All-

recently acquired the L-39 and cannot help but think about ways top make it "better." lots of variations in wingtips on these planes, so I'm reaching out to the brilliant engineering minds here for advice.

I have stock tip-tanks; they are roughly 10-12" diameter and 5 feet long (they hold 26 gal fuel). for fuel purposes, they are worth having. for flexibility, it's not a huge ordeal to remove the tank and add a wingtip (making it more mission-specific). probably a few hours.

many racers remove those tips, and either add an angled tip, or in one case, a winglet (see the racer in desert camo). I wont race, and don't plan on flying airshows, but I like the flexibility. to me, it's about doing mod's and getting performance, but I don't care about competing. I've been in enough airshows.

Lear 25s had a trailing winglet on the wing tank. Weird. Tradewinds turbine puts conventional winglets on the tip-tank of the BE36 (sort of what I was thinking).

Would I get the benefit of a winglet, if I added it to the tank instead of simply replacing the wing tip?

what does wind-tunnel data show? I don't know that any of the L39 tips I've seen have been wind-tunnel tested.

thoughts? anybody in the SE have a wind-tunnel?

Congrats on the new ride, Doc. No experience outside of a few hours in an L-39 with tip tanks and a thirsty engine!

Been a while, but recall that winglets and their cousins are designed to more gradually transition the wing tip vortices to the free stream. At least when their goal is reduction in induced drag and hence fuel consumption.

Seem to recall that some larger winglets can save 3-4% fuel. Suspect there may also be aerodynamic (handling and stability) value to some designs.

You might be able to get someone to run a proposed design in OpenFoam or other CFD code to see what a proposed design predicts. A wind tunnel would work but IMHO much more overhead and less accessible.

What specific mission parameter are you trying to improve with a different wing tip?

faster climb rate, lower fuel burn. preferably, lower fuel burn in climb and cruise.

I've found a few papers on winglet design, and while they all reduce fuel flow and improve climb, looks like at least 1 paper demonstrated a reduction in TAS in cruise. that's no good.

I'm an aero engineer by degree, but have spent most of my career in structural analysis so I'm only marginally qualified to answer such questions...

Having said that, winglet design is very tricky, and usually done to optimize a particular phase of flight, such as cruise since most jets spend most of their time and fuel there. Adding a winglet can lead to other issues like extra wing loading (and might require structural improvements/weight) or perhaps a lateral/directional instability if not done well.

For an L39, I'm not sure what improvements could be gained. To me, it seems most are toys to go have fun, do some aerobatics, and even race as you mentioned. Unless you want to use it for XC travel, I can't see it helping, and if you lost your tip tanks then that might hurt your range more than a winglet could help.

I can't speak to the examples you posted, but don't underestimate the power of marketing with this sort of thing! You didn't show the Lancair IV, but it originally had a clean wing and later had optional winglets. I think everyone bought those because they look cool, but in that case, they made the plane slower and added a bit of a dutch roll instability. They sure looked cool and lightened some wallets though!

My advice would be to just enjoy what you have and not to be tempted to go down a potentially very expensive rabbit hole that might not pay off. I do say that as an owner going down my own aero clean-up drag reduction rabbit hole on my Mooney, though!;)

The really hot combo on the -39 is removing the tip tanks (typically 46 gals), putting on the carbon wingtips, and adding the 100 gallon tank in the wing center section. It is in the D section of the wing. About 6-8 feet either side of aircraft centerline.

This gets more range (more fuel), better roll and handling without having the tip tank and fuel weight in the wings, and faster without the tip tank drag and is racing wingtips. I can’t imagine the Reno guys would remove their tip tanks if the planes were faster with the them. Also, the shop that works on our plane (Code 1) says the race tips are faster - who knows.

I would love to have this mod on our plane. With the Garret 731, it would have terrific range.

Zeke

agree 100%. but that fuel tank mod is pretty pricey!

How much is Code 1 asking for that mod these days? That last I knew was 3 years ago and they said it was $60K. We never did it so I don’t know how real that is. I can say that Code 1’s estimates are….. optimistic. The upgrade on the 731 ended up substantially more expensive than originally estimated.

Z.

i thought I heard them say $90. i wouldn't do it for that. probably would do it for 60 though.