High Ceiling Strategy

[Airco2020]

Anyone have advice on how to prepare for a high ceiling at state (80-90 ft) with only low ceiling (20ft) practice space? My current plan is to not back off on winds as much, but not sure what that number should be, and maybe add some length to my motor. Should I be doing anything different on the trim? Slightly increase angle of incidence on the wing to keep climbing? Move wing position forward to increase climb? I'm just not sure how to approach this?

[coachchuckaahs]

Look through prior posts this year about partial motor flying. You can get very close using a fraction of your motor.

Much of our testing is with half Motors, with a stick replacing the rear half of the motor. The stick should with half the full motor weight, and be half the hook to hook length.

We are currently testing limited penny planes for an AMA contest next month. These are slightly larger planes, but with only 3.1g. We initially tested in a gym with 20' ceiling, but the event had 100'. Therefore, we used a 1/5 motor (0.5g instead of 2.5g), and a stick about 7.5" long weighing 2.0g. Then we multiply the height and the duration by 5 to get a good estimate.

This had served us well in past SO events, and again this year.

Coach Chuck

[Polar]

I just finished my state competition with approximately a 60 ft ceiling height. The facility was actually open the day before the actual competition for competitors to test fly, so I did many tests before hand. I did do partial motor tests prior in my high school gym, and scaled the motor when I got to the state ground. However, the results I achieved in the 60 ft ceiling were better than 3 times my 1/3 partial motor tests. I did increase the angle of incidence slightly to get the plane to the ceiling, and I also changed rubber density while maintaining the same length. However, one trend I noticed was that if the angle of incidence is too high, or there is too much torque, the plane will stall. You may be able to wind to a high torque, but you'll want to back off to the max torque that doesn't cause stalling problems. Overall though, partial motor tests do give you a general idea of what to expect when you scale the motor to the full length.

[jander14indoor]

An alternative to backing off (and giving up that energy) in high sites is to give the prop a little down angle to avoid a power stall.

Jeff Anderson
Livonia, MI

[Airco2020]

[quote="Polar"I achieved in the 60 ft ceiling were better than 3 times my 1/3 partial motor tests. I did increase the angle of incidence slightly to get the plane to the ceiling, and I also changed rubber density while maintaining the same length. .[/quote]

When you say your results were 3 times better do you mean time was better or you went higher than expected?

Increase in rubber density?

[Airco2020]

An alternative to backing off (and giving up that energy) in high sites is to give the prop a little down angle to avoid a power stall.

Jeff Anderson
Livonia, MI

Interesting....just the opposite of what I was thinking to achieve greater height! So you're saying that you can leave the launch torque higher by angling the prop down just a little?

[Airco2020]

So I got some practice time and used the partial motor technique. That is a great solution! So many more tests in my 1hr in the gym.

I'm using 1/4 motor and the rest with a stick matching the weight of the motor size I've had the best luck with. I'm not getting the climb I need to reach anywhere near the top of the 90ft state site. I have some more test time this week and I'm looking at some of your suggestions. Increase wing incidence. I don't have a good measure for where it sits now, but my question is how do you decide to keep increasing wing or do you decrease incidence in the stab? I have less incidence in the stab, but there is some. It's at the FF recommendation (FF kit, built according to the directions). Is it just testing, or is there some rules of thumb on the difference etc.

[coachchuckaahs]

Airco:

Good job on exploring partial motor testing. Yes, it also gets more testing in a given time slot! Be sure to measure/characterize the RISE, and not just the ALTITUDE. as it is the rise that should scale. For our State, competition was in a 24' gym, so we looked for about 11-12' rise from a floor-level launch.

Increasing decalage alone, if the plane was properly trimmed, will/should induce a stall (on this year's tandem, this may not be highly apparent, it may be more of a mush in letdown). However, adjusting the CG forward and adding a little decalage to re-trim the plane increases the Static Stability Margin (SSM). Our experience with some past years is that increasing the SSM may improve the torque vs. climb characteristic (i.e., a plane that did not respond well to higher torque did so better when SSM was increased). The expense here is that you increase the drag configuration, which can be costly in cruise and letdown. So, there will be an optimum as evidenced by the stopwatch.

This all assumes you have a wing without unintended warps, and your motor stick is not flexing. A flexing motor stick will add down elevator (negative stab incidence which may reduce or eliminate your climb. We fought this one year, had a plane that would not accept much torque at all (dove). Once we recognized and fixed the motor stick issue, behavior was much better.

Note that I addressed decalage above, and not wing incidence. Decalage can be increased with adding wing incidence or reducing stab incidence. The angular relationship between wing and tail set the low-power flying characteristics. So what is the difference? Lets say you increase the wing AND stab by 1 degree each. Your decalage has not changed. So what is different? You have essentially added 1 degree of down thrust. Such down thrust may be useful in controlling high climb rates at high torque. However, if you have excessive SSM, the source of high climb could be the SSM. All of these factors work together!

On SO planes when we wanted more climb, we have worked on the above areas, focusing on motor stick stiffness, then SSM. We have not strongly played with down thrust. (Note: depending on your prop hanger, high torque motors may also flex the prop hanger and induce excessive down thrust. We use the Harlan hanger, and for high torque like this year, we end up bracing the front of the bearing with a little balsa wedge)

Continue to experiment, take good data.

We are seeing some similar issues in preparing Limited Penny Planes for Arizona next week. Very small changes in decalage are having strong influences on climb rate. These are unintended changes because we are dis-assembling the planes between sessions. I do not see as strong an impact on the heavier SO planes.

Coach Chuck

[Airco2020]

Airco:

However, adjusting the CG forward and adding a little decalage to re-trim the plane increases the Static Stability Margin (SSM). Our experience with some past years is that increasing the SSM may improve the torque vs. climb characteristic (i.e., a plane that did not respond well to higher torque did so better when SSM was increased). The expense here is that you increase the drag configuration, which can be costly in cruise and letdown. So, there will be an optimum as evidenced by the stopwatch.

Coach Chuck

First of all, thanks again for all the advice!!!

What is the best way to move CG forward? I don't want to add any weight I'm already just over. Moving the stab forward?

[coachchuckaahs]

Since you are still at experimenting stage, add some clay to the nose.

Once this is tested, and if it works, then you can worry about getting back down to weight.

Think of the motor stick as a beam in bending (and buckling). It has the highest loads in the middle. So you can sand both height and width at the two ends to reduce weight (or just at the tail end to move CG forward). You could sand both ends a bit, then add clay to get back up to 8g.

This year's plane should be build-able in the 6g range, giving 2g of ballast to move around. Not sure which kit you have. It it is FFM, it has carbon wings, so the big weight issue is in the motor stick. Judicious selection of MS wood, and careful reinforcement can get you a lot.

But focus on the moved CG and increased decalage first, see if it solves your problem. Then go to weight reduction to make up for clay.

Coach Chuck