Flight B/C

[poonda]

(division b, scratch build)
kind of a weird "issue" with trimming:

i'm flying with a slightly aft cg (1cm behind TE) and the plane is behaving pretty well except it starts climbing at literally 50 winds.
incidence and shimming are super conservative, glide looks good, 24cm ikara with 3/32 motor, 8.1g, and for some reason it climbs like 10 feet off a quick hand wind.

i was just wondering if this was happening to anyone else/if it wasn't optimal behavior?

If you could supply more information about your airplane, I could put your data in a neutral point calculator and answer your question thoroughly.

1. Wing width (chord)
2. Wing span
3. Stabilizer width
4. Stabilizer span
5. Wing weight
6. Propeller weight
7. Weight of fuselage/tail assembly (without propeller or wing)
8. Nose length (nose of motor stick to wing leading edge)
9. Distance from wing trailing edge to stabilizer leading edge

Very quick climb sounds like the CG is too far back (depends upon the info above though).

Brian T

oh sure thank you! i private messaged you

[bjt4888]

https://drive.google.com/file/d/1rV0VU2 ... p=drivesdk

Our performance at invitational today with a time of 1:46, any ways to cross 2 min or more

Cypher,

Good job so far. Do you have flight log data that we can help you analyze (rubber motor length and weight, max torque, max turns, backoff turns, launch torque, flight time, climb height).

Have you read the document I posted and watched the winding video (4th post in this thread)?

Brian T

[Astronomyguy]

I don't have any specific questions for this post but I feel like posting a progress update/venting about our performance at the Princeton Invitational today (I was at the same competition as CypherKat). Any general advice regarding my flights would be welcomed, though.

1. I've been practicing pretty much every day since my last update, and they have more or less been in the 40-80 second range. I polished up the imperfections on my plane one day (fixed rips with scotch tape, made a new HS with less loose mylar, etc. and found that it no longer flew well with my previous washin setting, combined with the fact that I did not precalibrate the decalage and CG from before the fix. The next day, I was able to hit an amazing time of 2:26 with the improved plane (CG, decalage, washin fixed) despite the plane being harassed by the HVAC system. I think it could have gone over 3 with a better venue in those settings. On the very next flight, the HVAC blew our plane into the basketball hoop, where it remained for a couple of hours until someone was able to get it down at the end of the school day, but then someone stepped on it and broke the plane in a couple of areas.

2. I fixed it up later that day (our last day before the invitational) but did not get to test the plane until the invitational itself. Using the plane's behavior from a couple of dud flights, I was able to get it up and running, but I did not have much time to practice as the 10-minute official time blocks took up the entire block back to back, with no space in between. In a large ceiling (30 ft) and necessitating a tight turn, I tried to trim my plane so that it would climb well. For my first official flight, the circle ended up shifting and the plane crashed and broke. Using CA I repaired it within 2 minutes and got a second official flight in. I moved the CG back and it had a better climb, but the circle shifted and it crashed again. Both times were around 1:10/1:20, but the second one definitely could have neared 2 minutes if not over it had it not crashed.

Here are a couple of things I noticed:

1. One of our rubber bands (Apparently, our only near-max mass one with a proper knot, bad decision to use and mix up my rubber previously) broke after I tied and winded it. I don't think this was a lube problem, it was one strand that repeatedly broke free from the knot or snapped (not sure which). I had to borrow rubber from our A team and constructed a good rubber band from .094 weighing 1.95. I wound and unwound it twice to break into it (is this the right thing to do?) and that was how our official flight was saved.

