Although I have the wingtips that came with the kit, I decided to make my own. This was partly for aesthetics (to minimize the GRP content), & partly for fun.  If anyone is unfamiliar with the theory behind Doctor Hoerner's ideas, there is much on the web. Try this for one such article: http://soaringweb.org/Soaring_Index/1965/PDF/1965_Aug_16.pdf

I added the same increase to the span as the rounded wingtip, which is approx 10" per wing. This gave me an angle of 37 degrees for my Hoerner tip.

So the first task was to draw the standard wing profile, and then plot a section through it cut at 37 degrees. This resulted in the shape as shown (with the vertical end rib for comparison). Surprising, isn't it? Hope it fits!

  

Next was to extend the leading edge ply to accommodate the Htip (short for Hoerner wingtip). There is just enough overhang on the outboard end to give adequate lap for the extension.

The extension was cut slightly oversize, & butted to the original ply with a doubler glued to the underside.  

Then the new Htip was offered up & held level with the upper wing surface, with the lower edge flush to the underside of the wing.  Against all the odds, the profile fitted inside the extended L/E ply quite well - only really needed to cut out to clear the doubler. Yay!

Since I fully expect to groundloop at some point in my adventures, I decided to incorporate a skid to the underside to protect the wing when the inevitable happens. This also can be used as a tiedown point. (I plan to fully fair the struts, & hope to make a separate fairing to cover where the struts join the wing, so it will be a bit harder to attach tiedowns to this more logical hardpoint.)

I made the skid from scrap 1/4" ply, and glued it to the vertical rib. The Htip needs a cutout to clear this, and the edges that are proud of the Htip are shaped after the glue has set.  I'm bushing the hole with some spare ally bushing so I can use a shackle for tiedown lines.

If I don't like any of this, I can always cut it off later - the joys of a wooden homebuilt!

With the Htip correctly positioned, I glued the lower edge and allowed it to set. Incidentally, I'm using WEST with microfibre filler in much of this mod, as it's better for gap filling. You can see the fillets formed in some of these (& later) pix.  

Then I cut some ribs from scrap ply, and glued in place. The rib positions coincide with hardpoints on the wing end rib.

I added some mini spars from RS4 scrap to the two ribs nearest the outermost tip section. These are not needed for aerodynamic loads, they are there to give a bit of extra rigidity in the event of hangar rash.

Note also the top of the ribs are not straight. The outer couple of inches are at upper surface level, with the inboard section a bit lower. This is to provide a base for the ply upper skin (see next post), but so as not to interfere with the flatness of the fabric when the tip is skinned (they would form a spanwise ridge if left at full height).

I cut some 1/16" ply strips & glued them to the Htip upper edge. This greatly stiffens the structure, & gives a surface to bond the wing cover fabric to later on.

I temporarily fixed a 1/8" spacer to the outboard end of the aileron. Then I cut a 1/16" ply vertical web to fit flush to this, with its profile matching the Htip & the aileron upper surface.

I WEST'd this in place & left it to set. This only needs an hour or so with WEST & my shop temperature typically 80 - 90!

Then I cut & glued the end closing skin, again from 1/16" ply. With the glue set, the triangular pointy end bit (note the technical terminology) is incredibly stiff.

I refitted the aileron, and lightly dressed the Htip edges to give adequate clearance. I don't want a howling gale through the gap in flight (draggy), but I need enough clearance to ensure it doesn't foul. (I don't fancy a sideslip all the way down to the ground).

I rounded off the trailing edge, and made a small epoxy fillet along the edges to stiffen it all up.

Some may notice the grain on the end of the aileron goes the wrong way. That's because I accidentally used the wrong bit for the Htip end pieces. D'Oh!

So there it is!  All that remains is to clean it all up, and then form a large radius curve on the underside. The upper edge is deliberately left fairly sharp, all of which is to encourage the tip vortex to form properly.

The finished Htip is quite adequately stiff & strong, and its weight is comparable to the GRP wingtips. I'll cover the top surface with fabric when I do the rest of the wing.

A few more misc pix to finish off.

That tip really looks good. I bet that you were a serious model builder in your previous life and the workmanship shows. Don't you just love working with wood!

Thanks Dick!

Actually, I was a boatbuilder at one time.  And yes, wood is just so nice to work with. Especially now we have epoxy glue.

Remember: When you do the inevitable ground loop your tail wheel will be on the ground (if you haven,t already broke it off) and the rear part of the wing will hit the ground.

I think you need to move your wingtip skids to the rear of the wing, just ahead of the aileron..

I have the droop tips and I have had both of mine on the ground BUT not at the same time...  It only skinned the paint off the rear 3-4 inches and didn't break anything..

