YC-14 Build (From MikeysRC)
- Thread starter dezertdog
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dezertdog
Senior Member
Found a little more time to wrap up the body. When it stops raining I'll bring out the sander and make things nice and curvy.
Currently researching airfoils and deciding what I'm going to do with the tail. The "flying tail" I used per the plans on Mark I was a ton of work, and I was never satisfied with it. Round 2 might go better with some experience under my belt, but for simplicity I may just go with a standard stabilizer/elevator.
I like the looks of the T tail as it's more "scale", but having a low stabilizer might give me better response at slow speeds with the prop wash going over it. Have to think on this one a little more. Weight as pictured is 1.4lbs.
Haha I just realized it looks like it has a little moustache. I swear it's part of the logo on the foam.
Currently researching airfoils and deciding what I'm going to do with the tail. The "flying tail" I used per the plans on Mark I was a ton of work, and I was never satisfied with it. Round 2 might go better with some experience under my belt, but for simplicity I may just go with a standard stabilizer/elevator.
I like the looks of the T tail as it's more "scale", but having a low stabilizer might give me better response at slow speeds with the prop wash going over it. Have to think on this one a little more. Weight as pictured is 1.4lbs.
Haha I just realized it looks like it has a little moustache. I swear it's part of the logo on the foam.
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dezertdog
Senior Member
Hard to stay away when it's this close.
Rudder is now functional, though 1" foam presents some issues with clearance. There are some interesting angles involved to make it swing a full 45* each direction.
All still blocky, I'm leaving sanding till all the interesting problems are solved.
After some reading and thought, I decided to go experimental on the airfoil and build a KFm3. Based on the info I could gather, this variation is great for heavy lifting and known for a very slow stall speed. Sounds perfect.
Figures are 12% thickness, steps at 50% and 75%. I decided at the last moment to extend the ailerons out another inch and extend the chord to give more roll authority. The plans weren't written for 1" foam and the ailerons just didn't seem to give enough down travel as designed taking thickness into consideration.
It was tricky to rip the foam thin enough for the steps. I ended up having to clamp a make shift fence onto the band-saw and slowly split 2.5" wide strips to .5" and .25" (the saw guard is only 4.9" tall, so the 50% step being 5" was a no go in one shot). The front (left) piece here is only taped on here as I am going to figure out a way to route the motor wiring under it before I glue it down permanently.
Keeps looking more like a plane! Still pondering the elevator solution...
Rudder is now functional, though 1" foam presents some issues with clearance. There are some interesting angles involved to make it swing a full 45* each direction.
All still blocky, I'm leaving sanding till all the interesting problems are solved.
After some reading and thought, I decided to go experimental on the airfoil and build a KFm3. Based on the info I could gather, this variation is great for heavy lifting and known for a very slow stall speed. Sounds perfect.
Figures are 12% thickness, steps at 50% and 75%. I decided at the last moment to extend the ailerons out another inch and extend the chord to give more roll authority. The plans weren't written for 1" foam and the ailerons just didn't seem to give enough down travel as designed taking thickness into consideration.
It was tricky to rip the foam thin enough for the steps. I ended up having to clamp a make shift fence onto the band-saw and slowly split 2.5" wide strips to .5" and .25" (the saw guard is only 4.9" tall, so the 50% step being 5" was a no go in one shot). The front (left) piece here is only taped on here as I am going to figure out a way to route the motor wiring under it before I glue it down permanently.
Keeps looking more like a plane! Still pondering the elevator solution...
dezertdog
Senior Member
The thought crossed my mind...
1" just didn't look like it gave enough deflection. I can always limit the throw in the radio if it's too sensitive. I sacrificed some wingspan having to make the body wider to accommodate a bomb drop door so I'm reasoning that the extra chord length gained also helps makes up for that with more lift. That and I think they look kinda stealthy cool.
The rudder could have benefited from it maybe as it's hinged at an odd angle. I had to bevel the top and bottom 15* to get it to swing full travel each way. I actually made two rudders, and in the end I decided to go with a 45* on one side to limit the turbulence generated. My thought is that one side of the rudder will have super laminar flow and the other turbulated, instead of both sides turbulated. I'm thinking that I'll get less drag with one large 45* than two smaller ones.
My thoughts were the same about the ailerons, that I didn't want to interrupt the flow over the top airfoil tip, to avoid making an odd vortice that would interact with the KFm steps. Who knows though really, I don't have a way to prove this theory out.
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It's true, you'll get really laminar flow on one side, but! what happens when you have more drag on one side of the airplane than on the other side? I don't think it will be that bad, I'm just excessively detailed with stuff like that. You also have more rudder surface on one side than on the other, which will make your rudder control be more responsive in one direction.
You could tape a piece of paper over the hinge, but fastening it only on one side - that way it can adapt to the shape of the turning rudder creating a super laminar flow. It would kind of work well - with more complex systems you can get a perfectly laminar flow system, but it's not worth the time in a foam scratchbuild.
dezertdog
Senior Member
You make good points, I swapped it back to the center hinge / dual 45*.
