Flight B/C

[bjt4888]

Back again after another flight session! It was only one hour so I didn't get to fix much, though.
I'll link videos here when they finish uploading--restricted access to Coach Chuck and Coach Brian as of now.
Questions:
1) How do you tighten up your turn? I added some more stab tilt(2.66deg now) and decreased washin and rudder offset, and am seeing a very large circle that tightens up under high torque but loosens to at least 23+ feet when descending/cruising.
I don't want to add more stab tilt(3+deg seems rather extreme), and more rudder offset will make things worse judging from past adjustments. bjt4888, I did see some of the wing rib skew you mentioned previously, but after reducing washin, it skews to the left. Not sure how to adjust further--I originally thought less wash-in would help but I've had to decrease it to avoid right turn, to the point where it barely climbs. If the CG and decalage settings are within typical range, the :41 second flight video looks both like the washin is too great (swing to the right initially) and too little (inadequate climb). There has to be something misaligned to do this. Am I remembering correctly that the earlier flights with this airplane looked good and the behavior in the video started suddenly and recently? One thought: how soft is the motor stick wood? If you attempt to twist it, can you? Usually when I am inspecting the FF kit motor stick wood, I'll bend, twist and weigh and calculate density. If density is below 4.5 lb cu ft (and sometimes it is), we add carbon fiber reinforcement (but we also don't use the moveable wing saddle, which would get in the way of the wing saddle lying flat on the fuselage side). I prefer to use 5.8 - 6.5 lb cu ft wood as this density is always stiff enough for SO. My one student that built the stock FF kit was getting diving behavior with all other trim in the correct range, we fixed by doing the following: 1. tighten the truss thread till it almost bent the motor stick up. To do this we removed the thread from the post and added 1/16" hard balsa to the post top and sanded to about .040", used a fingernail to make a tiny groove in the hard balsa and pulled the truss thread tightly up onto the new longer post. We then glued with Duco so that we could remove the thread to readjust, if necessary. 2. We increased the tailboom incidence about 1.5 degrees. This will bring the nose up a little and reduce the need for all the decalage to come from the wing incidence angle. I saw Josh recommend this on the FF kit and liked the idea. This worked. If your motor stick is twisting, then under the motor's torque, the twist will increase the washin. Look at washin before winding and installing the motor and after installing the wound motor. Is it different? If so, this is a problem and the motor stick wood is too soft and needs carbon reinforcement.
2) How do you make the plane climb efficiently? On 0.26oz torque the plane climbs to around ~18ft and I feel like that's rather low... It actually might be 15, as I'm eyeballing. I've increased wing incidence to the maximum, along with washin(probably 1/16-1/18" right now)--any more washin and it begans to turn right... no washin and it dives. I'm also seeing a straight line and slight tilt to the right at the beginning of flights, and then a steep climb and turn into a left circle. I remember reading that this shows too much washin, but without washin the plane refuses to climb. Is there anything I can do to get rid of this? See above. As you've seen trimming your other airplane, there is one (pretty much) ideal trim for best duration for each combination of washin, decalage, CG, propeller pitch and rubber density. Once you have this (and nothing is flexing that shouldn't be and nothing is misaligned) climb rate is first adjusted by washin (1-2 mm additional usually will make a difference in climb rate) and is next adjusted by propeller pitch and matching rubber density (more pitch will usually give a slower climb; but, watch out for too much pitch which leads to poor horsepower during let down; the stopwatch tells you this) and by adding more launch torque (increases initial climb rate and overall climb height; launch torque and climb height are close to a linear relationship.
I know what factors adjust these, but it seems like those settings are maxed out. It's just very strange behavior in comparison to previous planes... For 2), I suspect there may be some slight positive incidence to the stabilizer(tailboom joint might be crooked) but I can't confirm this at the moment. I don't think the motorstick is bending--It's relatively short, and I wasn't winding to high torque, either. I do recognize that there is instability at the end of the flight--I'll be fixing that in my next flight session, as tonight's was rather rushed.
Thanks!
Gosh I thought a second plane would be easier, but it seems like this one has even more problems than the previous. Every airplane has it's own character. Even if build lighter looking better, sometimes there is something that is not right (ex. soft motor stick, or tricky to see misalignment).