2. Our plane couldn't climb more than a couple of feet above where it was thrown. I think this is just a CG/decalage not being optimized issue, as well as torque not being optimized (we finally got our torque meter assembled but couldn't get it to work)

3. Circle shifting. Really is a sucker; the plane usually settles after a circle from launch and it remains stable until it suddenly shifts by 2 ft + in one circle, then crashes into something the next few. The plane seems unaffected during these shifts. I think I could start with a smaller circle, but the circle always gets wider as the torque decreases. How can I keep the circle constant during the cruise/descent? HS tilt is the obvious solution but it cannot be readily adjusted in competition. Would adding some clay on the inside edge work to lessen the circle by inducing roll? Plane stability could also be increased but because it would involve moving the CG more backward than optimal and lessening the decalage, the plane would fail to make use of the monster 30 ft it has. Preferably the plane should be able to climb to the ceiling and start a slow descent in a tight, unmoving circle. How would I trim for this?

I'm pretty angry because I knew that our plane was better, and we were just one place away from placing. Adding a minute (or maybe even 90 seconds) to our flight time wouldn't have been impossible if our luck had been better (plane wasn't broken the day before, more time to test, and rubber bands cooperating). But this happens, and it wasn't the worst of all outcomes so I'm satisfied. Just a bummer that this was the last invitational and we didn't get the gold.

Flight Log is now privated and sent to the coaches (times nearing 2:30 are too valuable to share the trimming for!!)
Flight Videos: https://drive.google.com/drive/folders/ ... share_link
(Best flight at the invitational was not recorded but it was basically the first one except it climbed higher and crashed higher, in a different spot)
Note that the 2:26 in the school gym also had a significant circle shift. We got extremely lucky that it did not crash into anything, most flights of that caliber in that venue aren't as lucky.

We also had another flight that was probably even better than the 2:26 - it was a 1:27 that clipped the ceiling of the gym before losing all of its altitude, somehow avoiding all obstacles along the way. This was unrecorded, and yet again circle shift was the culprit of the altitude loss.

[bjt4888]

I don't have any specific questions for this post but I feel like posting a progress update/venting about our performance at the Princeton Invitational today (I was at the same competition as CypherKat). Any general advice regarding my flights would be welcomed, though.

1. I've been practicing pretty much every day since my last update, and they have more or less been in the 40-80 second range. I polished up the imperfections on my plane one day (fixed rips with scotch tape, made a new HS with less loose mylar, etc. and found that it no longer flew well with my previous washin setting, combined with the fact that I did not precalibrate the decalage and CG from before the fix. The next day, I was able to hit an amazing time of 2:26 with the improved plane (CG, decalage, washin fixed) despite the plane being harassed by the HVAC system. I think it could have gone over 3 with a better venue in those settings. On the very next flight, the HVAC blew our plane into the basketball hoop, where it remained for a couple of hours until someone was able to get it down at the end of the school day, but then someone stepped on it and broke the plane in a couple of areas.

2. I fixed it up later that day (our last day before the invitational) but did not get to test the plane until the invitational itself. Using the plane's behavior from a couple of dud flights, I was able to get it up and running, but I did not have much time to practice as the 10-minute official time blocks took up the entire block back to back, with no space in between. In a large ceiling (30 ft) and necessitating a tight turn, I tried to trim my plane so that it would climb well. For my first official flight, the circle ended up shifting and the plane crashed and broke. Using CA I repaired it within 2 minutes and got a second official flight in. I moved the CG back and it had a better climb, but the circle shifted and it crashed again. Both times were around 1:10/1:20, but the second one definitely could have neared 2 minutes if not over it had it not crashed.

Here are a couple of things I noticed:

1. One of our rubber bands (Apparently, our only near-max mass one with a proper knot, bad decision to use and mix up my rubber previously) broke after I tied and winded it. I don't think this was a lube problem, it was one strand that repeatedly broke free from the knot or snapped (not sure which). I had to borrow rubber from our A team and constructed a good rubber band from .094 weighing 1.95. I wound and unwound it twice to break into it (is this the right thing to do?) and that was how our official flight was saved.

