Stalling and Bad Stability

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BalsaFerret
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Stalling and Bad Stability

Post by BalsaFerret » March 20th, 2019, 6:04 pm

Hi all,

Since my state (MA) doesn't allow kits, I'm only allowed to build a handmade plane for the comp. I've currently built planes from similar to FF kit items, although with a shorter wing chord (6.7 cm). However, because I'm relatively new to flying, I still have some trouble trimming and debugging the plane during practice trim sessions.

As of now, with similar trims to my last plane that achieved 2:00, I've been only getting ~:30. The only thing is that I still don't have a good feel for the event and that I need some more intuition to this event. As said in title, my plane is stalling and wobbling, basically anything you wouldn't want for a plane to fly well. As soon as I launch it, it basically immediately stalls, and continues flying, continuing this cycle for the whole flight, similar to a wave like fashion. Alongside that, it sometimes wobbles left to right (roll), causing stability issues in general. I've tried a variety of variable to change, but none of them have had a paramount increase in times; this includes prop, angle incidences, cg, and torque.

I have 2 sets of wings and 1 motor stick right now. The wings have 1/8"-3/32" warp and so do the stabs. However they perform differently, one set flying better than the other. The wings are angled at 3/32"-5/32", but the stab is at an angle of -1/32". The motor stick is 14", with and cg is usually 2.2"-2.5" behind the rear wing post. For propellers I've used mainly 24 cm non-flaring ikara props with an untouched pitch of 30 deg, but occasionally seeing better stability with flaring ikara 24 cm props. It seems that generally the lower the pitch the better it flies because of the faster speeds. My motor is 0.109" 45cm, winding to 125-130ish after break in, backing off to torques of 5.5-7. Torques any lower will not produce enough lift, unless I'm doing something wrong. Rudder is 1/8" left as well. If there is any other data that I need to provide I'll do the best I can to do so.

Lastly, I have a few other questions unrelated to my current planes. I'm wondering if there's a relationship between motor length and winds, whether or not it's linear and if there's a graph I can refer to when I'm cutting new/different lengths of the same width. I've been told by bjt that for my 45 cm .102" safe max winds were 126 and max was 147-ish. Right now, I'm experimenting with thicker rubber, 0.109" to be exact, with the same length, but I'm only ballparking to how many turns I can wind to without snapping (I'm guessing somewhere 110-115 ish with minimal break-in). Secondly, for wing covering, is it normal for the mylar to "curve" downwards between the ribs? I presume the mylar should be flat, but is it bad if they curve in? I've tried to prevent this, but after making the mylar tight on the covering cradle, it seems that I can't make it flat, but rather create some tension and hence curvature in the mylar.

I appreciate all feedback from anyone, thanks!

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Re: Stalling and Bad Stability

Post by CrayolaCrayon » March 21st, 2019, 7:39 am

Your CG might be too far back, or your incidences might be too high (just skimming over what you wrote here).

I'd wait for detailed feedback from BJT.
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Re: Stalling and Bad Stability

Post by coachchuckaahs » March 21st, 2019, 9:13 am

This does sound like a straightforward trim issue. Either too much decalage or too far back cg. Your escalate sounds reasonable, but may be a little high. Try reducing that, or move cg forward. Look very carefully at warps. A warp combined with some wing flex can result in a dynamic increase in decalage.

The second issue sounds like Dutch roll. The ffm kit is sensitive to wing wash in, and too much can result in Dutch roll. Increasing vertical fin area can reduce this sensitivity. Our self design uses no wash in. If your motor stick is flexible in torsion, it can add a lot of wash in to the left wing. On our recent LPP build, we actually put flight right wing wash in, which changes to left under load. I have not seen this on SO planes, since MS is usually stout.

A soft motor stick in bending can lead to decalage chambers in flight, but usually this results in diving under load, then stalling on let down.

On the covering, are you wrinkling it? Then place it on a frame using lip balm, and stretch only enough to remove large wrinkles. You do not want tension in the covering that may warp the wing. Some drooping between ribs is normal. If the covering is bending the spars, it is way too tight.

A video may help diagnose

Coach Chuck
Coach, Albuquerque Area Home Schoolers Flying Events
Nationals Results:
2016 C WS 8th place
2018 B WS 2nd place
2018 C Heli Champion
2019 B ELG 3rd place
2019 C WS Champion
AMA Results: 3 AAHS members qualify for US Jr Team in F1D, 4 new youth senior records

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Re: Stalling and Bad Stability

Post by jander14indoor » March 21st, 2019, 10:24 am

BalsaFerret wrote:Hi all,

<SNIP> already addressed stuff

Lastly, I have a few other questions unrelated to my current planes. I'm wondering if there's a relationship between motor length and winds, whether or not it's linear and if there's a graph I can refer to when I'm cutting new/different lengths of the same width. I've been told by bjt that for my 45 cm .102" safe max winds were 126 and max was 147-ish. Right now, I'm experimenting with thicker rubber, 0.109" to be exact, with the same length, but I'm only ballparking to how many turns I can wind to without snapping (I'm guessing somewhere 110-115 ish with minimal break-in). Secondly, for wing covering, is it normal for the mylar to "curve" downwards between the ribs? I presume the mylar should be flat, but is it bad if they curve in? I've tried to prevent this, but after making the mylar tight on the covering cradle, it seems that I can't make it flat, but rather create some tension and hence curvature in the mylar.