I'm going to leave the ailerons though. The top hinge builds in some differential and as I said before I'm not sure how it would interact with the KFm step.
The ailerons are fine that way, you are very right - the turbulence from a hingeline may affect the KFM vortex.
dezertdog
Senior Member
On a different topic, coming up on motor mounting. I've learned a lot since Mark I about thrust angles and from what I gather, I had it wrong... Though I was using a counter rotating setup, I had a square thrust angle which probably played a big part in my tail heavy issues.
I see that for instance on the Hobbico Twin Star, each nacelle points outwards to the wing tip, and I also see that for a Clark Y style airfoil (which is the most analogous design to the KFm3 I could think of) the thrust angle is about 2-4* down pitch.
So that being said, current thought is to shoot for that combo (2-4* down angle, canted outward). If anyone is a twin thrust angle guru I'd love to hear some input.
I see that for instance on the Hobbico Twin Star, each nacelle points outwards to the wing tip, and I also see that for a Clark Y style airfoil (which is the most analogous design to the KFm3 I could think of) the thrust angle is about 2-4* down pitch.
So that being said, current thought is to shoot for that combo (2-4* down angle, canted outward). If anyone is a twin thrust angle guru I'd love to hear some input.
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I'm not a twin engine thrust angle guru by any means, but I am what I call thinker - I deduct a lot of stuff, and I do wrong conclusions sometimes, but I'm right pretty often...
I believe that the horizontal thrust angle is there to compensate for the torque of the motors. However on your plane I understand that you are using props spinning in different directions, so the side torque should be cancelled between the motors.
However vertical thrust angles are there to correct the nosing up/down tendencies of the plane, without the need to trim the elevator too much (causing drag). I can't help you there, though I'm inclined to think this:
Motors behind the CG - up thrust angle makes the plane nose up, down thrust angle makes the plane nose down.
Motors in front of the CG - up thrust makes the plane nos down, down thrust makes the plane nose up.
Remember there's a possibility that I'm wrong.
I believe that the horizontal thrust angle is there to compensate for the torque of the motors. However on your plane I understand that you are using props spinning in different directions, so the side torque should be cancelled between the motors.
However vertical thrust angles are there to correct the nosing up/down tendencies of the plane, without the need to trim the elevator too much (causing drag). I can't help you there, though I'm inclined to think this:
Motors behind the CG - up thrust angle makes the plane nose up, down thrust angle makes the plane nose down.
Motors in front of the CG - up thrust makes the plane nos down, down thrust makes the plane nose up.
Remember there's a possibility that I'm wrong.
dezertdog
Senior Member
I think that's correct. I'm basing that off this vid @ 7:00
http://www.youtube.com/watch?v=RtgtcZ8C7UY
dezertdog
Senior Member
Two things I could guess about an outward thrust angle on a counter rotating twin seeing that Hobbico uses that setup. If you had a single motor failure in flight, it might be easier to land (more likely on a nitro though). Also I'm thinking it might add some stability in flight having each motor thrust away from the other.
The only benefit I can think of for that is that the prop washes don't cross each other creating turbulence. Which in turn may have a slight effect on stability.
dezertdog
Senior Member
Wrapped up the horizontal stabilizer and elevators today. Ran out of a single pieces of foam large enough so I used some extra 1/2" thick ripped sections from the KFm wing parts, butt glued them side x side and was able to pull it off.
Decided to go with the tried and true low design instead of the flying tail or the T tail. They will be right behind the prop wash and should give much better response at slow speeds. Not sure yet if I'm going to try for a single servo control for both surfaces, or grab the old bomb bay servo and just run dual.
As for the tail now that I'm not going to run the high elevator, I've been considering making the rudder taller or dropping the overall height of the tail. Not a huge priority though, it can wait till it flies and I see how responsive it is.
I also salvaged all the electronics from Mark I and tossed it all on the scale, all up weight right now without battery is 2.4lbs.
Decided to go with the tried and true low design instead of the flying tail or the T tail. They will be right behind the prop wash and should give much better response at slow speeds. Not sure yet if I'm going to try for a single servo control for both surfaces, or grab the old bomb bay servo and just run dual.
As for the tail now that I'm not going to run the high elevator, I've been considering making the rudder taller or dropping the overall height of the tail. Not a huge priority though, it can wait till it flies and I see how responsive it is.
I also salvaged all the electronics from Mark I and tossed it all on the scale, all up weight right now without battery is 2.4lbs.
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dezertdog
Senior Member
Working dual elevator check. Not sure if this was over engineering or not, but I really wanted to see if I could come up with a way to use a single servo, hidden inside the body for elevator control. Basically there is a servo inside that has a vertical push rod to the, whatever you call it, that transfers the motion via a 1/4" dowel to the control arms. I'm able to get full extension each direction. The real question is which weighs more, that wood or another servo. It was fun to build anyway and worst case can always be pulled out and two servos dropped in it's place if it doesn't work out. Seems pretty robust, I can really hammer on the stick and it keeps up.
