Plane Won't Climb Upwards?

[TSOlympian]

We've just finished building our plane and it won't climb upwards when we launch it.

The wing has a chord length of 6 cm, and laid flat it's 46 cm long but there are polyhedral joints in the center rib and at the tips so the horizontally projected span is 40 cm. Each rib is slightly cambered, and the posts are offset by about 1 cm so that the left side is longer than the right to counteract the motor torque. The back post is also angled to the left so there's a wash-in effect though it's not that apparent.

Our horizontal stabilizer is a 25 cm x 5 cm rectangle with cambered ribs, and our vertical stabilizer is just a trapezoid under the plane with a stiffer tab (not paper) near the back to help it turn.

The motorstick isn't really offset from the tailboom, instead they're two parallel overlapping sticks that we've taped together at the moment so they can be adjustable while testing. We were thinking of sanding and gluing them together after we find the right angles and lengths for the fuselage. The washout for the horizontal stabilizer comes from mounting the stabilizer on posts where one is higher than the other. At the moment, there isn't any horizontal tilt to the stabilizer (the left and right chords are even). The wing is also angled so the leading edge is mostly higher than the TE.

We're also using an Ikara 24 cm propeller, both the flared and regular props without any sanding yet. We were going to cut/adjust the propellers after testing just to see how it flies at the moment without any adjustments

The plane won't climb when we put about 300 winds into the motor lubricated with green dish soap. The motor we're using is a 1/8" strip tied in a loop. We also tried launching off of an overhanging auditorium balcony (just to see how it looks coming down) and it does turn, but it just doesn't go up.

We thought a possible issue might be the fact that the wing isn't evenly offset front and back. The back post is further from the center so the entire wing sort of points to the left, but that also shouldn't be affecting the climb, right? We were also wondering if the distance between wing and stabilizer had something to do with it, since they're kind of far apart right now. Also possibly the overlapping motorstick and tailboom might be messing with the location of the CG and adding unnecessary weight? We haven't had a chance to find the CG and calculate the neutral point yet.

Does anyone know why it won't climb?
Thanks in advance.

[DoctaDave]

300 turns is just not enough. You're probably on the very low part of the torque curve. Try 700 and see what happens.

I would advise not to use 1/8" rubber and to trim your prop the next time you test. I don't even use that thick of rubber when I go for the dime bonus. Something like .08-0.087" rubber will probably be adequate.

Try Armor All for lube, your motors will last longer, and it can be bought cheaply at home depot, Target, Walmart etc. Or even better, use 100% silicon oil if you can get your hands on some.

The CG and distance between the stab and wing should not affect whether or not the model will climb, but if your plane is excessively overweight it may have trouble climbing. I still suspect the weight is not an issue, as my plane with the 3 dimes weighs over 15g with the rubber and still will bash into the ceiling.

Try to use a torque meter as well if you can purchase or make one, as that would help a lot in getting consistent flights and allowing us to help you diagnose the issues with your model.

I'm fairly certain that the issue here is caused by the lack of turns you're putting in. 300 is really nothing even on a 1/8" loop. The model probably runs out of turns so quickly that any sort of climb would be over within seconds. If putting more turns doesn't work, I would suggest increasing the wing incidence a little incase your model is under-elevated.

[jander14indoor]

First, agree with DoctaDave, too thick rubber, not winding anywhere NEAR hard enough. You should be using rubber of about 3/32 thick and winding 1000 to 1500 turns on these motors. Rubber motors have a non-linear relation between turns and torque. At low turns like that you disproportionately low on torque, and you need torque to climb.

I will disagree on his comments about weight. He's right, you can get a heavy plane to fly. BUT, its a LOT easier to get a light plane to fly. Make sure your plane weighs close to 7 gm.

Adjustments/trim. You really should start close to what's recommended, you plane is more likely to fly close to desired and you will only need to tweak it to optimize trim. Then you can focus on matching the prop and rubber, your real task.

General comment to help understand why so many adjustments related to turn. These planes fly at a variety of speeds and with a varying range of motor torque. The various things done to help turn work differently at different speeds, some more effective at high speed, some more at low speed. You need ALL of these adjustments in place to maintain a consistent turn radius. Everytime before you launch you need to check that these settings are OK.

