too effective and that can lead to uncontrollability and over controlling. (A side benefit is that I get used to seeing it move under its own power and so when it does eventually fly I am not stunned, surprised or panicked).

The mention of undercarriage does mean wheels and it is simply because not all of my designs are for aircraft with undercarriages or rudders but I fit undercarriage and rudder for initial testing and development.

The 3 prototype build is because after a few accidents just one prototype would start to look severely secondhand and could be so badly distorted and heavy from repairs that I would no longer be testing the original design. When making design changes I implement the changes on all aircraft remaining unless the flying plane would not be suitable to upgrade due to damage or similar.

When I say I build 3 I initially mark out 3, cut out 2 and assemble 1. This way I can reuse the same electronics etc from the first to the third. Any major part changes can be implemented on version 2 and 3 easily and of course version one would be set in stone so to speak and used for salvage if required. If a trouble free development occurs I do build the remaining aircraft and gift them to others.

Perhaps you could find a test and development buddy in your area to help you with your design and testing.
 

FoamyDM

Building Fool-Flying Noob
#19
Hai-Lee thanks again for the detail on the Baby flights, Undercarriage, and the 3-builds. I know this helps me, and I hope this helps some other noobs. A partner is crime always makes for a better time.

Do you have any pictures of the "undercarriage". I think it would be helpful to see. (not needed but a nice add).