Climbing too fast

_deltaV
Member
Member
Posts: 48
Joined: February 25th, 2015, 5:36 pm
Division: C
State: FL

Climbing too fast

Postby _deltaV » February 26th, 2016, 12:30 pm

Coming to ask advice from the veterans as this is an unusual problem. My gym that I test in is 26' but rafters run along the entire ceiling making the clear airspace about 22'. In addition to this, the basketball hoops hang lower than this, about 20' up and there's a retractable divider curtain that also hangs low and inconveniently bisects my flight path.
A few details on my plane: 5% camber on the ribs, the OA length is a little less than 2' and the surface area for all the lifting surfaces are maxed out. I'm using .08" rubber from Dave Ziegler and the flaring ikara propeller cut to this year's rules. I generally go up to 1200-1300 winds, and have found the launch torque to be about .67 in/oz.

My problem is that the climb is fantastic but doesn't level off when I reach the top. It seems as though i've built something potentially reaching 30' but that has to fly in ~24' ceilings. I've had problems getting planes stuck both during testing and competition. So far what i've tried is decreasing the decalage but the plane starts to stall or doesn't climb, so i've stopped trying that. The only other idea I has was thinner rubber that would burn off torque quicker. .0625" doesn't produce enough torque for sustained periods, so I ordered .07" and .075" from Dave. Do any of you veteran flyers have any ideas if that will work or on how to make the ascent more gradual?
Boca Raton High School
- Helicopters - Microbe Mission
- Chem Lab - Experimental Design

"Remember kids, the only difference between screwing around and science is writing it down." -Adam Savage

jander14indoor
Member
Member
Posts: 1560
Joined: April 30th, 2007, 7:54 am

Re: Climbing too fast

Postby jander14indoor » February 26th, 2016, 2:12 pm

Probably yes, too fat rubber. Are you landing with a lot of turns when you don't get stuck? You shouldn't need .67 in oz to reach the ceiling that low, I'm getting there with more like 0.4 in-oz. Until you get other rubber to try, try winding to 0.67 and back off to say 0.4. Adjust until you just reach the ceiling. DON'T just wind to the lower value, you'll get a LOT less turns into your motor that way. Wind to higher torque and backoff.

Jeff Anderson
Livonia, MI

bjt4888
Member
Member
Posts: 552
Joined: June 16th, 2013, 12:35 pm
Division: C
State: MI

Re: Climbing too fast

Postby bjt4888 » February 27th, 2016, 9:18 am

dfaris,

.08" rubber is actually on the thin side for this year's airplane unless your propeller pitch is quite low (below 12" at the 3" radius").

What is your airplane center of gravity (in inches or mm from the wing trailing edge) and what is your propeller pitch?

Jeff Anderson is right-on that your launch torque is very high. After initial very low power fights (maybe 500 - 750 turns with no backoff turns with .08" rubber) to determine if decalage angle is correct for the particular CG you have selected (initial stalling corrected by wing incidence reduction to remove stalling) the first flights with full windings should be wound to the maximum torque that you believe you want to fly in competition and backed off to a pretty low torque value to avoid hitting the ceiling. A typical wind would be to 1.0 inch ounces or maybe 1.1 (maybe about 1,200 or 1,300 turns). Breaking torque will be around 1.3 or 1.4 or occasionally much higher. So a winding to max torque of 1.1 would be about 85% of breaking torque. You should wind one motor till it breaks (off the airplane, of course) to confirm and get your own data on this as every batch of rubber is slightly different. A typical number of backoff turns for the rubber you are using would be about 180 to 225. This should give you a launch torque of about 0.3 inch ounces. This launch torque should result in a max altitude of about fifteen feet, or so. Subsequent flights should also all be wound to the same maximum torque (1.0 or 1.1) and backoff turns should be reduced each flight (which will increase launch torque) until you find the optimum launch torque for your site (just under the rafters). After completing this cycle of flights, the interesting fine tuning begins.

Brian T.
AMA since 1972 off- and -on

bjt4888
Member
Member
Posts: 552
Joined: June 16th, 2013, 12:35 pm
Division: C
State: MI

Re: Climbing too fast

Postby bjt4888 » February 27th, 2016, 10:20 am

dfaris,

One more thought. The most effective way to reduce climb rate significantly is to increase propeller pitch. One propeller pitch is increased, you will probably find that you can use thicker rubber than you are currently using and gain duration and efficiency. If you study successful plans like Cezar Banks "Leading Edge" and Bill Gowen's design from last year (detailed in last year's Hip Pocket forum) and comments earlier in this wiki about possible successful pitch ranges, you will have a good start.

Brian T.

_deltaV
Member
Member
Posts: 48
Joined: February 25th, 2015, 5:36 pm
Division: C
State: FL

Re: Climbing too fast

Postby _deltaV » February 29th, 2016, 1:47 pm

My propeller pitch is hard to say, but it is whatever the ikara flaring propellers are. If i'm correct, they flare at launch to burn off some of the torque and slow the climb, and then flatten out with less torque and spin faster at lower winds. I did torque measurements on the .081" rubber and found the torque around 1300 winds to be about .67 in/oz. Numbers in the 1300 range seem to be the lucky numbers for us. The rubber I tested varies in thickness as seen by some motors being longer than another. I found the breaking torque to be about .9-1.0 in/oz and found the number of winds taken to break in the high 1600's. Near full torque launches climbed almost vertically, while our launches at 1325-1350 winds produced the slow gradual climb we were looking for. The plane circles 3 times before it reaches the divider curtain at 23'.
Boca Raton High School
- Helicopters - Microbe Mission
- Chem Lab - Experimental Design

"Remember kids, the only difference between screwing around and science is writing it down." -Adam Savage

bjt4888
Member
Member
Posts: 552
Joined: June 16th, 2013, 12:35 pm
Division: C
State: MI

Re: Climbing too fast

Postby bjt4888 » March 1st, 2016, 9:40 am

dfaris,

Good data. I don't see that you are mentioning backoff turns. See Jeff Anderson's mention of this concept in an earlier post on this thread. This is a very important thing to do properly to fly Wright Stuff at a high performance level. Correctly backing off turns to reduce torque for launch will reduce the climb height. The concept of winding to a predetermined maximum torque and then backing off to a launch torque is described in detail in last year's Wright Stuff wiki. Read these sections if you haven't already done so.

In order to match propeller pitch to rubber cross section to get maximum duration it is necessary to measure propeller pitch. Freedom Flight Models sells a pitch gauge. The stock Ikara "flaring" propeller has a pitch that varies between 10.5" and 12.5" as measured at the 3" blade radius. This pitch range will give ok performance with thin rubber like .081". Much better performance will probably be achieved by increasing pitch to 13.5" to 14.5" and using slightly thicker rubber. Pitch measurement is measuring propeller blade angle. So, a 11.78" pitch, measured at the 3" blade radius translates to a 32 degree angle as the circumference of a circle with 3" radius is 3 * 2 * pi = 18.85" and the tangent of a 32 degree angle is 0.625 and 0.625 * 18.85" = 11.78" (theoretical distance that airplane would move forward due to one revolution of the propeller if the propeller were moving through air as if it were a solid; like a screw moving into a piece of wood).

Also, as described (and pictured and plans included to detail) in last year's Wright Stuff wiki, the stock Ikara "flaring" is actually too stiff to properly flare. One way to correct this is to sand the blades thinner as described in last year's wiki.

Have I said "last year's wiki enough"; hint, hint.

Good job. Keep going on the project

Brian T.


Return to “Wright Stuff C”

Who is online

Users browsing this forum: No registered users and 1 guest