OK, thanks, HG. I did bias its position aft - maybe not far enough. I'll let you know if they work when it happens!

Excellent work! Great pictures also.
Any idea what difference there's between the Hoerner and the Drop wing tip effect?

With all the beautiful work you've already done, I suggest that you don't wait until it happens...

I'm sure that you will have the wings on before you cover them...

When you do, just lift the wing at the tip under the front spar until the other wings touches the ground or maybe almost touches the ground and you will see where it will touch first then you will naturally feel better if and when it Happens.

You know what they say:   THERE ARE PILOTS THAT HAVE GROUND LOOPED AND THOSE WHO HAVEN'T YET.........

When I do my preflight I pick each wheel off the ground like I mentioned above..

Thanks Ricardo.

I don't suppose there is any great difference between the two.

I just prefer to make as much of my plane as I can.

And as usual, it's down to what you think looks better. I'm aiming for the retro look!

Bruce

Wow, what an excellent description using text and photos.  Very good illustration! Your workmanship looks great too. Now is the time to install that wingtip, can you suggest a way to attach it to a wing that is already built? Unfortunately I am trying to figure out how you arrived at the curvature of the wingtip.  Could you provide a broader explanation? I can imagine you drew a chord profile (end view of wing), divided the curve into a number of points, then extended lines from those points to an adjacent view then cut them with a 37 degree plane. Do you then project those points into an auxiliary view that is perpendicular to that plane?

Thanks for your contribution, this is great!  You are both a clever man and an excellent presenter!

Paul Fiebich

Yes Bruce its all been said UP thread ...well done

regards Arthur

Thanks guys.

Yes Paul, that's kind of how it's done.  I'll do a sketch & post it tomorrow which will prolly be easier than explaining with words. Also some ideas on retro fitting an Htip to an existing wing. Quite do-able I reckon.

Bruce

Nicw looking tips Bruce. is there a aeronautical advantage to this design or are they just cosmedic like the drooped ones?

monte  

Very nice Bruce! You have me rethinking my drooped wingtips. Theses would save a LOT of weight!

Bruce, great job, as I did with mine one two years ago. I am not so good in woodwork, I made mine with foam core and moulding. Hoerner wings are great enhancement worth the effort and money. It will give you better takeoff and landing performance.

Quoted from pkoszegi

It will give you better takeoff and landing performance.

I've read something very similar many times on various discussion forums and/or info provided by companies selling wingtips. It sounds good, but isn't much fact to go on to justify spending a bunch of time and/or money to build or buy wingtips . I'd really love to see some before and after numbers, figures that indicate any comparisons of stall speed, cruise speed, takeoff or landing distance changes or any other measurable parameter, with and without wingtips on either the same plane or an identical plane. The topic of performance benefits from the addition of wingtips might be better served in it's own thread though.

Thanks

Lynn

+/- 5 mph is a big deal with planes like this. you may not see a HUGE leap in performance but something that is noticeable....rate of glide ( engine off/idle), and stall speed being the biggest probably.

OK, as promised a brief description of how I arrived at the Hoerner tip profile. This is specifically for Paul, but may be of interest for anyone else considering making these wingtips.

The sketch shown below should be self-explanatory within reason. You may need to expand it on your screen to read it a bit more clearly. But I'll describe it a bit more anyway.

Basically, you start with a vertical section through the wing (the shape of your wing ribs). I've drawn a very bulbous wing section, for illustrative purposes.

Then you decide what angle you want for your Hoerner tip. This is shown as alpha on the drawing (sorry, can't find the symbol on this lappy).

Next you draw a base line perpendicular (90 degrees) to the inclined plane you just drew. This is the base line for your Htip.

Divide both the original baseline and the Htip baseline into any number of divisions. THESE DIVISIONS ARE IDENTICALLY SPACED ON BOTH BASE LINES (shown as 'x' on the sketch).

Now draw vertical ordinates from each division point, and number each ordinate. Where the ordinates cut the profile line, identify each intersection with a letter.

Now project parallel lines from each intersection across to the inclined plane.

From each of these new intersects, project a parallel line to meet with the Htip ordinates you drew earlier.

Now join the dots!

To avoid confusion, on the sketch I've drawn all of the forward half lines, but only a couple for the aft section of the profile. The principle is the same, just hopefully makes the sketch a bit less cluttered.

A couple of notes:

The more ordinates you draw, the more accurate your Htip profile will be. Where the profile curves tightly, several closely spaced ordinates will be needed. On the sketch below, I've drawn an extra line (labelled 1w) between 0 & 1 to help define the leading edge profile. Towards the aft end of the section, the profile is pretty much straight, so far fewer ordinates are needed.  Just remember to draw the same divisions on the Htip baseline.