EDIT: Videos processed! Ordered by duration(so flight 1) here on the forums is the shortest, flight 4) is the longest. Hope this isn't confusing, I'll rename the videos to help too)
1) The first flight. Barely stayed up--diameter is very large(See previous videos for size reference/estimation)
2) Added more power-0.2oz torque. Barely got off the floor, this was with new rubber too...
3) Ends by crashing into a box. No clue what torque was(I should probably start logging trimming flights..), but it's circling in an oval?? Very odd. Could be exaggerated circle shift I suppose? I'll check my joints again, just in case.
4) 0.26oz torque. Compared to my old plane(overweight by like 0.3g if I recall correctly...), it climbs less. Not sure how to fix this, and I'm afraid that the straight line it does in the beginning will be amplified when wound to higher torque, as I'm almost certain my states venue has a 25ft+ ceiling. I can't add more wash-in... If I add more incidence a) it gets to be rather extreme and b) it will cause the wing post to wing joint to snap. 1:41 at 17 ft. is good, but if added torque leads to no climb or right shift, then there is still something to solve. I am suspecting either a loose joint, wing skew (skew to the right is ok as this increases left turn; see Bill Gowen's Carbon Penny design with deliberate right skew; skew to the left is not good as this causes right turn. Is skew changing when the motor is wound and loaded on the airplane? The longer flight video is still loading and I can't download to view as my PC will not scan this video format for viruses. Probably it will be loaded later tonight.

Yikes. Huge improvement, so I'm pretty excited about that--it turns somewhat proper now, but the diameter is quite an issue...
https://drive.google.com/drive/u/0/fold ... 7Z3l76-IFM If you cannot access this, please let me know! Thanks again everyone

Cat,

Good questions and observations. See my comments in bold above.

Brian T

[bjt4888]

I went back to build logs and am correcting the typical motor stick density I indicated above. We usually look for 5.2-5.5 lb cu ft not 5.8-6.5 as I mentioned in the last post. This density will build light enough and should also have adequate stiffness for typical SO motors.

Brian T

[bjt4888]

Cat,

I viewed your longer video and the flight looks pretty good. Can you move the CG back slightly (50-70 mg clay from the nose to the current CG point), which might make it stall and require 1 mm less wing incidence.

This would likely make the circle a little smaller.

Slight negative incidence on the tailboom (we use 1 degree, which is 1.5 mm tailboom angled up at the tip if the tailboom is 4" long) also helps so that the wing incidence can be reduced and the airplane flies a little nose up.

Do both of the above.

Brian T

[coachchuckaahs]

Cat:

On brief comment. You note that late flight instability needs to be worked out later.

It it IMPORTANT to get cruise/letdown correct BEFORE working on climb. You do not adjust climb with incidence generally, as such adjustments affect cruise/letdown, which is generally the longer part of your flight.

That said, it appears in your last video, at least to me, that the plane is flying a bit "flat" and fast. You will be amazed how much getting the nose up a little will affect the time, as it slows down the prop. This may be at the expense of stability though.

Two things will affect the attitude: Decalage and CG. It sounds like you already have a lot of decalage. If more than 5mm (difference between stab and wing incidence), I would start by moving CG back until you pick up a stall at slow speed, then move it forward slightly.

A flat/fast plane will also tend to have a larger circle.

In working with a mid school last week (I know, the plane is quite different), initial flight did not circle well. Inspection found the right wing tip loose. Fixed that and circle was great. Something crooked or loose is often the issue with not circling.

Low speed circle is most affected by tail tilt (assuming nothing loose or crooked). Don't be afraid to add more if needed.

Once plane is set up correctly, you may be needing launch torques at 0.4 to 0.7 depending on ceiling height.

Any adjustments made (such as wash) for higher power flight (launch) will require re-addressing basic letdown trim before proceeding with high power.

You mentioned a left skew in the wing. Brian correctly indicates that this is bad and could cause circle issues.

Coach Chuck

[pumptato-cat]

bjt4888,
Yep! The behavior started recently--it used to fly circles perfectly.
I've been checking warp, not skew, after the motor's loaded. I'll definitely check for flex when I get home, though.
Oh, I wish it was 1:41.. the last one was 1:28, because it kept hitting things/stalling during descent. I was also flying on 80 winds(new rubber)...
Thank you so much! I'll try everything mentioned. But, wouldn't decreasing CG cause the plane to become unstable in the "letdown" portion of the flight? I'm already seeing instability but I will definitely try moving CG first. Maybe incidence will fix the wobbling.
I will definitely check wingtips. How much tip is too much? I do think one of my wingtips is very slightly leaning inwards, but I don't think it's enough to cause the current problems.
Also, sorry if this is a bit of a stupid question, but why would left wing skew cause right turn? If that's the case, wouldn't tilting the stabilizer so the left edge is above the right edge(as in rudder offset, not stabilizer tilt) also cause right turn?
I'm currently at school, and I will be flying my old plane today. I'll do some testing with my new plane as soon as I can(likely tomorrow/Friday). Flying every day before states rolls around Again, thank you so much everyone!