2. Our plane couldn't climb more than a couple of feet above where it was thrown. I think this is just a CG/decalage not being optimized issue, as well as torque not being optimized (we finally got our torque meter assembled but couldn't get it to work)

3. Circle shifting. Really is a sucker; the plane usually settles after a circle from launch and it remains stable until it suddenly shifts by 2 ft + in one circle, then crashes into something the next few. The plane seems unaffected during these shifts. I think I could start with a smaller circle, but the circle always gets wider as the torque decreases. How can I keep the circle constant during the cruise/descent? HS tilt is the obvious solution but it cannot be readily adjusted in competition. Would adding some clay on the inside edge work to lessen the circle by inducing roll? Plane stability could also be increased but because it would involve moving the CG more backward than optimal and lessening the decalage, the plane would fail to make use of the monster 30 ft it has. Preferably the plane should be able to climb to the ceiling and start a slow descent in a tight, unmoving circle. How would I trim for this?

I'm pretty angry because I knew that our plane was better, and we were just one place away from placing. Adding a minute (or maybe even 90 seconds) to our flight time wouldn't have been impossible if our luck had been better (plane wasn't broken the day before, more time to test, and rubber bands cooperating). But this happens, and it wasn't the worst of all outcomes so I'm satisfied. Just a bummer that this was the last invitational and we didn't get the gold.

Flight Log is now privated and sent to the coaches (times nearing 2:30 are too valuable to share the trimming for!!)
Flight Videos: https://drive.google.com/drive/folders/ ... share_link
(Best flight at the invitational was not recorded but it was basically the first one except it climbed higher and crashed higher, in a different spot)
Note that the 2:26 in the school gym also had a significant circle shift. We got extremely lucky that it did not crash into anything, most flights of that caliber in that venue aren't as lucky.

We also had another flight that was probably even better than the 2:26 - it was a 1:27 that clipped the ceiling of the gym before losing all of its altitude, somehow avoiding all obstacles along the way. This was unrecorded, and yet again circle shift was the culprit of the altitude loss.

Astronomy,

Quite a tale of perseverance. Good job repairing and getting pretty good competition flights!

Circle shifting over the course of the flight is from the ambient airflow (causing gradual drift) in the flying area. My teams will always try to schedule their competition time slot near the end of the day and try to visit the Flight venue to observe for the drift direction and overall effect. In some cases, when drift is a problem, they will decide to deliberately fly a little low (maybe 4-5 ft from the ceiling instead of 1-2 ft) so that they can complete a flight without hitting an obstruction. More than once, this strategy has won the competition for them. Also, their first official flight will almost always be planned for 3-4 ft below obstructions so that drift can be observed on their own airplanes. Then the second official flight they might shift launch location and adjust for more aggressive climb.

I can answer more of your questions later this afternoon unless someone else gets to you first.

Be proud of your diligent work to achieve a very good level in Flight.

Brian T

[boatdziner]

Astronomyguy:
Another reason that you may be seeing a change in circles could be that the circular pathway is mostly due to your prop angle compared to the motor stick. If the prop is providing thrust toward the left, as the torque decreases you will see your circles gradually get larger. This is what we are seeing with our current plane. The first two circles are about 2 meter radius and they gradually open up until the last few have a radius of closer to 8 meters. We have added a slight amount of rudder and decreased the angle of the prop mount to try to fix this issue. As long as the extra basketball goals aren't down in the gym, we have been having good luck.

Happy Flying.
Dan Brabec
Steamboat Springs, CO

[vjshe]

1. my current plane is a stinger kit from J&h aerospace, I was running into issues with climb rate at higher torque and wanted to make a flaring ikra propeller, I was unable to find any pictures/diagrams on how to cut props when I was browsing through the forums (The picture on page 6 of the 2015 design forum does not show up) could anyone provide me with a diagram/picture of how to cut a standard 24 cm ikra propeller.