As you project your lines, label as you go. If you've drawn lots of ordinates, this will help avoid confusion. And I like to circle the intersects to make them stand out.

This method will work for any angle, and you can cut a section through any shape.

Or - use a computer with a drafting programme!  (Not as satisfying though. . .)

Bruce

Quoted Text

is there a aeronautical advantage to this design or are they just cosmedic like the drooped ones?

Hi Monte

I believe there is an aerodynamic advantage to a Hoerner tip, but I also think they give a more "finished" look to a wing.

Google 'Hoerner tip' and you'll get lots of explanations (see link in my original posting for one such). But here's my understanding.

The greater the wingspan, the more efficient the wing (in respect of lift to drag ratio). But a physically longer wing is heavier, has more profile drag, and needs more hangar space.

So a Hoerner tip 'fools' the air into thinking the span is greater than it really is, without the disadvantages above.

It does this by forcing a vortex to form outboard of the physical wingtip. This vortex is formed by the slightly higher pressure air on the underside of the wing flowing around to fill in the slightly lower pressure air on the upper surface of the wing. This vortex forms a sort of invisible fence that keeps the outer portion of the wing working. With a plain wingtip (rounded or square), the last few inches of the wing are in confused air and not contributing as much lift as a similar length further inboard does.

The diagram below (from Zenithair) gives a comparison.



As Lynn says, lots of claims are made as to how much benefit accrues from such tips, but it's pretty clear there is some - or an awful lot of commercial companies are wasting their time making planes with more expensive wingtips than they need!

Winglets, as seen on nearly all commercial jets nowadays, aim at a similar improvement, though they work in a slightly different way.

Drooped tips are another form of Hoerner, working in a similar way to the type I've made.

Bruce

Peter, I read your post on how you made your wingtips a while ago. You did a great job, but you also pointed out the downsides of working with fibreglass. It was one of the reasons I decided to have a go at making my own in plywood!

Personally, I hate working with GRP. It's sticky, smelly, irritating, expensive, and so on.  Wood, I love.  And I have a wooden aeroplane, so I'd rather have wooden tips  

Bruce the Luddite

Sorry, Paul, forgot to mention my ideas on retro-fitting wooden tips to your wing.

I would make a 'rib' from 1/4" or 3/8" ply, the same profile as the vertical wing section. I'd remove the inner portion, leaving a strip maybe 3/8" wide all the way round (or maybe with a vertical soldier in the middle to fix the shape better). You could include a tiedown projection into this new rib profile if you wanted.

You could then either take your wing off the plane, & put it on a workbench, or make a short section of dummy wing. You'd clean the paint off your wingtip down to bare wood, 1/2" wide all the way round the edge, as far as the aileron.

You'd glue the new end rib onto the existing wingtip. Then profile the bottom half of this new endrib to suit the Hoerner tip angle, and proceed as my original post. I would glue the leading edge D section as per, but not put any extension ply on the inboard section (still need it on the inclined Htip though). The fabric would overlap onto the existing end of the wing.

You'll need to reduce the thickness of the new endrib on the inboard face to give adequate clearance for the aileron allowing for the 1/16" ply triangular web you'll glue in there. Or you could terminate the 3/8" rib just forward of the aileron, and make the pointy bit at the aft end the same way I did.

If you really wanted to play, you could make the whole tip on a slight dihedral - say 5 degrees. You'd probably enhance the vortex effect, and it would give greater lateral stability in flight (if you need it!)

Have a go - it's fun!

Bruce

Bruce, thanks for the additional clarification of how you created the wingtip profile.  Your drawing is almost exactly what I sketched based on your original text and my understanding of creating a true view projected from the angular cut. You have made a three view drawing; a wing profile view (front view), an angular cut view (right side view) and an auxiliary view. Thanks for confirming my interpetation of what you did. In addition to boatbuilding, are you an engineer or draftsman?   One additional qustion, why the angle of 37 degrees? Is there something unique about that angle or is it just for appearance and not wanting the wing's length "grow" too much?

Again, an outstanding job of describing your project both in text and photos.  I enjoyed reading it and if there is quantifiable improvement as stated from your actual flying results and those of others who have done this, I will be encouraged to make the modification on my plane.

Hey, by week's end, our temperature in Kansas is supposed to rise to 50 degrees, my minimum for comfortable open-cockpit AirBike flying! Guess what I am looking forward to doing during the next couple of days?

Paul Fiebich

Glad it helped, Paul.

No, nothing magic about 37 degrees. Just looked about right, & gave a similar span increase as the factory wingtips.

Yes, I worked for quite a few years as design draughtsman, mainly in aircraft but also hovercraft & cars.

Then I got into designing ultralights, and from there the ARV Super2, a British light aircraft.

I'd be glad to hear how you get on making the wingtips!

Bruce