[coachchuckaahs]

Cat:

It appeared to me that during cruise and letdown the plane was flying flat/fast. Therefore it either needs more incidence or move the CG back. Either way gets the nose up. Moving CG back reduces stability, while raising wing LE increases drag. If your decalage is already at 5-6mm, I would be moving the CG. Note that this year C div plane is VERY SENSITIVE to CG location, so experimenting with that after getting all the bugs worked out is beneficial.

The wingtip "tip" is not a big issue, but an angle to the side (like rudder) would be bad. We have tip dihedral, so the loose joint acted like wash-in on the right wing.

Skew, or the wing turning to the left, essentially is giving right thrust and right rudder (if you think of the wing as the reference). The wing is what flies, so if the MS is angled to the right relative to the wing you are adding right thrust but perhaps a little left rudder.

Not following you on the stab "tilt". Tilt is when you look from the rear of the plane, and the left tip of the stab is higher than the right tip. The stab is lifting, so it lifts toward the right, pulling the tail to the right, or the nose to the left. Rudder offset is looking from the top of the plane, the left stab tip is further forward than the right, inducing some rudder (left yaw) relative to the wing. If the wing is skewed left relative to the MS, then really the MS is skewed right relative to the wing, adding right thrust.

Coach Chuck

[pumptato-cat]

Thank you so much!!!! I was looking at this completely wrong, it makes a lot more sense now
I think I will try moving CG first--adding a shim for negative stab incidence is rather annoying but I'll definitely try it after. Does adding stabilizer incidence also add drag, just like adding wing incidence?

Also, gosh I'm so happy right now--just got news that last year's EWS/WS was held in a 30ft ceiling. Strong AC, but the ceiling height is almost exactly the same as where I practice in, with the width and length also similar! Hopefully they use the same venue as 2022.

[coachchuckaahs]

I like to talk about "decalage" rather than "incidence". Decalage is the difference in incidence between the wing and the stab. So in that sense adding negative incidence to the stab is very similar to adding positive incidence to the wing. Look at the TOTAL difference, and if this is getting large, then you probably need a CG movement.

Generally, we run 0 on our stab and 3-6mm on the wing. If more than 6mm, I am looking for other issues (CG, warps, etc.).

So, what is the difference between adding positive wing incidence and negative stab incidence? Thrustline. Lets say you have 2 degrees of wing incidence, and 1 degree of negative stab incidence. Now move it all to the wing so that you have 3 degrees wing incidence, 0 on the stab. The Decalage stays the same. However, relative to the wing/stab combo, you have now rotated the front of the MS down 1 degree, or added 1 degree of down thrust.

SO, my general starting point results in a few degrees of downthrust, which can help keep the climb under control. The thurstline will only significantly affect the flight path at the highest torques, and so flying F1D in the salt mines (no backoff turns) the thrustline becomes critical. A lot of thrust angle can also hurt efficiency in cruise.

But, if you find yourself getting well beyond 6mm of decalage (3 degrees or so), in MANY CASES it is time to look at CG. At least in my experience with this weight class, LPP (3.1g) and even F1D (1.4g). I don't know if it is universal, but it has served me well as a starting point.

The Mid School I am working with set up the wing at 4mm (2 degrees), stab at 0, and CG for near neutral SSM, and the plane flew with NO ADJUSTMENTS. Stab tilt is about 1cm from center to tip (3.5 degrees). We have a single center fin and it was offset about 6mm (3.5 degrees). We then jumped right in to optimizing rubber/prop combinations. We also took time BEFORE gym time to look over the entire plane for warps, twists, etc. The stab is perfectly flat (LE and TE parallel), the wing is mounted straight, etc. etc. The covering is applied snug but not tight using Coach Brian's methods, and trimmed to the spars (no flappy bits hanging off). Using the Glue Management techniques in my video, the students built to 5.56g, whereas their prior effort was 19g! Most, if not all, of the difference was glue!

