Wednesday, December 21, 2011

More on "Hoerner Wingtips" in ultralight aircraft

Please note: James' blog has moved to a Wordpress site. To access it, please visit All posts have been transferred to the new site, and all new posts will only be accessible via Wordpress. Thank you for your interest!

My previous post cited the work of Dr. Hoerner in the development of wingtips which produce smaller vortices, which consequently produces less drag, which consequently allows more speed (for a given thrust), and consequently a better rate of climb....

Clearly the Hoerner wingtips are a desirable thing.  While there are other ways to produce effective vortice reduction (for instance, vertical winglets or even flat plates), the Hoerner wingtip is more easily fabricated, and frankly, in my opinion, just looks darn sexy.

Soooo, it's worthwhile to spend a couple of minutes reviewing Dr. Hoerner's research, so we can guess what positive results these wingtips will give us.

Below is a basic illustration of a wingtip vortice, which I have copied from Dr. Hoerner's most excellent book, "Fluid Dynamic Drag".

Vortice at wingtip, from Fluid Dynamic Drag, by Hoerner
This illustration shows how high pressure air is spilling from the underside of the wing out of the wingtip, causing the vortice.  (The villain isn't the vortice, it is the fact that a vortice is created by the loss of high energy air from the bottom of the wing.  The evidence and creation of a vortice only proves the problem.)

Dr. Hoerner reviewed several different kinds of wingtip configurations, including square wings with sharp edges (which is exactly what our Belite ultralight airplane has), along with round rectangular edges, sharp rounded wings, sharp full wings, and more.  He also reviewed vortices and drag  reduction around wingtip fuel tanks (which are not in our current Belite innovation plans.)   With each kind of wingtip shape, he calculated and presented the effective span of the wing -- in other words, a longer wing is a better thing -- and if you get that length 'for free' (by using better wingtips), that is a really good thing.

The very worst wingtip is a rounded wingtip, with a round cross section.  The very best wingtips are ones which have a sharp cross section.

And interestingly, the square wingtip with a square edge is nearly as good as the very best wingtip, but not quite.  It is worthwhile to quote Dr. Hoerner on this topic:

"Theory predicts minimum induced drag for elliptical lift distribution across the span....   however, directly in one case... that a rectangular wing (with sharp lateral edges) does not have a higher drag due to lift than the elliptical wing."

Wow.  Score one for Belite, and the game of innovative improvement hasn't even started!

Back to Dr. Hoerner's writing:

"It is seen that the most effective plan forms are the rectangular, the moderately tapered ones and those which have a long trailing edge....   In other words, to make the span of the rolled-up vortex system, or the effective span of a wing of given basic shape, as wide as possible, it is favorable to keep the tip vortices apart from each other as far downstream as possible."

Now if I look back at his charts, I do see that sharp wingtips have an advantage over the square wingtip with square edges, and all of those have huge advantages over rounded  wingtips.

So the Hoerner wingtip is taking it to the next level.

Let's find some more interesting remarks from my new best friend, Dr. Hoerner.  He makes some comments about the most ideal wingtip shape, which I (and others) now call "Hoerner wingtips":

"In case of [Hoerner wingtips], which is the most favorable one concerning small drag due to lift as well as to minimum sectional drag, it appears that one additional effect is the bent-up shape.  Experimental data.... on a small-aspect-ratio wing, confirm that this feature is important.  Combination of [Hoerner wingtips] with a moderately tapered plan form is believed to be the most effective..."

So things get a bit 'guess-work-eee' from here.  But we think that we know the following:

 * sharp wingtips reduce the size of the vortice -- which is what the Hoerner wingtip is all about
 * the addition of the Hoerner wingtip spreads the trailing edge, adding span, and further separating the vortice
 * sharp wingtips have less drag at higher angles of attack, which is where ultralights spend more time at than higher speed airplanes

Hey, wait a minute.  I better provide some documentation for a couple of those points.  Here's another illustration from the book which helps:

Improvement shown with Hoerner wingtips -- with square wingtips not far behind!
The illustration shows, on the top right hand side of the curve, the slightly higher coefficient of lift achieved with the Hoerner wingtips.  Square (Belite) wings are close behind, with round wings falling off considerably.

Here's a final helpful illustration, showing the cross section of vortices with sharp vs round wingtips:

Decreased vortice size with Hoerner wingtips (a) vs round (b) or wingtip tanks (c)

Finally, some conclusions...

