900 Skyraider

[krob]

After successfully building and flying the 900 series T28, I'm ready to try another. This time hopefully coming out with a model that will be lighter and fly longer.

[krob]

I'm using some lighter balsa in this kit to try to make it lighter than my last 900 series, and as you'll see later it worked on somethings. However, it didn't work on the elevator. The lighter picture is actually of the kit wood and not the contest balsa. The problem was glue application. I had some accidents with over application of glue on the contest balsa, so it came out heavier.

[krob]

Good weight savings cutting the fuselage sides out of lighter balsa! I moved the motor peg up about an inch also.

[krob]

AUW is 24 grams. It seems balanced, but I wouldn't be surprised if I needed to add a gram or two to the nose to get it to fly right. I concentrated on making the rear of the plane light, but making the front stronger. No matter what, you always need to add weight to the front, so I tried to beef the front up. For instance, on the leading edge of the wing I used two 1/16" basswood squares laminated together for strength. I also used a basswood main spar even though this is behind the CG. (On my last model, the main spar broke twice; I figured that this is a spot for extra strength.) To lighten the back, I made the side formers from contest balsa, and I moved the motor peg up about an inch.
I'll get pics and vid up when I get it trimmed out. Might take ahwile, cold and snowy out right now.

[Phugoid]

Nice job.

Cold and snowy here too!

[krob]

Thanks, The photo doesn't show the things that didn't work out too well, though. This was my first attempt at taping off a canopy, and I had some bleed through. Also I used decal setting solution for the first time, which certainly makes for a better finish. Unfortunately, I need a little more practice with it, and the krylon sealer seemed to melt the decals a little in some spots. I think it's because I didn't let the decals dry all the way, before spraying.
I tried a few trim flights today, and it's off to a good start. Although, The 1/8" rubber seems a little under powered for the weight. Need to change up the motor. Snow makes for great padding for trim flights! Unfortunately, you only get a couple tosses till things start getting wet.

[Phugoid]

I think 3/16" rubber would be a better bet....

[krob]

I need to order some wider rubber. My Lhs doesn't carry any. In the mean time, I doubled up 3/32". That keeps it up in the air, but still seems a hair underpowered. I'm going to order some 3/16" and 1/4". With the doubled 3/32", I got about a 10 and 15 second flight. A third flight (not on video)was much longer, but only half because it flew off the edge of the cliff at the edge of the field.
I haven't had to add a trim tab yet, which I find interesting. I actually need more of a left turn. Initially, when the motor is at max torque, it starts a left turn, but after a 180 it just flies straight. I built in a little right rudder and put 1/16" washout in the left wing and 1/8" in the right wing, so that hopefully the left wing produces more lift, too shallow up the left turn. With a stronger motor, it might just all work out.

http://s836.photobucket.com/albums/zz284/onekrob/?action=view&current=100_1059.mp4
http://s836.photobucket.com/albums/zz284/onekrob/?action=view&current=100_1056.mp4

[Phugoid]

I think that you need some right thrust, and left rudder. I think the initial left turn is down to prop torque rather than aerodynamics, This will get worse with wider rubber of course.

Also a bit of downthrust would not go amiss, with the wider rubber it might stall under power and it would also calm down the steep initial climb maybe?

[scigs30]

Krob, There are many ways to trim a rubber powered model and once you find a way that works, stick with it. Your model flies pretty good, I would add more rubber. If one strand of 3/16 is not enough power, then go with 2 strands of 1/8th. If your plane is under 30 grams, then one strand of 3/16 should be enough. Also use a 6 inch Peck Prop, that should help a lot. I apply the same amount of washout to both wings to help with stalls and provide a stable flight. I use a trim tab under the left wing so my plane flies in a left circle.

[krob]

Thanks for the info guys. I made a 12" loop of 3/16" out of rubber from the guillows kits, and it seems to be the right amount of power. I couldn't get a long enough flight to see if I should make any trim adjustments. I made up two loops and wound one up till it snapped to see how much I could get, and it was only about 320 winds. The one I stuck in the model I wouldn't wind past 250 to be safe. I'll order some good rubber.

[Phugoid]

I think that's a 6" Peck prop anyhow isn't it? I find It's always harder to trim with a larger prop due to the bigger torque reaction.

FYI krob I'm getting 600 winds on my Trojan which is very similar with 3/16" rubber 1.25 times prop to hook which gives a 20s odd motor run with the standard prop. I'm still waiting for better weather to try that one......

[krob]

yup, it's a 6" inch peck prop. I heard the bigger props slow the motor down, and I need a little more torque. I think when I get the contest rubber, if it won't stay in a left turn, I'll try taking the right rudder out. Any reason you go 1.25 length hook to peg, Phugoid? I've been going 1.5 just cause I read it some where. If I could get 600 winds, I think I could get a descent flight.

[Phugoid]

1.5 times is fine too, 2 or 3 times can be used even! It was to give you an idea of the winds I got with the 3/16" rubber. I think that it is fairly close to making some decent flights. What you might need is more power - not torque.

Your 6" prop will make your motor run longer, deliver more torque but less power, than the smaller one in the kit

This is how I figure it out in my head.....

Imagine the rubber has 600 winds on it, it will have a set amount of energy irrespective of the prop that is attached. Now with the little prop the energy is released in 20s, and with the big prop it is released in say 30s. Since Power is the rate of release of energy the little prop will deliver more power but for a shorter time.

To give your model more power you can therefore decrease the size of the prop, but this gives you less duration, or using the same prop then increase the energy, ie wider rubber.

I think the only way to counteract the prop torque is to use side thrust, this is because its effect stays reasonably in proportion of the prop torque throughout the duration of the motor run ie as the motor power varies. This is unlike say a rudder adjustment which gives you a relatively fixed effect.

So if my guess is that if you put in some right side thrust your model would fly straight and then right as it is set up at the moment, therefore some side thrust and enough left rudder will make it fly left in a slightly widening circle.

Sorry if I've told you anything you already know.....and this is only my opinion/random thoughts

[krob]

Thanks, I immensely appreciate all the info. I've got much to experiment with. My goal is a thirty second flight.