Sorry to get off track. Bottom line, if decalage is out of hand, go to CG. Trim increasingly until you see stall, then back off. (Note: Some have reported that the Div C plane does not clearly stall. In this case, trim until the nose is too high and the plane "mushes" then back off a tad). You should clearly see the nose slightly up. Play in this range and the stopwatch will tell you if you have gone too far.

Coach Chuck

[gz839918]

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Hey there,

My name is Justin and I've been following this forum for the last couple months and it's really an awesome environment.
A little about my flight: Stinger 2023: 
Old plane was in trim but wear and tear led to propellor cracks and other such things. It was overweight (10.8 grams) but still scored ~1:30 at comps.

Info: ~1000 turns, mild AC, 25 ft ceiling, shim 3/32 inch

New flight: I bought (another) JandH stinger kit and I built it as light as I could but the weight still came out to 9.1 grams (with ikara prop assembly) + super ultra mylar

Current performance: (this is gonna get a little confusing)
New fuselage + new wings: < 30 seconds
Currently, it is stalling really bad at like 2 seconds and then diving down. We tried cracking and re-gluing the nose to get down thrust and turn (2 degrees left + 2 degrees down) like our other planes, but it made no difference. We did stab tilt (maybe 15-20 degrees) and it boosted the performance slightly (30 seconds) but still had the gist of stalling and diving.
Visualization of our flight: It takes off, climbing (pretty well) for 2 SECONDS!!! Then it slows down A LOT and front wing height increases over back wing A LOT, then it falls. If it doesn't hit the floor the first time, it will continue swooping for 10 seconds.

Old fuselage + new wings: ~1 min
When we tried with our pre-trimmed fuselage from the old kit, (which is overweight + prop cracked in every direction possible), and also added the stab tilt (15-20 degrees) and it did a little better. 
However, I really don't want to be using my old fuselage and optimizing it because I know that we can do a lot better with a new fuselage and prop. 

A couple quick questions: 

1. Will increasing the height of the front wing (leading and trialing edge) by 0.5 cm have a profound effect? When assembling the plane, I made the front wing height higher for experimental reasons and can't figure out if this is making it stall.
2. Should I apply washin or washout to make it stop stalling? I felt like by making the leading edge lower than the trialing edge should make it not lift so much and hopefully control the plane.
3. I have tried using no shim as well, and still stalls and dives repeatedly, so if you know how to fix this, please let me know.
4. Should I try increasing the length of the front wing to maximize performance?
5. Should I sand down the fuselage a little to try evening out the weight?

I have another plane left in the kit, so if there is anything I should do differently when I built it next, please let me know. 

I would appreciate any help, thanks so much!

 - Justin

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Throughout the season I've observed that some teams launch their plane 1ft off the ground and let it climb up. These teams tend to get a longer flight time, which is logical. 

I would like to do this as well, but am having a hard time getting it to climb as soon as I let it go. 

Also, whenever I wind it up 900+ turns, the plane, as soon as I let it go, zooms really fast and starts going incredibly nose high to the point where the plane is vertical. Then it falls 3ish ft before starting to fly pretty normally again (but this fall is not ideal). Reading the previous posts of this thread, I undertand that the rubber motor should generally have around 1200+ winds. How do I reach this amount without the plane traveling incredibly fast? When I wind it up with only 800 turns, it seems to do fine and is pretty stable (but has trouble climbing up and using the whole space). 

Any other advice would be appreciated. I will try to provide photos and videos soon.

Thanks!

Hey folks!

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[pumptato-cat]

Hey gz839918,
I have already responded to epicSus's post. I think that got buried, too...

Hey coachchuckaahs,
Ohh, I see. I think the FF kit has some down and left thrust already built in. Incidence is currently at ~5mm for the wing, and 0mm for the stabilizer(although I might have glued the tailboom at an angle. I looked carefully and used a ruler, and there's a slight gap, but I'm not sure if it's my imagination or not...).
There is some left skew. I originally thought left skew turned the plane left... It's very slight, and I might be seeing it wrong(1-2mm?) but I'll attach pictures when I get the chance to. I'll mess around and see if I can get it to warp, with no skew...
I'll add negative incidence to the stabilizer and go from there, thanks! CG is definitely off by quite a bit, and I'll take you and Coach Brian's advice and move CG back. Although I'm still pretty worried about the end of the flight(The stalling is back) I'll just fly and see how it goes.