2. When I tried to use the rubber max turns calculator (https://tinyurl.com/3b5cjud8), I got 2441 turns, my rubber bands snapped at around 1000 turns(around the knot area) with a 1:15 winder. I use armorall as lubricant, one O ring, and break in rubber bands by stretching them out and holding for 5 minutes.

3. My plane rises very fast at the beginning of my flights, it sometimes stalls because of this, I believe I need to make flaring propellers to resolve this issue but is there anything else I can do? I get flight times of 2:05 seconds in a 20 foot ceiling

4. I am also unable to find the Bernie Hunt design spreadsheet. I looked online for the yahoo group but was unable to find it. Is there any way I could get a link to the sheet?

[bjt4888]

1. my current plane is a stinger kit from J&h aerospace, I was running into issues with climb rate at higher torque and wanted to make a flaring ikra propeller, I was unable to find any pictures/diagrams on how to cut props when I was browsing through the forums (The picture on page 6 of the 2015 design forum does not show up) could anyone provide me with a diagram/picture of how to cut a standard 24 cm ikra propeller.

2. When I tried to use the rubber max turns calculator (https://tinyurl.com/3b5cjud8), I got 2441 turns, my rubber bands snapped at around 1000 turns(around the knot area) with a 1:15 winder. I use armorall as lubricant, one O ring, and break in rubber bands by stretching them out and holding for 5 minutes.

3. My plane rises very fast at the beginning of my flights, it sometimes stalls because of this, I believe I need to make flaring propellers to resolve this issue but is there anything else I can do? I get flight times of 2:05 seconds in a 20 foot ceiling

4. I am also unable to find the Bernie Hunt design spreadsheet. I looked online for the yahoo group but was unable to find it. Is there any way I could get a link to the sheet?

VJshe,

1. See here for a picture of our modified flaring Ikara. This prop, once tuned for flaring flex and matched to rubber density, is good for 10-15 seconds additional duration.

https://drive.google.com/drive/folders/ ... share_link

2. Look at my fourth post in this thread for a link to an article I wrote for NFFS Digest on Science Olympiad Wright Stuff rubber winding. It includes the formula we use.
3. Rapid initial climb is typical of this year's airplanes. All 18 airplanes that my 18 students built this year climb about 3-5 ft in the first flight circle and then calm down to about 1-2 ft climb per circle after that. Test increased pitch on the symmetrical propeller and possibly lower rubber density to control climb rate.
4. I'll see if I can post a copy of this spreadsheet somewhere on the web. I'm thinking that the Files section of the Indoor Free Flight Facebook group would be a good repository. I'll have to ask the group monitor about this though.

Brian T

[bjt4888]

Astronomyguy:
Another reason that you may be seeing a change in circles could be that the circular pathway is mostly due to your prop angle compared to the motor stick. If the prop is providing thrust toward the left, as the torque decreases you will see your circles gradually get larger. This is what we are seeing with our current plane. The first two circles are about 2 meter radius and they gradually open up until the last few have a radius of closer to 8 meters. We have added a slight amount of rudder and decreased the angle of the prop mount to try to fix this issue. As long as the extra basketball goals aren't down in the gym, we have been having good luck.

Happy Flying.
Dan Brabec
Steamboat Springs, CO

The primary reason for turn circle to get larger over the course of the flight is motor torque getting less from start to end of flight. Motor torque causes the airplane to roll and yaw (turn) left and to pitch slightly down. Counteracting trim settings like left wing washin and left wing offset keep the airplane flying level (ish) and nose up and with reasonable turn circle and minimal to zero roll at the beginning, high torque portion of the flight. Later in the flight, as torque is less, these setting push the circle bigger. Another factor is that rudder offset has more effect during the higher speed early portion of the flight.

Flight circle growing toward the latter portion of the flight is normal and you need to balance the early circle size with the later circle size (using all turn parameters). Propeller pitch and rubber density and launch torque also can have a significant effect on circle size.

