C of G for 1/4 Scale Zlin (Falcon)

[Chris Hurst]

Can anybody tell me the correct C of G for the 1/4 scale Zlin sold by Falcon Aviation?

I've tried to contact Richard by email and phone but perhaps he is away.

The book says on the wing joining block but I stupidly built the wing without marking the position, I have a Roto 35 fitted and it currently balances 11cm back from the LE at the fuselage. I'm sure this is well too far forward.

I've also tried using an Excel c of g calculator which suggests about 18 cm back which would need quite a lot of lead on the tail. I'm nervous about my calculations so any help would be appreciated.

Thanks

Chris

[Tony Collins 1073]

Chris, 3 years ago I took my Zlin to The Euroflugtag do in Germany and made the maiden on the thursday before the show.
Mine is the exact same as yours even with the Roto 35 in it. The C of G was set according to the instructions in which incidentally the landing is described as needing almost full up elevator by the end of the landing. I thought that was strange but went according to the instructions. On the first circuit it felt awful and I decided to land immediately.
The approach was at normal speed and altitude but I did not seem to have very good elevator control. If I eased of on the elevator just a tad it wanted too drop to quickly it and I decided to overshoot. my altitude was about 6ft and I opened the throttle and it just flicked into the ground. It was not going too slowly for a normal overshoot. The front of the aircraft was broken off at the engine mount former.
Not to be detered I rebuilt it and moved the C of G back. John Greenfield kindly did the test for me and it flew better although not perfect. John managed to knife edge it but it was dropping on knife edge, even with masses of rudder deflection but still it flew and I decided to take it to the German show again.
One other thing was that as I didn't have retracts the undercarriage was sitting further forward and made for very bouncy landings bordering on almost tipping over. As I make my own aluminium undercarriages I was able to modify one to sit back under the leading edge of the wing. This was at Johns suggestion. Anyway, in Germany I met up with a good bunch of lads from the Chesil club and told them of my problems and they suggested that Mark who is one of their 3D pilots would test it for me. However they checked the C of G first and Said it was much too nose heavy [ Still ] What a great bunch of fellows. Led by Phil Parmenter they literally took over and I ended up with 3 Bits of sheet lead screwed underneath the fuz as far back as it would go. Mark took it up and it performed very well. At the next Cosford event Steve Holland flew it for me and did his rolling circle like his big Zlin. He went right down the strip on knife edge but apparently it wouldn't climb on knife edge. When someone asked him how it flew Steve said " Much better than I thought it was going to"
I then took it too the Chesil club and flew it in a cross wind. I was told it looked good. Better than I felt.
I have marked the the C of G underneath the wing and it is 13.5 cm from the front of the wing . The bit that sits back under the fuselage.
I hope that this is of some help to you. The final position will depend on your flying preferences of course.
At least I may have saved you the problems that I have been through.

Good luck.

[Chris Hurst]

Many thanks for your detailed reply Tony.

Can I just double check I understand you correctly?

Is your 13.5 cm measured from where the wing meets the fuselage underneath? i.e. the front of the underpan stuck over the wing joining bandage?

Another question, how much flap would you suggest for landing and do you have any elevator mixed with flap?

Thanks

Chris

[Tony Collins 1073]

Hi Chris, Yes it's the joint where the fuz meets the wing underneath.
As far as the flaps are concerned, I have set them on a two position switch at about 20 degrees and 40 degrees but these have yet to be tested so the elevator mix which I intend to use has not been decided.
Have you got much to do before your maiden?

Cheers Tony

[Chris Hurst]

Thanks again.

Only add some lead to the back end and get the engine to a point I'm happy with. I'm struggling with that as you may have seen in my other post!

Chris

[Tony Collins 1073]

Hi chris. Sorry but that plea for help got past me. I've had a look and it seems that if some folk can't offer any sensible specific help than they shouldn't be posting.
Anyway you may not know but carbs and engines happen to be something that I have spent a lifetime on, so I will try to help you.


There are several things that could be at fault but if you got the engine second hand and someone has been inside then

the extremely fussy settings of the metering inlet valve and pop off pressures [ the pressure at which the valve opens ]

may have been upset. There is a special guage for setting the pop off pressures and the spring strength has to be

synchronised with the height of the metering lever which also requires a precision guage. If these are wrong by even a

little you can run either too lean or too rich and the engine can overheat in spite off the settings that you have tuned the

engine externally with. The mid range is also affected with incorrect pop off pressures. The diaphragm which operates

the metering valve lever should be flexible without leaks other wise it cannot open the metering valve correctly and

becomes very touchy with a slight variance of atmospheric pressure. These can stiffen up with age. The addition of a

cowling can produce this difference in pressure. Ideally you should have a pipe fitted to the diaphragm cover and bought

out level with the carb intake so that they both see the same pressure. with the Zlin having a rear mounted carb this

should not be a problem unless you have a right on the edge situation for your fuel metering. The only other part to give

a problem [ assuming you have no blocked jets ] is the twin one way valve membrane if it is not seating properly.

As you can see it is not going to be easy for you especially without the setting up equipment. If you can't sort it then

the best thing you can do is pop the engine in the post to me and I will put it on my stand and sort it so that all that

you will have to do is put in your plane and fly. It'll just cost you parts if any and post.

Cheers Tony.

[Chris Hurst]

Thanks again Tony, that is a kind offer.

Since writing the engine post I've spoken to IAD and he has give me a couple of suggestions. In particular the engine doesn't seem to be pumping fuel as well as it should.

I hope to be able to try these on Friday or Saturday. If / when that fails perhaps I can get in touch. I'm in Cambridge so not too far away. How do I contact you?

Regards

Chris

[Tony Collins 1073]

Hi Chris. If you need me just send me a PM.