LET Model JS1 is here!

[lenb]

Good news Tom. Its always nice to get the first flight out of the way so you can concentrate on enjoying it.

I noticed in the photos there is something on top of the fuselage behind the canopy, what is that?
I apologize if you have already addressed this.

Congrats on the new addition to your hanger

Len

[Stew2]

I think that is called an extractor vent? I think I remember Dean Gradwell posting pictures in his Antares thread on RCGroups when his son had a steady hand to cut the hole in the top of his full scale...29, maybe? I believe the theory is that it is a vent designed to let any cockpit pressurizing air a way out and to keep the flow laminar on the fuselage. Apparently sailplane cockpits become pressurized from air entering via cool air vents and the tow release in the nose. A pressurized cockpit pushes air past the canopy seals and triggers turbulent flow down the fuse, adding drag... There are probably as many debates about this being actual or not as there are socks in a top dresser drawer! But for us really cool scale guys, it is a really cool looking scale detail. Of course, if the vents are not actually open, then they really do just add drag for us..... I think they are really neat looking and know for sure that I couldn't tell the difference in performance if my model sailplane had one or not.

For some aero details, I think the extractor vents are a pretty interesting design. As I understand them, the desire is to have a vent that will allow air to flow out laminarly, and will develop no or very little low pressure at the exit to prevent a suction at the exit to pull air out of the cockpit. That is why they have a little ski ramp at the front of the vent, to hop the flow up over the actual air exit, making the flow think it is following the contour of the fuse and thusly staying laminar along it, and at the same time giving a flow path that cockpit air can mix with the outside air and boundary layer not triggering any turbulence.

The stuff the full scale guys go through to minimize drag is very interesting. Like leading edge bug wipers....

[tewatson]

It is a vent, and functional (open).

[tewatson]

Discovered an issue with the landing gear. This is the optional WEMO gear, which I ordered with the plane mainly because it has a cool internal drum brake. It is a clever design that also allows manually extending it further than normal for FES takeoffs...it does this through split actuating arm halves that lock into position with a series of ball detents. Retracting/extending the gear resets the arms into the normal position. My plane does not have FES, so does not use the extra extension capability. Turns out the system can unlock and move both ways, and if it moves in the wrong direction, the gear does not fully extend and will collapse. Found this out on the maiden landing; luckily on grass, so no damage whatsoever.

Given the design ingenuity, it is quite puzzling why it was not made to shift only in the right direction. I did not take it apart to look, but possibly the detents are relieved to ease movement in one direction and not the other. It obviously has to be secure enough to not pop loose on touchdown, and this makes sense because the "normal" position is locked in place by six balls, and when shifted, only two. I do know that it somehow shifted on the maiden flight (takeoff and landing are on video, and clearly shows the gear was properly down on takeoff, and was only down about 3/4 of the way for landing.) Did not notice this on approach, as my focus was on glide path and control.

It only malfunctioned once (and I have no idea why), but gear problems are not cool so I fashioned a stop bracket from 90 degree aluminum stock to prevent it from unlocking in the wrong direction. No changes to the gear assembly itself, other than two slightly longer 2mm screws to retain the plate.

[yyz]

I think you should be a technical writer. Such clear and concise explanations for complex subjects.

[tewatson]

11 years of writing and defending regulatory compliance testimony. It’s a sickness of mine.

[ARUP]

Put a patent on that restrictor or have the gear manufacturer give you some credit where due! Very nice workaround!

[tewatson]

Update... After a few outings with the plane, it was still exhibiting some TE issues on the main wing panels (very limited flap throw and inner aileron double-centering) and it was affecting overall performance.

Inner ailerons - as predicted during the original build, the OEM linkage was just not working and even with re-shaped wipers there was still some double-centering. I decided to execute the same retrofit as for the tip ailerons (detailed in posts 29 and 30), plus another round of wiper thinning. The freed-up surface with improved linkage geometry now moves smoothly and precisely.

Flaps - there were two factors in play: First, the wipers were still too thick and bound up prematurely for landing mode. Second, the OEM linkage setup used longer than needed servo arms and as a result, less than 40 percent of total servo travel could be used, thus reducing precision (particularly for small camber changes.) Replacing the servo arms and another round of wiper thinning allowed doubling the servo travel while increasing available landing flap by 5-6mm.

Overall flight characteristics are greatly improved, and the added landing flap throw is a welcome change.