Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Hunting on Small models

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Old 11-25-2017, 10:13 AM
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skaliwag
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Default Hunting on Small models

Probably the same on all airplanes. DC3 does it. Piper Seneca does it but really noticeable and disturbing on smaller models?? Root causes, ??
Model in question is a low wing. 24 inch span. 5-3/8 cord. 5/8 thick. 130sqin. 7oz empty. Sliver Build Thread - A TD .020 Project Post 102
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Old 11-26-2017, 08:25 AM
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Rodney
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Define what you mean by hunting. Is it in the yaw or pitch axis?
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Old 11-26-2017, 08:55 AM
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Yaw. Fish tailing
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Old 11-27-2017, 04:29 PM
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It kind of sounds like dutch roll but the plane doesn't look to have all that much dihedral. You might try temporarily adding some additional fin material from the canopy to the base of the fin like on a bubble canopy P-47.
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Old 11-27-2017, 11:11 PM
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That is one nice design (and build)! At first glance, it has a short tail moment arm and a powerful engine/prop on a quite long front moment arm, so the prop might destabilize the model in the yaw axis. If that is the case a dorsal fin or a bigger vertical tail could help. That's speculation, though, I didn't put that model in a stability calculator.

What I would expect is a problem with the wing airfoil. It looks like true flat-bottom (e.g. Anderson SPICA), what is of course customary for these small models. These are great airfoils for slow flying models but not for a fast model. At lift coefficients of 0.45 and lower, there is a bottom side stall producing much drag. And as always, the airflow departures happen to be on one wing first (randomly), making for a strong yaw moment.

A fast model like this one flies at low lift coefficients, say 0.2 or so. You wrote it flew very slowly on the maiden so that could have been normal (top side) departures (stalls) randomly left or right. If you get the engine running properly, you could try normal slow flight and see if the hunting is still there and then fast flight and see if it is there again. Just a thought...

Last edited by UStik; 11-28-2017 at 12:49 AM.
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Old 11-28-2017, 06:49 PM
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*-*

Last edited by skaliwag; 11-28-2017 at 07:05 PM. Reason: *Double Post
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Old 11-28-2017, 07:03 PM
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Herr UStik, Is there a "Stability Calculator" downloadable on the web?
Only a minimal amount of dihedral, perhaps 1/2inch in a foot at most.
Wondering if some washout would help in slowflight.
Can you recommend a suitable airfoil for this Genre of model airplane? 24' span 5' or so cord, 130 sq" area. 25w of power. 4>5" prop.
My brain is full of these Squiblets.
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Old 11-29-2017, 02:44 AM
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Rodney
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Do you not mean 24" spam and 5" cord? 24 foot span is an awfully bit model.
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Old 11-29-2017, 04:06 AM
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The calculator I'm using is Plane Geometry; it's not free but cost only a nominal fee. It has some limitations since it's actually made for gliders. It doesn't take into account any propeller effects, you would have to estimate them yourself. Otherwise, it calculates stability coefficients which can be compared to those of similar models. You might try dihedral at certain airspeeds and would have a pretty good C/G estimate for a desired airspeed. It's not trivial, though.

Washout has no effect on stability but on stall, and of course it might help IF random flow separations are the problem. As a suitable airfoil the true Clark Y comes to mind. (It has still mostly flat bottom so the wing build is easy.) But I wouldn't build a new wing. If and only if the airfoil makes for problems (still to be found out) I would try turbulators (trip strips) first.
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Old 11-29-2017, 05:45 AM
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Default Spam

Originally Posted by Rodney View Post
Do you not mean 24" spam and 5" cord? 24 foot span is an awfully bit model.
24 foot of Spam would be awfull flimsy.. meant 24 inches
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Old 01-01-2018, 11:33 AM
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Back in the days of long wire antennas, I thought the antenna whipping around behind my half-A planes was causing that.
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Old 01-06-2018, 11:10 AM
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I'm late to the party but I tend to agree with USTik on alll points to some degree or other.

It's clearly intended to be a fairly sporty model and one which would be flown upside down much of the time. As such it should not have a thick lifting airfoil. But the pictures seem to indicate a fairly simple and thin airfoil. So I think it's OK in that regard. Although at lower speeds the wing may be seeing various near stall things going on which causes some yaw wander and oscillation.

Where it should be more obvious is when it's in medium cruise speed turns. If the tail tends to hang a bit low in the turns that's a great sign of too low a vertical tail volume coefficient. Namely the fin area x the tail length in relation to the wing area and span is too low so their is some yaw stability issue.

There's information out there for equations for calculating the vertical coefficient and even charts showing a reasonable range to stay within and using real plane examples of good values. I'll bet that this one is too far to the low side of things. I'd include links to these things but for some reason half my web sites are down at the moment. Not sure if it's some big web thing or if there's big storms causing technical issues.
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