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[Stuart Mellor]

Photo of my O/D contra in flying test bed. Only a few flights to date, but more importantly - no issues with the contra! in spite of o.t.g. full bore tests.


One big issue though - I simply mounted the unit in an old model with the side thrust as built in for a single prop set up. The model now has a severe yaw due to the sidethrust -  I suspect much more than would have been the case with a single prop with the wrong sidethrust.
Never expected that effect with a contra...... I thought side thrust effects would be much reduced. At least it put the wind up a few club members on take off.

Next project is to remount on rubbers with no sidethrust  -& a rear mount - so I can tweak the sidethrust in future.

If it all goes to plan! - I intend to mount the unit in my Essence, when I've trimmed it out on a the Axi/single prop.


Then - will revisit my modded variable pitch design - now I've a better idea of the engineering involved.
Thanks to John Chadwick for the unexpected photo.

Stuart

[Stuart Mellor]

Question -based on my post above:-


Are contras more sensitive to side thrust than single props?

[Malcolm Harris]

Two separate things going on here Stuart.


One, the two spiral slip streams effectively cancel each other out so really no sidethrust needed to counteract the "push" on one side of the fin. So because you've got some sidethrust built in there is a big vector pulling the model to the right.


Two, contra models are extremely sensitive to rudder. My schedule flying rudder throw on the contra Galactika is only around 6mm deflection each way at the trailing edge and I still fly with lots of expo to tame this! So any tiny out of trim rudder deflection has a big effect. Even with the 18MZ trim rate at its lowest setting one click of trim is nearly too much.


Last thing, don't waste your time trimming your model as a mono prop and then fitting the contra, as EVERYTHING will be different. Even things like aileron differential change so bung it in and getbtrimming.


Malcolm

[Stuart Mellor]

hi Malcolm. Thanks for that. Just based on your comments- you've highlighted the downsides - so give me some advantages!


ps -  I built the Essence for a single prop - so work needed first!

[Malcolm Harris]

Pulls like a tractor


M

[Stuart Mellor]

Surely there's more! What about downleg braking etc?

[Nigel Armstrong]

Congratulations on a superb bit of engineering Stuart!

[steve plummer]

Wow I sore this contra at the 2018 natts.
I must say it looks awsome as if it came out of a specialist machine shop.
Well done Stuart your a clever man hope it works as well as it looks !!
Steve p

[Peter Jenkins]

Stuart


Having been flying an Agenda with CRS contra for this year I am hard pushed to say whether it is better than my Citrin with a single prop.  I did find that the Agenda felt "heavier" to fly in comparison with the Citrin but I suspect that is more to do with the aerodynamics of the Agenda than the contra.  The Agenda may have greater aerodynamic damping as a result of the design.


The one upside I've found so far is that you get the same snap speed to the left or the right.  You don't get that with a single prop unless you fiddle with the aileron deflection in the snap to give the same snap speed.


I abandoned the idea of fitting a contra to the Citrin as it would have involved major surgery to the front end to get the length with the contra system back to the required max of 2 mtr.  I suppose the alternative was to get a Kunzit but the Agenda and contra came along so I went that route.  Now that the competition season is over, I shall go back to the Citrin and get up to speed with flying that again and then form an opinion.  At the beginning of the year, a back to back revealed that the Citrin felt "lighter" on the elevator than the Agenda. It will be interesting to see how it feels now that I am used to flying P19 (generally badly!)


The other downside of a contra is that if you bust a prop it's usually 2 that you need to buy!


Peter