How do you choose the best wing tip device?

[eightyknots]

There has been some discussion recently about the work of Hoerner. One thing that Hoerner lends his name to is the wing tip used in some aircraft, including recreational aircraft.

 

Chris Heintz (one of the CH701's designers) illustrates it like this:

 

 

If the Hoerner wing tip works as illustrated, you get something (in this case, wingspan) for nothing. However, that is 20th century technology, started by much earlier by Hoerner and adopted by Heintz/Tedesco in the 1980s on the Zenith aircraft.

 

Heintz claims that the Hoerner wing tip expanded the wing by 100-200 mm (per wing):

 

 

Lately, there are other wing tip designs found on RA-Aus style aircraft. One is advertised by a supporter of this forum, the Nynja. Here, the wing tip device is an upswept one:

 

 

One wonders what would be the best wing tip for 19-xxx high wing plane? Any thoughts??

 

 

[Bob Llewellyn]

There has been some discussion recently about the work of Hoerner. One thing that Hoerner lends his name to is the wing tip used in some aircraft, including recreational aircraft.Chris Heintz (one of the CH701's designers) illustrates it like this:

 

[ATTACH=full]28668[/ATTACH]

 

If the Hoerner wing tip works as illustrated, you get something (in this case, wingspan) for nothing. However, that is 20th century technology, started by much earlier by Hoerner and adopted by Heintz/Tedesco in the 1980s on the Zenith aircraft.

 

Heintz claims that the Hoerner wing tip expanded the wing by 100-200 mm (per wing):

 

[ATTACH=full]28670[/ATTACH]

 

Lately, there are other wing tip designs found on RA-Aus style aircraft. One is advertised by a supporter of this forum, the Nynja. Here, the wing tip device is an upswept one:

 

[ATTACH=full]28669[/ATTACH]

 

One wonders what would be the best wing tip for 19-xxx high wing plane? Any thoughts??

It adds bending moment. It adds a lot of root bending moment. Any effective tip device does. The winglets on one of the Cessna twins - 421 mebbe? - drop the fatigue life to ~1/4 of the life without winglets.

 

The Hoerner tip produces less bending moment than a winglet, and less benefit than a winglet; if you want the benefit of higher aspect ratio, build a bigger wing. There are far better places to improve most aeroplanes than the structurally arduous wingspan extension.

 

Airliners use winglets for hangarage reasons, not because they're better than a span increase.

 

 

[djpacro]

One wonders what would be the best wing tip for 19-xxx high wing plane? Any thoughts??

Its just one element of the overall design, so - best for what purpose, in what context?

[Dafydd Llewellyn]

The general drag equation is (in its most basic form) Cd = Cdo + (Cl^2)/(Pi x Aspect ratio). Cd is the total drag coefficient; Cdo is the "fixed" part of the total drag coefficient (roughly, that due to friction and form drag) and the second term is the induced drag. Wing span extension or winglets increase the effective aspect ratio and thus reduce the induced drag; in effect one can think of the wing as being a device to hold the tip vortices apart - the further apart they are, the less the induced drag.

 

However, when the aircraft is flying at something like twice its minimum drag speed, the induced drag term becomes very small; so if you have a lot of horsepower for the weight, you can fly sufficiently fast that a low aspect ratio has little effect on the overall drag in cruise; that's the basic philosophy of the RV series, amongst others.

 

But if you need to fly as efficiently as possible, this means flying only just sufficiently above the minimum drag speed to avoid speed-instability; and then the induced drag contribution amounts to almost 50% of the total drag. This is the situation for airliners and sailplanes; and if there is a physical constraint on the actual wingspan (hangar width, taxiway clearance, or the FAI class restrictions for sailplanes, for example), then it's possible to gain a few percentage points by a "virtual" span increase from winglets.

 

Generally speaking, the best form of GA wingtip for general use is probably the Dornier raked version of a Hoerner tip - i.e. it looks like a Hoerner tip from the front, but the maximum span is at the rear corner. However, "wingtip hot-ups" have been a popular gimmick, starting from the "conical camber" tips on the original Cessna 210 (which were actually there to slightly reduce the dihedral effect of the C182 wing, since the undercarriage was retracted; and there have been some pretty extreme examples.

 

If you want to see some examples of extreme aspect ratio wings, look up Hurel-Dubois.

 

 

[rgmwa]

Another example that may be of interest. The RV-12 (LSA) wingtip becomes progressively more concave from front (flat) to rear (scalloped). I assume it's a type of Hoerner tip. I don't know if it has a particular name, but it's surprising what an elegant shape you can get from a flat sheet of aluminium.

 

rgmwa

 

 

 

[djpacro]

This may add to the discussion:

 

http://soaringweb.org/Soaring_Index/1965/PDF/1965_Aug_16.pdf and

 

 

Consider one item in the recent development of the Xtreme Decathlon from the Super Decathlon. More power so rate of climb is good but another design objective was to increase the roll rate - reduction in wing span helped achieve that. The neat wingtip was removed leaving a blunt tip - compare effective aspect ratios between #1 and #5 to help determine what is lost. Then consider the reduction in profile drag. And a weight reduction which helped to compensate for the heavier engine.

 

Mix that in with the other design changes and the Extreme is a much better airplane than the Super all round.