Designs

[leetx]

jander14indoor:

The plan is recommending the 45 degree angle be at a particular radius to give a particular pitch.

Never flown this plane, but from experience, it should be in range of .070 to .096 width. That's a very wide range I know, but it should be in there somewhere.

For the large ikara prop, I know that a wider rubber is needed. We recently tested with a .093 rubber and got close to 2:30. I was wondering if others have experience using such a big prop with narrower rubber like .090 or .085 to increase flight times.

Thank you.

[blue cobra]

I'm not sure where to go from here. My planes specs are:

12.75cm chord wing
2 dihedral points with a middle portion 24 cm long and the tips angled at 20 degrees
28x10cm stab
6.93 grams
Also I'm using the 8-7/8in Ikara prop with .090 rubber with 90 winds (1350 turns).

It got just under two minutes and I really don't know how to improve. My WS mentor was with me when I flew, and he didn't really know where to go either. All he could suggest was ruby bearings for the propeller.

[jcollier]

I'm not sure where to go from here. My planes specs are:

12.75cm chord wing
2 dihedral points with a middle portion 24 cm long and the tips angled at 20 degrees
28x10cm stab
6.93 grams
Also I'm using the 8-7/8in Ikara prop with .090 rubber with 90 winds (1350 turns).

It got just under two minutes and I really don't know how to improve. My WS mentor was with me when I flew, and he didn't really know where to go either. All he could suggest was ruby bearings for the propeller.

How much have you played with wing incidence or stabilizer incidence to see how it affects the flight and time?

[jander14indoor]

<SNIP>It got just under two minutes and I really don't know how to improve. My WS mentor was with me when I flew, and he didn't really know where to go either. All he could suggest was ruby bearings for the propeller.

The basic plane you have should be capable of more. And two minutes is very respectable.

Now's where it gets hard. Unless you have noticeable friction in your prop hanger (usually indicated by noise) I wouldn't worry about the ruby bearings, never heard of them being used, not necessary to hit three minutes.

You have to start playing with the variables systematically and figure out which ones to tweak to improve. The idea is to maximize energy storage and use (matching prop and rubber) and minimizing drag (look for skewed surfaces, play with wing and stab incidences as already mentioned, play with cg location, open up the turn if there's room, etc.). This is where the flight log becomes invaluable. Make small changes systematically and check for improvements. Keep records so you can go back if things degrade. etc.

As a hint, that rubber size seems a little large for these planes and most IKARA props. Try a much thinner and longer motor that will take many more turns. This is also the range where winding to torque, instead of turns, becomes useful.

Hope that helps some.

Jeff Anderson
Livonia, MI

[blue cobra]

I'll try some .080 rubber with 100 winds and see how it goes. Also, moving the wing forward is equivalent to increasing AoA with respect to flight behavior? So could I decrease AoA and move the wing forward to minimize drag?

I've gotten no gym time from school. That two minutes was at a college one and a half hours away. If I get any more gym time, I'll play with it some more, and report back.

EDIT: How does tail volume affect flight? I calculated my tail volume to be 1.6, which according to the table below is a bit high.

AMA gas models 1.0 to 2.0
Mulvihill rubber 1.5 to 2.2
Wakefield rubber 1.4 to 1.7
Indoor rubber duration 1.0 to 1.5
Hand launched glider .6 to 1.1
Full size 1913 Moraine-Saulnier, Type 'L' .16

According to the formula Tail Volume=(Tail Area/Wing Area)x(Tail Arm/Average Wing Chord) you can decrease tail volume by decreasing tail arm or by decreasing tail area, OR by increasing wing chord/area.

Assuming I want to decrease my tail volume, my best bet would be to decrease tail arm then, right? (Since I can't increase wing area with the laser cut ribs I'm using)

[jander14indoor]

Careful about changing multiple things at once. Unless you understand something called Design of Experiments, you can't correctly sort out which factor gave desired results or maybe undesirable.

Moving the wing fore and aft is NOT equivalent to changing angle of attack. It changes angle of attack, but also other things, possibly giving uninterpretable results. I'd stick with changing just angle of attack if you adjust that.

Bummer on the gym time, does your coach understand the importance of testing in this event? Its what the event is really about, not designing and building (though those ARE important).

Someone else will have to answer about tail volume, though I'd be leary about shortening tail arm. Just from observation, the short planes just don't fly as well. They tend to be VERY sensitive to changes in stab, and unforgiving to ceiling bumps (lose a LOT of height).

Jeff Anderson
Livonia, MI

[carneyf1d]

Instead of decreasing tail lever arm by shortening the tail boom, what about having a slight droop boom?

[Frosti97]

It is amazing we did it this year and won first place in the magsig

[calgoddard]

I don't like drooping tail boom designs. The reason is that the plane is more difficult to transport in a box, unless the tail boom is removable, which is really above the skill set of most middle school builders. I have not seen any evidence that a drooped tail boom will significantly improve performance of a WS plane. I also don't like the plane landing by first dragging its delicate horizontal stabilizer on the ground. A drooping tail boom could be a nightmare if the plane lands on a light fixture and you have to get it off with a balloon or pole. Our team has never been beaten by a plane with a drop down tail boom, so why bother?

[smartkid222]

- for the Leading Edge kit, what is(are) the recommended rubber size(s) for this prop/plane combination?

It comes with three bags of rubber in the kit. I think they are something like .096-.106