I’ve read that the Sig Smith Mini biplane has a tendency to balloon upward upon application of throttle. I believe I read it on Cafeenman’s website airfieldmodels.com. I’m not going to be able to match his construction but I’ll do my best.

In my wandering on the web looking for photographs of different Smith DSA-1 aircraft I ran across Sky Classic Aircraft. They are offering new plans for the full size aircraft. They show the aircraft being built in different stages. I was astonished when I looked at photos of the wings. They were not under cambered or flat bottomed. The site also showed a 3 view and there low and behold was “Wing foil NACA 23013”, a semi symmetrical airfoil.

My question is this. Would this airfoil help eliminate the ballooning? It would be easy enough to cut new ribs. Would you do it?

Thanks,
John

Sig's web site says they use a semi semmetrical airfoil. The center of which is flat. Here is a the picture I saw on the Sky Classic website.

All aircraft tend to 'balloon upwards' (climb) with added power, depending on how the aircraft is trimmed. The airfoil has little effect on this other than some are more affected than others. The amount of camber is a good indication of how sensitive to power (or more correctly, Speed) the aircraft will be, along with the rigged difference between that tailplane and the wings. Simply put, the more the camber, and the greater the rigged difference, the more marked the effect.
Evan, WB #12.

I've always noticed a difference in ballooning between my flat bottomed trainers and symmetrical sport planes and ascribed the difference to the air foil shape. Obviously I know very little about aerodynamics.

What could be done to reduce the "ballooning"?
Thanks,
John

As pimmnz suggested it's all about aircraft trim. The camber of the airfoil doesn't cause the nose up ballooning. It's the CG location in concert with the related up trim in the elevator or stabilizer angle that produces the nose up pitch response when the flying speed is raised by adding power or performing a dive. So if you want to properly correct for this sort of behavior your FIRST line of attack is to edge the CG location back in small steps until the model performs more to your liking. Most of the airfoils commonly used by models will not play a part in this setting in that they all react the same. You can achieve good results with a flat bottom ClarkY style as readily as with a full symetrical airfoil.

The primary way of testing for the optimum CG location is to do a dive test. Trim the model to fly level at about 1/2 the rev setting (typically 1/4 to 1/3 throttle). From that push into and hold a 45 to 50 degree dive so the model is flying fast. Then release the elevator stick and obverse the reaction.

If the model responds with a sharp and sudden nose up rotation into a steep stall then your CG location is too far forward for serious sport flying. But if the model is a trainer then it may be just right to only slightly too much depending on the results. Trainers need to have a little more, BUT NOT TOO MUCH, built in pitch stability. But TOO much even on a trainer will frustrate the student by fighting their inputs all the time.

The "just right" result that you want to see is that following the stick release back to neutral is for the model to pull to level in about 100 to 200 feet and then similarly slowly raise the nose to a mild stall or even mush ahead a little and return to level flight after a couple of mild oscillations. Where you find your happy place between the 100 and 200 foot range is up to you. But overall this is a nice range for a casual sport model that will spend much of it's time upright.

The signs that the CG is too far back for most sport flyers is that the model has very little tendency to want to lift the nose. Or perhaps it stays in the dive. This will happen when you're just barely ahead of or right at the neutral point. This is the desireable setting for competition aerobatics or really serious sport flyers that want to make the model perform very similarly in any attitude and don't mind the extra work load of doing ALL their own pitch corrections.

Remember that it does this due to the CG IN COMBINATION WITH the elevator trim. As you move the CG back to do another test you MUST retrim the elevator for hands of level flight at 1/2 revs each time before each test.

So why do we use symetrical airfoils for a lot of sport models? Simply because a flat bottom or other higher camber value airfoil will not support negative G flight modes with the same charactaristics or ease that a low or zero camber airfoil will give. So to keep the model nearly or equally responsive in positive and negative G situations we choose a low or symetrical camber airfoil. But make no mistake about it. A forward CG will balloon the model just as badly as a high camber airfoil if the CG is too far forward.

If you have a "balloony" trainer you can try this for yourself. Shift the airfoil back in small segments of 1/4 to 3/8 inch at a time until it's got a really long recovery in the dive test. Then see how much more aerobatic it has become. And if it had a lot of downthrust to compensate for the pitch up under added power then you may even find that it's too much downthrust now and it actually wants to DIVE a little at full throttle.

B Matthews,
Thank you very much for the detailed response to my question. I do understand what you are saying and it does make sense.

I believe I will change the airfoil simply to have something different. I'll have the only Sky Classic Smith at the field.

I will seriously keep your advise in mind if it does "balloon".
John