- Your wing should be square to the motor stick, small amounts won't hurt much, but I mean small. If you can see significant skew, cut off the offending wing post and fix it.
- Make sure the left wing tip is at a higher angle of attack than the right (washin). It doesn't take much, but without it you will struggle to climb even when you have enough winds. This generally requires you to crack the front spar and reglue it so the left side kicks up about 1/8 of an inch. Once you start flying you will adjust this so the plane flys with only a SLIGHT left bank to level.
- Center of gravity is important, as a starting point for these planes it should be near the rear wing post. Adjust wing and stab angle of attack to fly consistently with the cg to the rear like that. Yes it is different than man rated planes and RC planes. But it really needs to be there so you horizontal stab helps with lift.
- I once designed a plane for WS teams that had and adjustable tail. For an experienced flyer, it worked OK, but was unnecessary. For new teams it just turned out to be a way to fail, causing inconsistent results making it harder to interpret and trim. Don't use tape or a rubber band to hold tail boom position, you are asking for problems. Glue that tail boom to the motor stick with the recommended offset. Its easy enough to change small amounts by warping, larger by cracking and regluing.
- Make sure the horizontal stab is perpendicular to the tail boom and tilted with the left side high (about 3/8 to 1/2 inch).
- Don't trim turn with the vertical stab or a rudder tab. Its TOO powerful and TOO draggy. Make it parallel to the tail boom and leave it alone.
- Most of your turn trimming will be done by adjustments to the tail boom. Angle from motor stick and stab tilt.

As I write this, I'm reminded about something I learned so long ago that I take it for granted. We all build carefully to make sure that the plane is as accurate as possible. However, its impossible (or at least unlikely) that it is built perfectly. More experienced builders will be closer to right more often, but still not perfect. As a result, you HAVE to learn to be comfortable with sometimes major surgery when trimming a new plane. It is NORMAL to cut a component half through, crack it and reglue to add a kink, or adjust a tilt. Become comfortable with this fact.

Jeff Anderson
Livonia, MI

[TSOlympian]

I just tried to fly it with close to 1000 turns on the motor and it still won't climb up...any more suggestions?
I noticed today that the propeller shaft isn't exactly parallel to the motorstick of the plane -- instead it's angled down a bit relative to the fuselage. Is it possible that that's why it's not climbing?

[bjt4888]

TSOlympian,

Please supply the location of the center of gravity with the motor installed (should be about 2" behind the wing trailing edge if a typically configured SO airplane) and all other dimensions (nose moment length (motor stick end to wing leading edge LE)), tail moment arm (wing trailing edge (TE) to stab leading edge), wing incidence amount (amount the front wing post is longer than the back wing post is approximate; measuring from a reference point like the fuselage top edge (if it is a straight line) to the underside of the LE and also same for TE; can also calculate the inverse sin and give us this in degrees)), thrustline offset in degrees, stab tilt, stab incidence angle in degrees.

What does the airplane weigh without the motor.

Trim the propeller to proper size (only takes about 10 minutes) and glue the tailboom to the motor stick so that the stabilizer is at a zero incidence angle relative to a reference line like the fuselage top line (makes for easier decalage calculation) and with about 2 degrees of left turn offset.

I agree with DoctaDave about rubber size. 1/8" is way too thick and is pretty much a waste of time to use for testing. 3/32" is even too thick. Freedom Flight Models supplies custom cut rubber, order sizes between .084" and .087" from them.

Great job getting your airplane built. If you can supply the above, we can give you very specific help. Also, you could take some close-up pictures of the nosebearing/prop shaft wing attach from the side and the tailboom

[TSOlympian]

An accident earlier this week actually destroyed most of the wings/stabilizers we were working on, and since our invitational is this weekend (i.e. tomorrow) we decided to make "emergency" wings out of Styrofoam plates. Our planes are kind of rudimentary and makeshift at the moment, though we're working to redo all the building with balsa etc.

At the moment, our motorstick is actually two 1/8"x1/8" balsa wood sticks glued together. The tailboom is another 1/8"x1/8" stick that overlaps with the motorstick for a bit and they're taped together. The entire fuselage is 53 cm or 21 inches long. While the entire fuse is one straight line, the horizontal stabilizer is angled so that the trailing edge is 1 cm taller than the leading edge. For the wing, the leading edge is about 1.5 cm higher than the trailing edge. The distance from the wing LE to the stab TE is about 42 cm.

