Decalage Sanity check

[Antonia]

Hi All,

Silly question time, I maidened my mini take anywhere 1/8 scale full composite DG 1001M from Royal Models yesterday and it was a near disaster. It took 60% or whatever 60 means on a Jeti transmitter display up trim to get it to pitch trim, that works out to ca 4 mm up on the elevator T/E, and that only gave me ca 3 mm up elevator remaining.
So I checked the decalage after getting home and came up with zero zero degrees

I didn't bother checking during the assemble as being a good quality mouldy I assumed this would have been sorted during the prototyping.....
Anyway 1.5 degrees seems to be rule of thumb for the decalage angle, with that in mind please have look at a few photos attached to make sure I've got everything the right way round, I set the wing under surface flat and zeroed my angle of attack gauge on the resulting chordline, and made a taped on packing piece to lift the tailplane to 1.5 degrees L/E down, please confirm if that's all correct, I can then fly it again to see if that improves the trimming before making the modification more permanent by re-profiling the tailplane seat.

[Antonia]

Also, I set the CofG to the result from the eCalc online calculator at 31 mm, my normal RC online calculator comes up with a different CofG location, what your thought on the discrepancy?

[B Sharp]

Hi Antonia.
I don't think that you are too far away. I possibly would have looked for a 2 degree angle, especialy when you are using a 15% static margin as your starting point. With that combo I think you are always going to end up with a bit of up elevator dialled in.
I would have possibly opted for a 2 to 2.5 degree angle and a static margin of 10 for starters. Some of my aircraft are flying quite happily with a SM of 5. Having said that I do like them to be a bit closer to a neutral trim.
I see that one of your calculators is giving a CG of 31mm and the other looks like 43 mm. I would probably err on the safe side with 31mm to start with but I suspect that you would likely end up with a pile of up trim after your first flight. I have found that your second calculator has given me mixed results in the past leading to a couple of wild rides on first test flights.
I hope my musings are of some help.
Brian.

[RobbieB]

Morning Antonia (or probably will be when you read this).

Just drawn up a quick planform in my aerofoil programme from your figures and it plots the AC (25% MAC) at 22.34813mm from the LE (I wouldn't be too fussed about the last two decimal places.......) projected back to the root. If you set your CG a little further back from that for your first (next) flightyou should be fine flying at 1.5 degrees. You will still probably still need a touch of trim to get the nose up at that position but you can then move the CG back progressively to suit your desired flying preferences after the initial flight which will then enable you to bring your elevator trim back to neutral.

That AC position is purely for the main wing as calculated by my aerofoil programme and is all I've ever used as a starting point for my models - never faffed about with static margins and all that stuff - keep it simple. It's not a million miles away from the CG programme figure anyway - 0.14mm.

One and a half degrees/two and a half degrees......hmmm, depends upon your flying preferences I suppose or how stable you want it to be. For me, a high incidence wing with a corresponding foward CG doesn't make for a nice model to circle in thermals.

[Ian Davis]

Hi Antonia

I went through the same process of C of G calc comparison some years back. The difference between the two you are using were a great worry to me at the time. I have moved to e-calc exclusively and find its results absolutely right.

I did discuss the differences with the author of R/C calc. but never got a useful answer. The problem in the calc. is associated with the derivation of the neutral point, but that is a far as I got with trying to understand it.

Ian

[John Vella]

Hi Antonia, Robbie and Brian are spot on . My only input is on setting up my ASH 26 was that I set the tailplane to - 2.0 deg and the C of G to 28 % MAC. I never got fixated on static margins as that can be discovered by the "dive" test. Regards John.

[B Sharp]

I deliberately didn't mention the dive test John as it seems to provoke all sorts of reactions from various people. Personally I'm a fan!
Brian

[Antonia]

Thank you for the replies gents

I didn’t dare think about a dive test on the first flight, as it did a spectacular dive down the slope on the launch with not a lot of elevator left let’s say it was an exciting event!...

I’ve taken onboard your comments, I’ve made two more packing pieces at 2 and 2.5 degrees, and put the glider back on the CofG rig and marked 35, 37, & 38mm CofG positions by moving the batteries rearward. I then should be able to try lots of different permutations to get her flying correctly, and I’ve added a little bit more up elevator movement also.

Just got to wait for the weather now, I’ll keep you posted as and when, thanks once again for your input

[Antonia]

I took a day off work on Friday, and went down to Whitesheet as we don't have any SW facing slopes locally, and wanted a good landing area as well,.. I know very picky

Thanks to your suggestions I now have a lovely flying mini scale DG 1001M, Brian you were right, I did need a little more than 1.5 degrees, ended up at 2 degrees at the same CofG, no elevator trim at all, it just flew away effortlessly. A really lovely soarer, caught a thermal as well, really pleased. Thanks for your input gents

[Barry_Cole]

So a nice easy lunch then??

Congrats, look great as always.



BC