PC 21 Scratch Build

[Jackson T]

Looking good!

Thanks!

 

[Quinnyperks]

I like it

 

[Jackson T]

I've cut out all the ribs, spars, LE's and TE's for the wing. I'm gluing up the left one as we speak!

I decided to go with a NACA 1410 airfoil instead of a Clark Y because I want something with a bit less camber to make inverted flight easier. We'll see how it goes!

 

[Jackson T]

Here's were we're at. I'm calling it a night for now, I'll hopefully do the right wing tomorrow.

 

[Quinnyperks]

Do you think that you will put retracts or flaps in it?

 

[Jackson T]

Do you think that you will put retracts or flaps in it?

I'll program some flaperons, but no retracts. Do they even make retracts this small? Here's a photo of the wing with my hand for scale.

 

[speedbirdted]

Do they even make retracts this small?

No, but making retracts using linear servos is not exceedingly hard...

 

[Quinnyperks]

No, but making retracts using linear servos is not exceedingly hard...

That is pretty small

 

[Jackson T]

No, but making retracts using linear servos is not exceedingly hard...

Hmmm, maybe not on this one, but sounds very interesting, thanks for the idea! Would it work with 9 gram servos?

 

[Jackson T]

That is pretty small

Sure is! It's missing an aileron though, which will add 40mm at the root and 25 mm at the tip, so it'll be a bit bigger than in the photo when it's on the plane. It'll also get a 2.5cm solid block on the wingtip to sand into a nice shape, which will add a bit more size again. The PC 21 has a span to fuse length ratio of 0.81, while most planes are more like 1.25.

 

[speedbirdted]

Hmmm, maybe not on this one, but sounds very interesting, thanks for the idea! Would it work with 9 gram servos?

It would, but I wouldn't do it. Servos last the longest when exposed to a constant load that has little sudden change - that's why they do things like actuate control surfaces really well. Landing gear has to deal with sudden large shocks, because no one can land perfectly smoothly every single time and expose the servos to no force whatsoever. Eventually, after enough shocks, they will strip out. This is why a while ago people used pneumatic retracts as in that case there are no gears to strip out (and many folks, myself included, still swear by them for large planes) not to mention the power and channel requirements being lower. Linear servos tend to be better as they have worm gears which are often made out of metal and they stand up to shocks better because a worm gear is very efficient at multiplying force in one transformation only - a powered worm gear can efficiently transfer force to a carriage that keys into it and moves along with the threads, but trying to turn the worm gear by pushing the carriage is much harder. This means that less force gets transferred to any other part of the system.

I've long thought that a way you could implement a normal servo into a retract setup without any risk of damaging it is to add some sort of "locking" mechanism. One that when the retract is in the fully extended position the landing gear leg itself somehow becomes locked to the wing or fuselage or whatever, and instead transfers landing shocks to said wing or fuselage and not the servo. The only times the servo would be taking any force from the landing gear would be in the moving position between fully extended and retracted, which should pose no problem as the only time the landing gear would encounter significant force while moving would probably be the pilot doing something stupid (like retracting the landing gear on the ground... which I have done), and of course when fully retracted, but there's no real chance of it receiving significant shock there. Just because I haven't been able to do it doesn't mean you couldn't!

 

[Jackson T]

Finished the right wing.

It came out about 2 grams heavier than the left wing. I'm attaching them together as we speak!

 

[Jackson T]

The wings glued together fine, now on to mounting the servos in the wings.

The arms stick out the underside of the wings, about 10mm clear of the fuse sides. Here's what it looks like all put together so far.

 

[speedbirdted]

I like it so far. I have a suggestion though - give it some kind of undercarriage, whether retractable or fixed, since landing on the servo horns is not going to let your servos live very long. I built a belly landing low wing plane once with the aileron servos on the underside of the wing. The first time I went to fly it, the landing stripped both of the servos!

 

[Jackson T]

I like it so far. I have a suggestion though - give it some kind of undercarriage, whether retractable or fixed, since landing on the servo horns is not going to let your servos live very long. I built a belly landing low wing plane once with the aileron servos on the underside of the wing. The first time I went to fly it, the landing stripped both of the servos!

I did think about that, but though it would probably ok. I think I'll re-evaluate now after reading your post though. Thanks for your input, it's much appreciated!

 

[leaded50]

you could use such ones too.... for not ripping servo.

 

[Jackson T]

Following @speedbirdted's advice, I decided to move the aileron servo arms to the top side of the wing. I had to cut some rib out to unscrew the horn, but the wing still feels nice and strong.

 

[speedbirdted]

Following @speedbirdted's advice, I decided to move the aileron servo arms to the top side of the wing. I had to cut some rib out to unscrew the horn, but the wing still feels nice and strong. View attachment 166283

Interesting way to mount the servos. I would have just doubled up the wing rub with like 1/16 ply or something and added some gussets, then just added a hole for the servo and screwed it into the rib.

 

[Ketchup]

It would, but I wouldn't do it. Servos last the longest when exposed to a constant load that has little sudden change - that's why they do things like actuate control surfaces really well. Landing gear has to deal with sudden large shocks, because no one can land perfectly smoothly every single time and expose the servos to no force whatsoever. Eventually, after enough shocks, they will strip out. This is why a while ago people used pneumatic retracts as in that case there are no gears to strip out (and many folks, myself included, still swear by them for large planes) not to mention the power and channel requirements being lower. Linear servos tend to be better as they have worm gears which are often made out of metal and they stand up to shocks better because a worm gear is very efficient at multiplying force in one transformation only - a powered worm gear can efficiently transfer force to a carriage that keys into it and moves along with the threads, but trying to turn the worm gear by pushing the carriage is much harder. This means that less force gets transferred to any other part of the system.

I've long thought that a way you could implement a normal servo into a retract setup without any risk of damaging it is to add some sort of "locking" mechanism. One that when the retract is in the fully extended position the landing gear leg itself somehow becomes locked to the wing or fuselage or whatever, and instead transfers landing shocks to said wing or fuselage and not the servo. The only times the servo would be taking any force from the landing gear would be in the moving position between fully extended and retracted, which should pose no problem as the only time the landing gear would encounter significant force while moving would probably be the pilot doing something stupid (like retracting the landing gear on the ground... which I have done), and of course when fully retracted, but there's no real chance of it receiving significant shock there. Just because I haven't been able to do it doesn't mean you couldn't!

Well, I haven’t ever tried anything with retracts, but maybe if the load was pushing straight down the servo arm the load wouldn’t go into the servo. For example, when the gear is down, the servo arm and the wire that is connected to the gear are both aligned so that the force is directed straight down the center of the servo arm and none of it is used to turn the servo. Of course this would only work for small planes though, because if the plane is too big the servo arm would just break. Also, I know that this might be hard to visualize, I just don’t have any drawings now. Sorry about that.

 

[Jackson T]

Interesting way to mount the servos. I would have just doubled up the wing rub with like 1/16 ply or something and added some gussets, then just added a hole for the servo and screwed it into the rib.

By "following your advice", I just meant I mounted the servo arms on the top. I had already mounted the servos in the wing, shown in post 97, so I had to go from there. They feel pretty solid in there, I'm not worried about them.