The center of gravity is a little in front of the TE of the wing.

I'll work on getting the other measurements/pictures, but are there any glaring problems so far with what I've listed?

[bjt4888]

TsOlympian,

Good job supplying data. Your stab and wing incidence angles are so large you are probably getting an "air brake" effect. One cm stab te incidence is equal to a negative 11.5 degrees and 1.5 cm of wing le incidence is equal to 14.5 degrees positive. The difference between these two angles is called the decalage angle, which you currently have set at 26 degrees. Reset the stab to zero incidence and reset the wing to 0.25 inches of wing le higher than wing te and your airplane should fly (as long as it is near the minimum 7 gram weight).

Send the other data requested and pictures and i should be able to provide more help.

Brian T.

[bjt4888]

TsOlympian,

One more bit of info I forgot to include. A typical decalage angle for this year's SO design rules is between 2.5 and 4 degrees. Your current 26 degrees is about ten times what should be needed. I'm also wondering how you got 1,000 turns into a 1.5 gram loop of 1/8" rubber. A 1/8" loop at 1.41 grams (probable actual rubber weight after subtracting .08g for two rubber o-rings) is only about 8 3/16" long and the turns estimation equation that we use (posted earlier, developed by John Barker), which slightly underestimates breaking turns, this rubber loop should break at about 930 turns.

When you supply the rest of the data, I can give you much better advice.

Good luck today.

Brian T.

[TSOlympian]

Brian T and Jeff Anderson,

Sorry about the late reply. I've been swamped with tests and assessments this week at school.

About the rubber, we're using a 15:1 winder and putting in about 50-70 winds. When were originally testing the motors to see what the breaking point was, we broke an unstretched one at about 76 turns. We've since been stretch winding and lubricating our motors to squeeze in fit these turns more comfortably, though these motors don't stretch very far before they break. We were going to try putting in 80-100 winds tomorrow, with stretch-winding and lubrication. It's very possible though that we've been miscounting winds, so we'll have to do that more carefully. Also possible but less likely is that our winder is actually 10:1. We'll double check on both of those factors.

Good news is we pulled through right before the competition! We managed build two more sets of wings (basla frame and microfilm, not styrofoam) which were both significantly lighter and also raised on wing posts that made the height and angle of incidence for the wing adjustable. We also followed your suggestion and completely reglued the horizontal stabilizer so it was flat on the tailboom, and only angled the wings. These changes also cut the changes down from 12 g to about 7.8 g. Thanks for your help!

Those changes together made our plane climb, although at the moment its a little shaky. It keeps rocking back and forth and side to side while climbing. It also doesn't go very high, only slightly above our heads, before it starts heading back down. Do you know why this might be or what we can do about it? Also if the weight reduction made a big difference in the plane being able to fly, will it become a problem later on when we try to do the dime challenge (and add weight)?

[jander14indoor]

Weight is CRITICAL in this event. Moving from 12 gm to 7.8 is a big first step. Long term, you really need to get to 7.01 to 7.05 gm. Seriously.
Yes, the payload will significantly affect flight time. You will need new trim settings and rubber/prop combos for each payload to minimize the loss. I cannot tell you if the bonus is worth it (I don't know for sure) you'll learn that from data YOU collect.

Now, to improve your current plane until you can build a lighter one. A 7.8 gm plane is capable of 2 minute flights and should reach most ceilings you have access to.

If you are still using that 1/8 inch strip, you won't get max time, but you should certainly get to the ceiling. Try the following, one at a time:
Raise the leading edge small amounts (1 mm at a time).
Add a little washin to your left wing. Your left wing should have a slightly higher angle of attack than your right. Typically done by cracking the leading edge spar just to left of center rib and lift the left tip of the front spar around 1/16 to 1/18 inch. Reglue spar. It will have a kink in it now at that point if you look along the spar with the left leading edge high.
Make sure you are winding your motor HARD. You can use winder turns instead of actual motor turns as long as you are consistent.
Try a different prop, higher pitch, larger blade area.

Long term, once you get it flying to the ceiling, you will get longer flights on thinner rubber (probably). Before you worry about the payload, make sure you can get a 7 gm plane to fly 2 plus minutes. If you can't, you are wasting time on the payload.

Jeff Anderson
Livonia, MI