C of G

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Pete Lewis
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C of G

Postby Pete Lewis » December 7th, 2008, 6:06 pm

I am currently fettling / rebuilding a 1/4 Scale ? Cessna 180/185 Skywagon
Possibly built from a Wendel Hostetler plan originally ? ( I don't have any plans )

Can anyone suggest where the proper C/G should be please ? , measured from leading edge .
and also a suitable Prop size for a zenoah 45 ? that I am currently shoe - horning into said Flying machine .
the engine is new and will need running in .
I posses some wooden Props circa :- 20 X 8 , 20 X 10 and a 21 X 10 .
Scale like flying speeds required


* Thanks again for the outstanding levels of help , not to mention generosity , previously given to me by L.M.A members and supporters .

I hope this Forum continues to raise the bar in terms of general decency , and will be as highly regarded as the outgoing one .

Best regards : Pete

Robin Woodhead
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Re: C of G

Postby Robin Woodhead » December 7th, 2008, 7:22 pm

Pete Lewis wrote:I am currently fettling / rebuilding a 1/4 Scale ? Cessna 180/185 Skywagon
Possibly built from a Wendel Hostetler plan originally ? ( I don't have any plans )

Can anyone suggest where the proper C/G should be please ? , measured from leading edge .
and also a suitable Prop size for a zenoah 45 ? that I am currently shoe - horning into said Flying machine .
the engine is new and will need running in .
I posses some wooden Props circa :- 20 X 8 , 20 X 10 and a 21 X 10 .
Scale like flying speeds required


* Thanks again for the outstanding levels of help , not to mention generosity , previously given to me by L.M.A members and supporters .

I hope this Forum continues to raise the bar in terms of general decency , and will be as highly regarded as the outgoing one .

Best regards : Pete


:idea: Try this site for the formular to calculate the CG http://www.scaleaero.com/CG_Calculator.htm

Chris Lane
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Re: C of G

Postby Chris Lane » December 7th, 2008, 8:29 pm

If its any help; the C of G quoted for my Hostetler Cessna 150/152 is 128mm back from the LE on a chord of 490mm - that is 26%.

Pete Lewis
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Re: C of G

Postby Pete Lewis » December 8th, 2008, 1:39 pm

Thanks for the replies.

Robin - I get the gist of the supplied link - sort of , but cannot get it to work for me , I was never the brightest pin in the box though....

Chris - would 26% of Root Chord ( mine being 410mm inc. flap ) be a safe starting point do you think ?
how critical is it on a Cessna type.

Any suggestions on Ideal prop size for the Zen 45 in this aircraft ?

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Dave Parry
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Re: C of G

Postby Dave Parry » December 8th, 2008, 1:57 pm

Pete Lewis wrote:Robin - I get the gist of the supplied link - sort of , but cannot get it to work for me , I was never the brightest pin in the box though....


Pete, if you can't click on the link Robin has sent you just highlight it then right click on your mouse, go to copy then pasted it in the address bar at the top hope this helps. Dave Parry Forum Moderator ;)
LMA Journal Editor :geek:
You can contact me at: lma.journal.newsdesk@gmail.com or Dave.Parry07 on Image

Chris Lane
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Re: C of G

Postby Chris Lane » December 8th, 2008, 9:09 pm

Pete, for a 'conventional. aircraft like this a C of G range of 25% to 30% is usually quoted. 26% is therefore well forward and a safe starting point. If you subsequently want to make the aircraft more sensitive you can reposition batteries etc later to move the C of G back. The Toni Clarke web site lists propellers for each Zenoah and your 20 x 10 and 21 x 10 will be OK to start with but a 21 x 8 should help bring the speed down. My F152 has a ZG45 at 30% scale so your quarter scale will be well powered!
Chris

Dave Tregaskes
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Re: C of G

Postby Dave Tregaskes » December 9th, 2008, 9:24 pm

Pete,

Try this C of G calculator, its easy to just fill in dimensions and get an accurate result.
http://adamone.rchomepage.com/cg_calc.htm
I hope the link works - its the first time I've tried this sort of thing !

Pete Lewis
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Re: C of G

Postby Pete Lewis » December 10th, 2008, 1:10 pm

Thanks again for the help guys.

Power wise , it appears as though this aircraft was originally designed for a Zen. 38 or similar - with rear exhaust .

I am using the Zen. 45 mainly because of budget , and the fact that I already have it .the props came from a swapmeet - undamaged .

