Gliders > PSS

MIG 29

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JohnPearson:
Yes guys, this is the Mig 29 I flew at bigL, a few years ago. The wing that comes with the kit is an RG14, good for ducted fan use, but not for slope flying on this model. I used the SD3020 and increased the wing by' I think' 14%. I will get a set of wings cut for you Fred when I get the CNC up and running again!

Flamingo Flier:
When I read John’s reply I must admit I was sceptical. How could a change of wing section make much of a difference to this model? Surely the low aspect ratio, short and enormous fuselage, double fins, huge tailplane and all the other lumps and bumps guarantee that the parasitic drag on this model is going to be so great that the comparatively little differences between one aerofoil and another would be insignificant. Surely this one would fly with any wing section. However I was curious and had a look at the RG14 drag curves. Sure enough the low drag range at low Reynolds numbers only extends from a CL of 1.0  to minus 0.2 so it’s not a thermal soaring section. It won’t fly slowly but it will fly inverted at high speed!  Maybe it was time to do some sums………….

The wingspan is 1.145m and the wing area is .226sqm giving a mean wing chord of near enough 200mm. The weight is given at 2.7kg. Doing the maths …..  at a cl of 1.0 (ie slow flight!) the model would fly at near enough 50kph. This will be exciting to land – too fast to land on a rough slope. The Reynolds number is over 180,000 so we can forget about the 50k and 100k plots. 2.7kg is far too heavy and a wing section that would fly slower (ie at a higher cl) would be an advantage.

Redoing the maths with a weight of 2kg and using a wing section that would still be in low drag at a CL of 1.2 the model would fly efficiently at 39kph and would still fly (with high drag) even slower so landings without damage might become more frequent. At 2kg the Reynolds numbers would still be respectable so the weight could be reduced even more.

SO:
John has a point. An aerofoil that works more efficiently than RG14 at higher coefficients of lift would reduce the landing speed and that would be good. However reducing the weight is even more important. I still think that in the range of cl’s from 0.8 to 0.3 (where most of the flying would take place) the differences between RG 14 and a more glider friendly section wouldn’t amount to much compared with the parasitic drag.

JIM

PS to John – I cannot find any drag curves for SD3020 – have you got any?
PS to Fred - Would you be tempted - as I would - just to reduce the weight and fly it at it is....... the difference between flying at a cl of 1.2 against a cl of 1.0 is less than 4kph and the high drag at slow speed might help in getting it down!!

billscottni:
If I remember right, John found the model too light and had to ballast it up a bit.

Flamingo Flier:
Hi Bill – so the plot thickens.
I wonder what weight John’s model was. Had he also started light (no batteries for the motor etc.)?  Did he feel it needed to fly faster in order to get the wing working or was it just lacking in response at the lower speeds? Would it slow down for landing? Was it difficult to throw it fast enough at the launch? Hopefully John will log again on soon………………
Or, just maybe, it’s only a pensioner like me that thinks that fast flying models are a pain to land and the thing will be fine for everyone else at 2.7kg!

JIM

billscottni:
Jim,
 if I remember right (and my memory ain't what it was!), the model was fairly unresponsive and wouldn't penetrate

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