Hip Pocket Builders' Forum

Outdoor Free Flight Forum => Power => Topic started by: wmazz on April 13, 2018, 03:52:58 PM



Title: Wing Tips (outer wing panels): Square or Tapered?
Post by: wmazz on April 13, 2018, 03:52:58 PM
I think it is well established that square wing tips (outer panels) have no
negative effect on the glide performance.

But is there an advantage to use tapered wing tips (outer panels) during
the power pattern?

What I am very curious about is if tapered outer wing panels provide more
protection against flutter, than square outer wing panels

The obvious answer is; if a square outer wing panel is built and covered correctly,
they work fine.

I guess what it comes down to: is the tapered outer wing panel more forgiving?

thanks


Bill M.

I am aware that a tapered outer wing panel changes how a wing stalls, and causes
the tips to stall first.

To help control this, I thought adding washout would help. I would prefer to build in
the washout using the technique used by the 1/2a "Time Machine."

Any opinions? I have seen a post where the question of building in washout was asked,
but nobody responded to the post. BM


Title: Re: Wing Tips (outer wing panels): Square or Tapered?
Post by: lincoln on April 14, 2018, 07:40:13 AM
A tapered wing will tend to be more flutter resistant, particularly if you keep the same span and use more chord at the root. The taper reduces the weight and torsional moment of inertia, raising the flutter frequency. The larger chord near the center increases torsional and bending stiffness. If you split the wing into thirds, tapering only the outer panels, with the tips maybe 1/2 the chord of the root, I suspect you wouldn't have too much trouble with tip stalls either, all else being equal. The correct washout depends on the amount and distribution of taper, and how the tip airfoil does at lower Reynolds numbers than the center. A tapered wing along the lines I suggest above ought to have less induced drag in the glide. There are lots of articles out there written about this sort of thing.


Title: Re: Wing Tips (outer wing panels): Square or Tapered?
Post by: flydean1 on April 14, 2018, 09:52:24 AM
wmazz, you've been asking a lot of good questions lately.  Evidently you are planning to design your own airplane.  That's fine if you've built a few proven designs, some good examples you have shown you have access to.  I can recommend the Pearl series, the latest by Don DeLoach is available in several sizes, from E36 to C/D Classic.  I've built a couple and they work EXACTLY as advertised, provided you build and trim them EXACTLY per plans and instructions.

However, if you insist on rolling your own, the best way I know how to address the questions you ask is to look into the background of proven designs.  Many have been published where the designer explains why the various parameters were worked into the design.  There are countless Symposium papers, Free Flight Digest, Free Flight Forum (UK), Free Flight Quarterly articles.

You've got some reading to do before you start cutting wood.


Title: Re: Wing Tips (outer wing panels): Square or Tapered?
Post by: alset on April 14, 2018, 12:28:47 PM
If I interpret the question and the graphic properly, the "tapered" wingtip is also known as "upswept". In the upswept case, according to W. F. McCombs in "Flying and Improving Scale Model Airplanes", this is equivalent to adding one degree of dihedral. Conversely a "downswept" tip subtracts one degree and a flat tip does not effect dihedral.


Title: Re: Wing Tips (outer wing panels): Square or Tapered?
Post by: lincoln on April 15, 2018, 04:19:28 AM
Reviewing the drawing, the "tip panel" is a lot shorter than I assumed it would be. So you could use more taper. As I understand it, you'd get a bit less induced drag if you kept the trailing edge straight and swept the leading edge back. (https://www.bydanjohnson.com/wp-content/uploads/2017/06/Xenos-3-view-art.jpg)(But I'm not sure it would be more flutter resistant that way. It makes the tip lighter, but it puts the remaining weight further back. If flutter is the main consideration, then the taper should probably be 1/4 at the leading edge and 3/4 at the trailing edge, keeping the 1/4 chord point of each bit of wing in a straight line.
(http://www.hippocketaeronautics.com/hpa_plans/data/thumbnails/72/tailwind_4-view.jpg)
When you put the taper that far out, I suspect no washout is required, unless you use an extreme taper ratio. This sort of taper WOULD reduce vulnerability to flutter, unless you extended the span.

I should point out that I have no power ff experience, but it seems to me this question is about stuff that's common to a whole lot of aircraft.

Mark Drela, for RC sailplanes, puts the washout further in, only 1/2 or 1 degree. To some extent, this is planform-specific, but it's supposed to make circling behavior more benign. OTOH, his models are designed to turn either way, so I'm not sure how applicable it would be.


Title: Re: Wing Tips (outer wing panels): Square or Tapered?
Post by: wmazz on April 18, 2018, 01:22:25 AM
This is my basic layout.

However, if you insist on rolling your own, the best way I know how to address the questions you ask is to look
into the background of proven designs.  Many have been published where the designer explains why the various
parameters were worked into the design.  There are countless Symposium papers, Free Flight Digest, Free Flight
Forum (UK), Free Flight Quarterly articles.

I am a NFFS member, and I have a good collection of Symposiums and the 1992 NFFS Book of Power Models.

I will have a question or two about your Pearl vs the AstroStar in the future.


Bill M.


Title: Re: Wing Tips (outer wing panels): Square or Tapered?
Post by: lincoln on April 18, 2018, 01:44:59 AM
Looks conservative to me. In the engineering sense of the word. That's good.


Title: Re: Wing Tips (outer wing panels): Square or Tapered?
Post by: ddock on April 18, 2018, 09:05:55 AM
Ronnie Thompson and Gerald Brown both fly tapered tip Pearl type models in all the AMA gas events. Smokin fast models! Both are on the podium all the time.
Denny