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I'm looking at plans to build a radio controlled (RC) glider that is launched by rocket boost mainly as a park flyer. I've been given a set of plans for a model that is a dihedral wing (see attached photo), and I am thinking converting it to a polyhedral wing, as I read that polyhedral wings are more stable for people new to flying as I am.

The new wing shape (I haven't drawn it out yet), would have a flat section over the fuselage acting as a partial cover for the rc components, then a 6 degree angle from there to mid point of the wing, and then another 6 degree angle to the tip.

Wing length and total angles/measurements would be the same, as would the airfoil (Which can be seen in the rib segments on the plan). My other option is simply adding the central flat fuselage cover section, and leaving the rest of the wing as is. (One note, the reason for the thinning of the rib sections in the front is that it is a partially built wing, there is a balsa skin over those parts, then the entire wing is coated in a lightweight monokote type material).

Is there any value in making this change to a polyhedral wing, or is it mostly a wasted effort that I shouldn't worry about doing?

Wing section (I have full plans if other angles are desired): enter image description here

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  • $\begingroup$ Ever break in the wing requires reinforcements since the spars and sheeting are cut. This increases weight and complexity. Just build it as designed. $\endgroup$ – Eric S Dec 11 '19 at 3:00
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In terms of in-flight behavior, the biggest change between dihedral and polyhedral (or tip dihedral) with the same overall stability is more roll response for a given yaw input -- this occurs because yaw-induced roll is caused by the skid presenting the outside wing at a higher angle of attack than the inside, and this effect is both more pronounced, per area, with the sharper angle of a tip dihedral or polyhedral tip panel, and is applied further from the centerline, so has a longer lever arm.

I'd agree, however, with the advice in another answer, not to modify your first kit. Build to plans and instructions. If you want to compare, get the kit built and the model flying correctly, then either buy another kit or buy materials (make photocopies or scans of the ribs and tip plates) and build another wing -- then you can compare the in-flight behavior for yourself.

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  • $\begingroup$ Yaw is movement around the vertical axis, so if I translate correctly, a poly wing will produce sharper turns and greater roll? So a simple dihedral wing is a "genter" handling characteristic? $\endgroup$ – JohnP Dec 10 '19 at 17:45
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    $\begingroup$ @JohnP almost correct. A poly wing will produce more roll than simple dihedral, other factors equal. Higher bank, coupled with some elevator input, can produce sharper turns, but the more usual use case is to give usable levels of bank from a smaller rudder or less deflection. $\endgroup$ – Zeiss Ikon Dec 10 '19 at 18:42
  • $\begingroup$ so... net effect for a new flyer who will likelier have a heavy touch on the controls would be a "twitchier" handling model with a poly vs di. $\endgroup$ – JohnP Dec 10 '19 at 19:16
  • $\begingroup$ Depends. Poly and tip dihedral are used on trainers -- but with small rudders and low flight speed (because poly recovers to level very positively). A twitch at rocket boost speed, and you're likely to rip the wings off. $\endgroup$ – Zeiss Ikon Dec 10 '19 at 19:16
  • $\begingroup$ Raising the wing tips higher may drastically affect the frontal center of drag, by raising it above the center of gravity. Especially for a rocket aircraft this could be absolutely disasterous. Look at the X-15. You want symmetry and plenty of stability. $\endgroup$ – Robert DiGiovanni Dec 10 '19 at 21:07
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If you are "new to flying" please do not start out by making modifications to your kit. Build it faithfully and to the letter as best you can. Secondly, rocket power may be too much for now. Take your model to a good hill and learn to glide it first.

Changing a dihedral to a polyhedral will only make your build more difficult. Biggest issues will be control throws and setting the CG correctly, followed by crosswind performance, which could be seriously affected by modifications. Stick with the plan.

It is highly recommended to find a "mentor" at a local R/C flying club, or even to join one. This will save many crashes, which end hours of building in seconds. Expect a few during the learning process.

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    $\begingroup$ Useful advice, and it answers the question embedded at the end, but it doesn't answer the title question. $\endgroup$ – Michael Hall Dec 10 '19 at 17:17
  • $\begingroup$ @Michael Hall really too many to mention, and impossible to quantify without testing. IMHO not a good idea for someone "new". $\endgroup$ – Robert DiGiovanni Dec 10 '19 at 21:03
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    $\begingroup$ Maybe not a good idea for a new builder, (as you pointed out) but for a new flyer? The OP stated "I read that polyhedral wings are more stable for people new to flying as I am." While admitting to being new at flying, he might be very accomplished at fabrication. In this case the added stability (if true) would be a definite plus, right? $\endgroup$ – Michael Hall Dec 10 '19 at 21:41
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    $\begingroup$ @Michael Hall yes, as a glider, added stability would be a plus, as a rocket plane, no. It is very important to have frontal drag, CG, and thrust going through the same point on a rocket (just look at the ones with out wings). I would also worry about cross wind effects and frontal drag change doing these mods. Raising Cdrag would negatively affect handling by increasing pitch up tendency, but could be an interesting substitute for tail downforce if done right. But I would follow the plan first. $\endgroup$ – Robert DiGiovanni Dec 10 '19 at 23:37
  • $\begingroup$ @RobertDiGiovanni - Realize that the rocket boost will be in the order of 1-3 seconds at a max load of 10 newtons. I realize that disaster takes less than a second (Believe me, I've had my share of spectacular launch failures on new design rockets), but the actual "rocket plane" segment is very short and almost exclusively for altitude. I anticipate around 4-700 feet on an average C/D engine. $\endgroup$ – JohnP Dec 11 '19 at 1:00

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