Trimming

[Toms_42]

Wasn't 100% sure where to post, but I have hit a wall. I am using a ffm 2015 plane, and can't seem to exceed 3 minutes in the air. I've been having the best luck with thin props and narrow rubber, but it almost always lands with nearly half of the winds on it. Here is a video. It is using a trimmed ikara prop, (not sanded, but cut to be narrower on the trailing edge of the prop), 1800 winds on 0.87 rubber, and launched with about 0.6 in oz of torque.

https://www.youtube.com/watch?v=GuXf0jXH3yc

Does anyone have any tips? The wing is at neutral lift relative to how it started when we finished the kit, the wing is 7cm from the nose, and the cg is about at the rear post. I have about a gram of ballast on the nose to keep it over 8g. (Sorry the video starts late, I had to launch it then walk back to film it. Flight time was 2:55).

[jander14indoor]

If you are really landing with half the turns on the motor you still don't have the prop/rubber combination dialed in.

Not entirely sure where I'd go with that, you have two factors basically, prop pitch and rubber width. Lay out a test matrix testing all combinations of higher vs lower pitch, thinner vs thicker. Not too much. Should give you 4 test points to look at. Test. Pick the best direction and explore further out in that direction.

To ensure this is organized, look up designed experiments, factorial design. Provides rigorous, statistically correct way to handle the data.

Oh, and 3 minutes is pretty good. Its on the edge of what I can get everyone to and to improve I'd have to do a lot of testing myself. Don't let that discourage you though. Dedicated SO flyers regularly beat my good times because I don't spend enough time on the event.

Jeff Anderson
Livonia, MI

[bjt4888]

Toms42,
First off, congratulations on 2:55; very good. What's your ceiling height to the suspended lights? I completely agree with Jeff Anderson that testing rubber thickness and propeller pitch is the way to go. You mention that you are winding to 1,800 turns and launching at 0.6 in oz torque. You don't mention max torque and backoff. Maybe you are already doing this, but if not, it is VERY important for maximum duration. The .087 kit rubber should make a motor of about 17.0 to 17.5 inches (or more if you're using nylon o-rings sliced from tubing). This motor should take about 2,400 turns (or more and at least 1.3 inch ounces of torque before breaking (test this and see for yourself, if you haven't already). A good full 85% full power (safe) winding would be 2,040 turns (136x15) which should get you to about 1.0 inch ounce. Backoff from this torque/turns setting should be about 120 to 150 turns (or so) to about 0.4 inch ounces for the initial test flight of this winding method. This winding method should help (if you are not already doing). Beware though that by winding to a 85% breaking torque and backing off to the same torque you are using now could (most likely) result in a much higher climb and you would be into your ceiling obstructions. When changing to this winding methodology, you will want to back off to a lower torque like .4 and then gradually use less backoff turns each successive flight which will gradually increment up your launch torque to find the safe altitude again.
One more thing; cutting the TE off of the Ikara "flaring" propeller reduces the effective pitch quite a bit (to about 10"). Measure it with the Freedom Flight pitch gauge to see for youself. After cutting, you will need to repitch (twist the spar for each blade near the hub with needle nose pliers per the description in the booklet that comes with the kit). My students are having good success with this propeller repitched to about 13" (measured at the 3" radius station, the prop pitch is not helical and varies over the length of the blade and this is not too critical at this stage of your testing). With the propeller repitched to 12", the .087" rubber will work pretty well. If you repitch to 13" or 14", thicker rubber may work better. My students are using .089" x 16.5" with 13" pitch with pretty good success (consistent 3:05 - 3:10 under a 23 ft ceiling and 3:10 - 3:17 under a 28 ft ceiling). Sanding the blades of this prop thinner per the Chris Goins "Double Trouble" design will get you another 15 seconds or more duration and may require thicker rubber yet.

Good luck and good flying so far,

Brian T.

[bjt4888]