The LMA motto of " too little power " etc. ought not to apply in this case then ........... I will endeavour not to rip the wings off !( jest ) may be a bonus for short field operation then ?

As the engine is new , but stored unused for some years , the Cessna will suffice as a suitable test bed , I hope .

A nice Warbird is my ultimate ambition for this motor , but my dreams often surpass my abilities .

Regards . PL

Dave Cooper
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Re: C of G

Postby Dave Cooper » December 22nd, 2008, 4:33 pm

Hi - just been consulting some of my Flight Physics colleagues here at Airbus...

A CG of between 25% and 32% of chord would seem a sensible range for this class of model. However, someone mentioned the use of flaps. This will alter the relative CG to CP (Centre-of-Pressure) position on take-off and landing. If using flaps with slats (not sure about your model), this can mostly be ignored, but flaps without slats can cause a pitch /trim change (often rearwards) on full deployment.

I think, on balance, I'd start at the forward position around the 25% mark and gradually 'creep' the CG aft until you find the "sweet-spot". Hope this helps.

Pete Lewis
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Re: C of G

Postby Pete Lewis » January 16th, 2009, 3:40 pm

Thank you Dave .

All information is most welcomed .

Yes this model has flaps, standard hinged not fowler or owt' fancy - but no slats . and are operated from a slider on the side of my TX , for the initial flights at least . the max angle being appx 40 degrees. this would hopefully allow me to operate flaps without letting go of the stick(s) if having to cope with a pitch change at the same time.

I also fly mode 1 , and have a partly functional left hand due to injury. coupled with very little signs of intelligence .....

I may yet put the flaps on a two position switch ? - advice welcomed on this aspect of flap operation please.

Do I take it that Airbus Tech's calculate these things to a much finer degree ? or do the test pilots have an " interesting " work load ;)

Regards. PL

Roy Hill 1855
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Re: C of G

Postby Roy Hill 1855 » January 17th, 2009, 8:29 pm

Good evening Pete, i fly "throttle right" and found it awkward to use a slider/rocker/rotating pot etc,but useing the 3 position switch on my DX7 (up/1/2down/ full down-flap),1/2 down for takeoff, full down for landing, works very good for me---just needed to add a touch of up or down mix as req,d. imo,best of luck,regards,Roy. :)

Pete Lewis
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Re: C of G

Postby Pete Lewis » January 19th, 2009, 4:50 pm

Thanks Roy.

Your opinion is worth quite a lot actually , especially to the un - initiated . Actual user experience is most welcomed.
I take it the mix you mentioned is down - elevator to counter any pitch change ?

After reading your reply it has only just occured that having a " variable" slider will give variable effects , and possibly unwelcome results.

I have never really needed to explore the extended functions of my Tx until now in three years of ownership, and prefer not to use mixing functions if at all possible , partly because I find the Manual so awkward.
But ,it's a good job that I have the mixes available if I so need them . so it's up to me to learn them.

I am now going to set Flaps on a three - position switch as you suggest , sounds a far better solution.

regards. PL

Roy Hill 1855
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Re: C of G

Postby Roy Hill 1855 » January 19th, 2009, 6:11 pm

Your welcome Pete,it seems that some require down ele,some need up, and some none at all,my cessna 152 is very neutral,flaps up or down, but my 185 required alot of down ele when using flap,it is very much a trial and error situation, but great fun trimming it out! In my experience up to 30 degree flap increases lift,and further(up to 40 degrees) increases drag,i use 20 deg for take off, and 40 for landing. Just remember,if overshooting,increase airspeed before raising flaps!!!
Happy days Pete,Roy.

David Turner
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Re: C of G

Postby David Turner » January 19th, 2009, 7:45 pm

Best to control the flaps via a slider or rotary switch. In that way, you will be able to extend/retract the flaps smoothly, slowly and in small increments.

Also, it's a good idea to mix in some elevator movement, so as to help you to control the pitch changes that are associated with flap' operation. These changes are likely to be significant, but you'll have to mix by trial-and-error.

All of the above, IMO, of course.

Pete Lewis
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Re: C of G

Postby Pete Lewis » April 4th, 2013, 4:33 pm

The Cessna met an untimely end last year , prior to that , thanks to members advice - She flew lovely . 3 positions of flap on a switch ( starting at 20% ) mixed with down elevator . On a deliberately nose heavy Model.

The slider method may have worked better ? but I never got to find out . Sadly


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