Hoerner Wingtips for the Sonerai II – an Introduction

This is the first in a series of articles on an interesting project I’m doing over the winter to extend the range of my Sonerai II by adding wingtip tanks. The original plan was to remove the wingtips and use them to make templates so that someone else might make Hoerner wingtips.

Why do that? We installed a Jabiru 3300 engine in our Sonerai II, which reduced the fuel duration by about an hour. The duration was already modest by our standards and we never took off without full fuel. With the Jabiru 2200 engine, we ran the aux tank dry at least once and had to switch to the 6-gallon main tank on the way home. Nasty! With our fuel consumption increased from 4.1 gph to about 5.1 gph, flights would need to be fast and short with total fuel of 16 gallons. We could never schedule a flight to an airport without avgas and hope to return without a fuel stop somewhere else. That’s quite limiting. It also affects the mission.

One of the moderators of the Sonerai group threw out the question, “Who wants Hoerner wingtips?” About a dozen members signed up at a modest price. But five years went by without a plug or mould being made. We plan to fly our Sonerai next year, so we needed to get going.

As we wrote above, we wanted someone else and someone better at composites to make them. Alas, the first chap moved to China and my ideas for Hoerner wingtips went with him. We asked another composite plane builder to have a go, but it looked like he wanted to make something more elaborate. The price also seemed to be elaborate, maybe $5,000 over 5 months – gasp!

We came home dragging the drawings behind, muttering, “Rats!” My budget is about $500.

Having removed the original wingtips and having the benefit of some conversations about making wingtips, we saw a way to make a pair. A few notes and dimensions were jotted down and work began.

By comparing the wingtips, we found that one of them had a kink, so the other one was chosen for the pattern to make templates. The wingtip was placed on a sheet of 3/4-inch plywood, and we marked the circumference and cut it out carefully with a jigsaw. It was a success, so a second one was made. You need two templates to carve the foam, one at each end.

Why would we not need to make the moulds smaller, you ask? Doesn’t the new wingtip need to be the same size as the one we removed? No – we learned something in the research process. The original wingtip fits very snugly between the wing skin and the outboard rib. The fibreglass wingtip is 1/32-inch thick and that just fits the gap.

What I learned was this: To replicate that profile, you’d need a precision female mould and a vacuum pump to get a very dry layup. We were making just two wingtips, so a female mould for each was not justified. Two foam plugs would be made, which would be sacrificed when the layup was removed from the plug.

By using 0.032 aluminum strips about 1.5 inches wide between the rib and skin, one could slide the new wingtips over the aluminum strips, and they came very close to fairing with the wing skins. A tiny bit of volume was gained, which was a plus.

Why not make a mould and go into production? At the time, we decided to make the wingtips, we surveyed the 750 guys who build, fly, or own Sonerais. Not a nibble resulted. So one pair of wingtips it is. (At this point they look great and some guys may regret their silence.)

What’s a Hoerner?

After World War II, a designer named Dr. Sighard Hoerner discovered that if wingtip vortices could be displaced, improved wing efficiency (total lift) resulted. The wingtips he designed were for gliders, which of course fly in a different speed range than racers. Accordingly, he found that drooping wingtips work best for gliders.

On a Sonerai, a suitable version of the Hoerner wingtip has been shown to displace the wingtip vortices about 8 inches outboard of the wingtip. The result is a 5 percent improvement in cruise speed. As we shall see, these wingtips do not droop. They have an upward bevel set at about 45 degrees. It works in the wind tunnel.

One or more of the Sonerai builders race their aircraft in Vee-class and are seeing around 175 mph. Very nice on 80 hp. The Jabiru 3300 engines are rated at 120 hp, but our model has solid lifters and shows around 127 hp on the dyno. Yes! We have hopes of similar results, with these wingtips, perhaps faster.

Why install that bigger engine? The truth is we do not weigh 150 pounds. Once we get into the plane, the climb is marginal with a passenger on board. On a hot summer day, the climb is questionable. The Jabiru 3300 engine causes mission and flight improvements in 17 different categories. If we put the 17 into broader areas, they are flight safety, passenger and fuel load, flight performance, and mission. All the things we bought the Sonerai to do became realities. The range almost doubles. Climb is exciting.

But that range and flight duration need more fuel. We like fuel reserves as well. You just cannot add wing and fuel at will to aircraft wings, and you should not do aerobatics if you do. Some pilots have been known to pull the wings off their aircraft after lengthening the wings. One advantage of the Hoerner wingtips is that they add just 3 inches each to the wingspan. With the wingtips/tanks empty, the wing loading is not noticeably changed.

There is a bulkhead in each wingtip, and that space will hold 3 gallons of fuel per side, or 6 U.S. gallons total. Thus we have gone from 16 gallons to 22 gallons, which at 5.1 gallons/hour, results in just a tad over 4 hours flight time.

Why not 10 gallons each? The addition of 6 gallons of fuel causes a weight penalty of around 40 pounds. Six gallons restore the original fuel duration. That is all the fuel we need.

One still needs to be careful about the manoeuvring speed with that fuel on board and more careful about turbulence, but in a racer, you tend to avoid the kind of weather Bonanzas seek. (We used to own and fly a D-35.)

At best, our knees and backs will now endure about 90 minutes. So we can fly out and back a flight of 3 hours duration without refuelling. Yes, one needs to transfer those 6 gallons of fuel to the aux tank. Turn the pump on and a few minutes later the fuel is there. For that matter, it could be done on the ground, before takeoff. Very nice, that.

If we go to the fly-in at Stanley N.S., we could do one of the stops south of Quebec City where no fuel is kept. Refuel from the tip tanks and be on our way, after a good stretch, that is.

Next month we’ll discuss the planning and calculation requirements for starting to make the tanks as well as cutting the foam plugs.