Toms_42,
One more thing to consider; with a nose moment arm of 2.75" and a CG near the rear wing post, your airplane is operating with a static stability margin of about 60% or 65%. This is almost triple what the design really needs and adds drag if a corresponding high decalage angle is used. The airplane will fly pretty good w/ 65% SSM (as evidenced by your video), but will have excessive pitch stability and drag resulting in less than optimal duration. What are the incidence settings for your wing and stabilizer? I am guessing that they are quite high based upon this SSM. The kit design SSM is 8.5%, which is quite low but very efficient and is typical of a long tail moment arm design like this. A more conservative SSM than the kit design (this is what we are using to hopefully improve recovery from ceiling hits) would be between 15% and 25%. This setting can be achieved with a 1.5" nose moment arm and a CG of about 2.25" behind the wing TE . Our 1.2 grams of ballast clay is usually just behind the CG or just in front of the CG or in that range (this is between 2" and 2.5" behind the wing TE). A SSM in the 15% to 25% range will require around 1.5 to 1.8 degrees of decalage (difference between stab incidence and wing incidence). The kit recommended settings of positive 0.65 degrees for the stabilizer and 2.3 degrees positive for the wing gives you 1.66 degrees of decalage. A number of my students Freedom Flight kits have trimmed out with a negative 0.65 degrees stab incidence and between 1.16 and .58 degrees of positive wing incidence (front wing post 1/16" to 1/32" longer than back wing post) and between 20% and 25% SSM. Lower SSM and corresponding lower decalage angle will reduce drag and increase duration, and, as long as the SSM isn't too low, the pitch stability should still be pretty good. Pitch stability has shown to be a bit of a challenge with a long tail moment arm 8.0 gram SO airplane.

Good Luck,
Brian T.

[someusername]

I am having some similar issues as tom, but I have a few more questions than he does (sorry this post is probably going to be very messy)

My best time currently is a 3:36 in about a 30 ft. ceiling using the 2015 FFM with the 8cm wide wing with 1500 winds on a 0.094 thick rubber (with armour all as lubricant) attached to a 24 cm default Ikara prop with a 40 degree pitch. I have tried using thinner rubber on the same prop with a 20 and 30 degree pitch, but I could not get the times to match that of the 0.094 rubber band. My average time using the setup that I stated before is about 2:00 to 2:30, but that is only with 1000 winds.

One major problem I am currently having is I am unable to repeat the 3:36 flight due to my motor stick not being stiff enough to hold the 1500 winds. I don't know how it worked the first time, but it did roll/dive before if started to climb normally (on the 3:36 flight). I have tried attaching carbon fiber to the bottom of the stick to make it stiffer, but that did not work. I am still trying to find the limit of how many winds can be put on before it can't gain any lift at the start. Any ideas on how I could fix this issue?

The second issue I am having is just being uneducated in how trimming a prop works. I see lots of people talking about using trimmed flaring props, but I never understood what they were doing or how it worked. I do have two Ikara flaring props that I bought for me and my partner to try out, but those didn't work as well as the default ones (flaring and default are set at 40 degrees for the pitch). do you think that learning how to trim a flaring prop and then using it would be useful in my situation?

The last issue is on how to correctly wind the rubber. Like the flaring props, I also see a lot of people talking about winding and then unwinding a few turns to get the maximum torque on their launch. How exactly do the back-off winds work and how do they help improve flight times?

[Toms_42]

Thanks for all the help guys! I'll definitely try and keep that in mind next time I can use a gym. Not sure what the height is to the lamps right now, but I'll check. And yeah, I think it is a prop/rubber match issue. Thanks again!

[jander14indoor]

<SNIP stuff I'll let someone else address>
The last issue is on how to correctly wind the rubber. Like the flaring props, I also see a lot of people talking about winding and then unwinding a few turns to get the maximum torque on their launch. How exactly do the back-off winds work and how do they help improve flight times?

You are taking advantage of the hysteresis of the rubber.
Briefly (you can find more detailed info by searching back through my past posts) the curve your rubber follows on winding is different (higher) than on unwinding. You fly on the unwind!!
The following are example numbers, but qualitatively correct. You need to determine the actual numbers for your motor size and batch.
- If you wind up to 0.5 in-oz torque on a given motor it might take 1000 turns.
- If you wind up to near breaking, say 1.0 in oz it might take 1500 turns. If you the unwind it won't take 500 turns to get back to 0.5 in oz. I might only take say 150 turns. - - You now have a motor wound to 0.5 in-oz torque, but with 1350 turns, a 35% gain in fuel in this example.
- Instantly longer flight, probably higher too because you'll have more time at higher torque early in the flight.

I've described an experiment/data collection every team who wishes to perform at high level should do. Make up a series of short, say 4 inch, motors, common width. Wind one to breakage, recording torque and turns at frequent intervals. Take another motor wind to 90% of max and unwind, taking data both ways. Plot all that. Wind another motor to 85% and unwind again recording data and plot it.
From this you'll know the max torque of YOUR rubber. You'll also see the hysteresis loop I mentioned and that the wind & unwind curves are pretty consistent. You'll see that by winding past target and unwinding you'll always end up with more turns.

