I have a Yak 54 with a 62in wing span. I have been wondering how would lengthening the wingspan to around 72 in would effect the performance, using a thinner airfoil shape and retaining the same root length and tapper. I plan to retain the original length of fuselage and use a 30 cc gas engine. Any takers on this?
Thanks for any input.
Herb

Herb, you would have to recalculate the MAC to locate the CG properly. You would also have to increase the area of the empennage proportionally. And one more, increase the moment arms of the tail and nose. In other words, you would have to scale everything up. Dan.

Thanks Dan, but by scaling EVERYTHING up I would just end up with a bigger Yak. What I was pondering buy just increasing the length of the wing and maintaining the same tapper out to the tips. The root would remain the same, but maybe thinning the airfoil somewhat. Im not expecting it to fly the same just wondering how it might affect its flying.

Herb,

Dan's answer is 100% correct, but let me put it another way for you. Just making the wing larger will result in an airplane that takes off and lands slightly slower, but the controllability will suffer and the airplane will be more difficult to fly. It also may not be able to recover from a spin.

Think of it this way: What if you modified your Yak by making the horizontal and vertical tails smaller and shortening the fuselage ? That's essentially the same thing as increasing the wing area. How would it fly ? Certainly not an improvement !

Dick

I'm not sure that it would be more difficult to fly. But it would not want to roll as quickly as the original span version. It would also need a further forward CG location unless you make the tail bigger too. All in all it would not be nearly as aerobatic. And the snap roll speed would suffer as well due to more span with no other change.

+1

Think high aspect ratio, as in motor glider...

The aspect ratio would increase, of course, but it would be nothing like that of a motor glider.

Since the taper remains the same, the wing area would not increase very much, therefore the CG would not have to change much, or alternatively, the tail volume would have to be slightly increased, either by a slight lengthening of the fuse or a slight increase in stab area, or a combination.

If the plane is currently balanced conservatively, as is often the case, then it's possible nothing would have to change at all.

The roll rate would decrease, but that's not necessarily a disadvantage. Models generally roll much faster than full scale anyway.

The thinner airfoil would have some consequences, depending on the choice. But it's not like the design of a Yak 54 is cast in bronze, or necessarily the best possible proportions at any scale. What did you have in mind with your change?

Jim

There should not be any need for enlarging the tail since tail authority should be just fine, this is after all an aerobatic aircraft, the control surfaces are large enough. since you are only increasing the span by a relatively small amount, as stated, the roll rate will be the biggest change.

CG should not change much, but to be safe bump it forward a tiny amount until you get the feel of the new config. I would not change the airfoil much, considering the taper I would keep the tip thicker percent wise.

A thicker tip profile will reduce the tendency of a tip stall on a tapered wing. You may want to consider keeping the same chord length at the tip, thus reducing the taper ratio, slightly, and less tendency of tip stall. This would require more work because you will have to build a new wing. simply adding more wing at the tip will work but you will end up with a shorter tip chord and probably not like the performance.

Sure tip stalls are fine when you want to snap roll and other maneuvers but there are times when you do not want them, such as when you are at a high rate of speed low to the ground and make a tight turn.

Peace, Wolf