Q.  So what is all of this worth to a Belite ultralight aircraft?

A.  We believe the aerodynamic effective span will increase by a little over 2 feet.  However, we are cheating a bit, because the Hoerner wingtips will add about 20 inches of real span all by themselves.  The increase in effective wing area is 7+ square feet!  (with an original wing area of about 108 square feet).

Q.  What performance increases will result?

A.  Hopefully I'll see an improvement in climb rate, a reduction in stall speed, a reduction in takeoff roll, a reduction in landing roll (I'd like to beat my personal measured 100.5' landing record -- which I know I'll be able to do [because I've already done it in another Belite, before I even started talking about Hoerner wingtips   :-)  ])  and more...

Q.  What weight penalty do the wingtips entail?

A.  Substantially less than a 8 ounces per wing, vs. our existing square design.   Really!

Q.  What if they don't work quite that well?

A.  Then I'll get to figure out why, and I'll have the coolest looking ultralight airplane on the planet.

My thanks to Dr. Hoerner, and his wonderful book.  I believe my quotes and illustrations taken from his book fall under fair use doctrine.

I am also indebted to Mike, who is building this wonderful test airplane with Hoerner wingtips for me.

If you want an overview of what else I'm working on (and willing to talk about) you can read it here:

Upcoming 2012 Product Innovations at Belite Aircraft

-- James Wiebe, 2011 EAA August Raspet award recipient

Monday, December 19, 2011

Upcoming 2012 Product Innovations at Belite

Please note: James' blog has moved to a Wordpress site. To access it, please visit All posts have been transferred to the new site, and all new posts will only be accessible via Wordpress. Thank you for your interest!

"Upcoming 2012 Product Innovations at Belite"

 an overview of what's coming...
 this post focuses on "Hoerner Wingtips"

One of the absolute joys of my pseudo-job is that I get to investigate, implement and test aerodynamic improvements to my Belite ultalight airplanes.  As a result, I have come up with a goal of decreasing drag on our aircraft so that our very smallest engine option will allow cruising flight at 62mph.  (And the larger engines will require throttle limitations in order to stay within Part 103 -- or registration as experimental aircraft.)

When all is implemented, this will provide important improvements to takeoff and landing performance, along with substantial improvements to climb rate, reduced stall speeds.

Because our aircraft is so similar to a conventional high wing aircraft, I have identified several opportunities for drag reduction which are already available on many 'commercial' certified aircraft.  These opportunities include:

a)  lift strut drag reduction via fairings  --  all airplanes should have this
b)  wingtip drag reduction via reduction of wingtip vortices
c)  increased Lift / Drag in wing airfoil via subtle improvements
d)  reduced drag due to improved cowling design
e)  And maybe there will be more.

(none of these future options have been made available, or even priced yet)

I've already implemented and test flown a Belite with lift strut fairings, and I will provide a full report soon.  Hint at the outcome:  It was awesome.

I'm also working on a changed cowling design -- it will take a few more months and I'll have that one ready for report.

I've been reading through a variety of aerospace engineering reports on wing airfoils.  I find that my engineering and math background are an excellent helper for me as I've learned more and more about airfoils.

As for wingtip vortices, lots of people already know that Dr. Hoerner tested many different types of wingtips, in order to select and document those which increased effective span and reduced drag by reducing or eliminating those nasty wingtip vortices.  I happen to have gotten my hands on a copy of a couple of his books (they were loaned to me by a friend at my Church) and they have been delicious reading for my ultralight aircraft aerodynamic engineering education.

So, here's some photos of a set of wings in construction progress, showing Hoerner wingtips being fabricated.  These will end up on a plane which is being built to exhibit at Sun N Fun in 2012.  No opportunity for improvement is being overlooked in this airplane.  The wings in question are carbon fiber spars, with aluminum ribs, and the Hoerner wingtips.  You may also notice that they are also being prepared for VFR night flight via the addition of wingtip lights.  (The airplane is being built with a heavy 4 stroke engine, and will be registered experimental, thus allowing night flight.  It will be also be flyable as a legal FAR Part 103 airplane, albeit with a different engine.).  (This particular set of wings is being built by builder Mike.  Mike is awesome.)

So here are the pictures:

Belite Ultralight Aircraft wing, under construction with Hoerner wingtip

Detail of Hoerner wingtip, showing nav light fairing

Front view of Hoerner wingtip, with much smooth work still to do

Quartering view of Hoerner wingtip

View of Hoerner wingtip, prior to adding foam to front cell

Trailing edge quartering view of Hoerner wingtip

View of Hoerner wingtip from outside rear