BrianT

[coachchuckaahs]

Astronomyguy:
Another reason that you may be seeing a change in circles could be that the circular pathway is mostly due to your prop angle compared to the motor stick. If the prop is providing thrust toward the left, as the torque decreases you will see your circles gradually get larger. This is what we are seeing with our current plane. The first two circles are about 2 meter radius and they gradually open up until the last few have a radius of closer to 8 meters. We have added a slight amount of rudder and decreased the angle of the prop mount to try to fix this issue. As long as the extra basketball goals aren't down in the gym, we have been having good luck.

Happy Flying.
Dan Brabec
Steamboat Springs, CO

Adding just a little to what Coach Brian said. There are many factors that impact circle size. Each factor MAY impact the circle differently at different speeds or torque.

In my team's experience, if the plane is well-trimmed, and no excessive drag issues (such as excessive wash), the following are the key factors, in order of impact on the flight:

1. Left thrust (prop). This impacts the plane at the highest torques. We have found it critical on F1D in the salt mine, where we do not unwind at all. IN low ceiling SO flights the torque at launch is not usually high enough for left thrust to make a large impact.
2. Left rudder offset. This impacts the climb more than the letdown. Adjust offset for the early to mid part of the flight. It does impact the whole flight, but more at higher speed.
3. Tail tilt to the right (as viewed from the rear). This has a greater impact later in the flight, though it impacts the entire flight.

So it appears to me the best place to start is to add a little more tail tilt. This may be more difficult this year with the short tail moments. However, one approach we have used is to wet the rear of the motor stick/tail boom with water on your finger tips just ahead of the stabilizer. Then apply a twist while drying the boom with a heat gun. Just twisting without water can cause breakage, and will relax back out quickly. Adding water without heat will make the plane temporarily tail-heavy, and it does not lock into place until the water is dried out.

Coach Chuck

[coachchuckaahs]

Astronomy:

I was off for the weekend and just catching up.

One point I would like to re-iterate on your notes is the critical importance of adjusting decalage/CG for cruise and letdown, NOT for climb.

The plane spends a lot more of its time in cruise and letdown than it does in climb. While it is tempting to adjust CG/Decalage to improve climb, this is usually detrimental to the rest of the flight. A dramatically wrong CG (tail heavy) will definitely cause issues such as climbing steeply on launch, but this is because the plane is not stable enough, and small things make big impacts. In fact, more often with a marginally stable plane we see diving on launch due to very slight motor stick bending that would normally not impact the flight. This year, with short motor sticks, bending is less likely an issue.

The CG and Decalage then is adjusted for cruise letdown. For a given CG, increase decalage until the plane starts to stall, and then decrease a little bit to just remove stall. I understand that some versions of this year's plane do not exhibit a strongly noticed stall, so you may just need to experiment with decalage and monitor the resulting durations. Our planes, not from a kit, did indeed show a stall when forced.

Move the CG initially for stability. One way (maybe risk) is to intentionally cause a light girder hit and see how it recovers. We have even at times caused a simulated hit with a steering pole. If it dives substantially, your CG is too far back, try it further forward.

Once you are happy with stability and decalage in low power flights, then increase launch torque through fewer unwinds to adjust your climb. If it races or dives while rolling inward, adjust for more wing wash. Changes at this stage, including wing wash, tail tilt, rudder offset, etc. require re-optimizing decalage before continuing.

Now once you are getting near the ceiling, make SMALL changes to the CG, re-optimize decalage, adjust torque to get the altitude, and re-time. Small changes in CG, especially on C division, are having large changes on time due to the short tail moment.


All of this requires detailed logs and lots of flights!

Another note: You mention "increasing plane stability" by moving CG back. This is NOT correct. Moving CG forward and increasing decalage will increase stability and make it recover from disturbances faster. Also, adding clay to a wing tip to adjust circle size is not a good idea. While it may cause the plane to roll in, it is not addressing the root issue, and so you will have several forces working against each other causing more drag.

Coach Chuck