Frankly, you should do this experiment with every motor width you try to fly with.
Oh, note. The max torque only depends on motor width, so you know the breakage torque of a larger motor. The turns is linear with the motor length. A 4 inch motor that breaks at 1500 turns will break pretty close to 3000 turns for an 8 inch motor. Etc.

Regards,
Jeff Anderson
Livonia, MI

[bjt4888]

someusername wrote:

One major problem I am currently having is I am unable to repeat the 3:36 flight due to my motor stick not being stiff enough to hold the 1500 winds. I don't know how it worked the first time, but it did roll/dive before if started to climb normally (on the 3:36 flight). I have tried attaching carbon fiber to the bottom of the stick to make it stiffer, but that did not work. I am still trying to find the limit of how many winds can be put on before it can't gain any lift at the start. Any ideas on how I could fix this issue?

The second issue I am having is just being uneducated in how trimming a prop works. I see lots of people talking about using trimmed flaring props, but I never understood what they were doing or how it worked. I do have two Ikara flaring props that I bought for me and my partner to try out, but those didn't work as well as the default ones (flaring and default are set at 40 degrees for the pitch). do you think that learning how to trim a flaring prop and then using it would be useful in my situation?

See page 6 of the "Design" thread for a picture of a trimmed Ikara "flaring" propeller. The back portion of the blade is cut off using a sharp exacto knife and a metal straight edge. You could also cut using a good pair of scissors. After trimming, repitching is necessary as the trimming of the back portion of the blade reduces pitch to about 10".

From your description of the 3:36 flight I am deducing that the duration achieved is benefiting from the fuselage flex during the early part of the flight. This is a desirable flying characteristic for low ceiling indoor duration rubber if you can control it. For certain national competition classes, the motor stick is deliberately selected or trimmed/sanded so that it will flex slightly when a fully wound motor is installed. When the motor stick is flexed/bent in this condition it causes the tailboom to angle downward a little from its normal attitude, thereby reducing the stabilizer incidence and overall decalage angle. Also, the wing washin/washout and incidence will usually be changed by motor stick bending. The end result of this will usually be a rapid, highly banked initial power phase with very little or zero climb (or even a loss of altitude) for maybe one or two circles. As the flight progresses and motor torque/tension reduces and the motor stick bending reduces and goes away, the tailboom, stabilizer and wing return to their original positions and the flight attitude begins to look normal with a gradual climb. As you have seen, this type of flight attitude can be very desirable. Reproducing this type of flight requires keeping careful records (good data and notes) of all variables and especially the rubber thickness, length, number of winds and torque value that produce the best result. There is some risk using this flying style. If the launch torque or other critical trim is a little off, the airplane may bank or dip quickly to the floor. I have seen this strategy used successfully where the airplane is launched from well above head height so that it can bank hard and descend for several circles (and a good amount of duration) before it recovers and begins to climb normally.

I would not recommend the extreme version of this flying strategy for SO, but my students are using a subtle version by reducing left wing washin to a minimum so that the airplane banks and climbs a minimal amount for about one circle and then climbs normally.

Good luck and good science,

Brian T.

[DoctaDave]

For some reason during the descent of my flights, the wing swivels back and forth, it looks like it banks to the right and then goes back to a level position, and then swivels back into a bank, and then back level again. It does this a few times before leveling off and resuming normal flight. My plane does this one to two times during the flight and it causes me to lose a bit of altitude. Could this be because the torque from the motor, and then extra lift on the left side of the wing are almost equal and thus constantly switch back and forth as the stronger force?
Also something that may or may not be related is that my plane sometimes acts as if it is stalling during its descent, but the nose doesn't really go up. It just slows down, dives a little (sometimes it dives about a foot and other times it can even dive 5 feet), and then resumes normal flight.
Besides from these problems the descent of my flight is quite slow and very smooth.
Both of these things happen in the majority of my flights and have caused me to get times a lot lower than I really should be getting.

My plane has a very smooth ascent and takes about a minute to reach a ceiling height of around 24', and then cruises for another good minute and a half without any problems. Then the descent gets wonky and gives me inconsistent times.

I am using the FF model with a flaring ikara prop sanded to the specifications from Chris Goin's Double trouble plans. I am also using .091 rubber with 1800 winds, and about 350 dewinds.

Does anyone else have similar issues or know how to solve these problems?

[jander14indoor]

You might be on the margin of stalling, try moving the cg forward a VERY little.
Also sounds sort of like Dutch Roll which has to do with amount of dihedral and vertical stab relationships. My memory is pointing to too much fin or dihedral, but I'm not sure. I'm not sure which way to tell you to move, do a little digging around the web you should find something.

Thanks,

Jeff Anderson
Livonia, MI