I appreciate all feedback from anyone, thanks!
Yes, for constant cross section, turns is linear with length to same torque/breaking point.
There are some formulas out there for turns per inch vs rubber cross section, but it is pretty statistical in nature and depends a lot on which batch of rubber. Best just to test it. If you aren't winding to torque, just wind a short loop (say 4 inches) to breaking. Winds to breaking (assuming similar break in and usage) is then linear with length.

As already stated, some drooping/flapping in the mylar is normal. In flight, it will balloon upward to more closely match the curve of your ribs. And either way, it isn't real critical, more important is to have it wrinkled and loose enough that it doesn't distort the wing frame.

Jeff Anderson
Livonia, MI

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Re: Stalling and Bad Stability

Post by bjt4888 » March 21st, 2019, 3:00 pm

BalsaFerret,

Good job supplying details!

Coach Chuck and Jeff Anderson have given excellent advice for determining your airplanes issue and resolving.

One additional question. Although very low pitch propeller will fly with lots of power, you may have noticed from other comments on this wiki that the recommended pitch/diameter ratio for best duration is probably in the range of 1.4 to 1.7. The 30 degrees you mention, if measured at the 2.25” radius that the FF tool is set to, would be a ratio of 0.87. Very, very low. Best for your airplane would need to be determined by testing. Also, of course, change in pitch would mean testing different rubber densities and lengths to match.

Sorry for the late entry into this discussion. Finished our Regional on Saturday and have four WS teams and two ELG teams that are prepping for States in five weeks.

Brian T

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Re: Stalling and Bad Stability

Post by BalsaFerret » March 21st, 2019, 7:38 pm

coachchuckaahs wrote:This does sound like a straightforward trim issue. Either too much decalage or too far back cg. Your escalate sounds reasonable, but may be a little high. Try reducing that, or move cg forward. Look very carefully at warps. A warp combined with some wing flex can result in a dynamic increase in decalage.

The second issue sounds like Dutch roll. The ffm kit is sensitive to wing wash in, and too much can result in Dutch roll. Increasing vertical fin area can reduce this sensitivity. Our self design uses no wash in. If your motor stick is flexible in torsion, it can add a lot of wash in to the left wing. On our recent LPP build, we actually put flight right wing wash in, which changes to left under load. I have not seen this on SO planes, since MS is usually stout.

A soft motor stick in bending can lead to decalage chambers in flight, but usually this results in diving under load, then stalling on let down.

On the covering, are you wrinkling it? Then place it on a frame using lip balm, and stretch only enough to remove large wrinkles. You do not want tension in the covering that may warp the wing. Some drooping between ribs is normal. If the covering is bending the spars, it is way too tight.

A video may help diagnose

Coach Chuck
Thanks for the advice, I think my issue is pretty much dutch roll, and you were basically right on the washin for the wing and the stab. I reduced both warps and added on a piece of 1/64 balsa onto the rudder. It's reduced the rolling a lot, but it is still doing so seldomly, so I'm not sure if I should retouch the warps again or is it something else?

I am wrinkling the covering to a very fine consistency. I just wanted to know if any drooping of the covering is detrimental in a major fashion, but as said, it shouldn't be that big of a deal. I am, though, making the mylar completely taught when covering, so that the covering next to the ribs is a bit stretched. However, it doesn't warp the wing in any sort or bending the spars. I might try to make it less tighter so that not much tension would take place.

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Re: Stalling and Bad Stability

Post by BalsaFerret » March 21st, 2019, 7:44 pm

jander14indoor wrote:
BalsaFerret wrote:Hi all,

<SNIP> already addressed stuff

Lastly, I have a few other questions unrelated to my current planes. I'm wondering if there's a relationship between motor length and winds, whether or not it's linear and if there's a graph I can refer to when I'm cutting new/different lengths of the same width. I've been told by bjt that for my 45 cm .102" safe max winds were 126 and max was 147-ish. Right now, I'm experimenting with thicker rubber, 0.109" to be exact, with the same length, but I'm only ballparking to how many turns I can wind to without snapping (I'm guessing somewhere 110-115 ish with minimal break-in). Secondly, for wing covering, is it normal for the mylar to "curve" downwards between the ribs? I presume the mylar should be flat, but is it bad if they curve in? I've tried to prevent this, but after making the mylar tight on the covering cradle, it seems that I can't make it flat, but rather create some tension and hence curvature in the mylar.

I appreciate all feedback from anyone, thanks!
Yes, for constant cross section, turns is linear with length to same torque/breaking point.
There are some formulas out there for turns per inch vs rubber cross section, but it is pretty statistical in nature and depends a lot on which batch of rubber. Best just to test it. If you aren't winding to torque, just wind a short loop (say 4 inches) to breaking. Winds to breaking (assuming similar break in and usage) is then linear with length.

As already stated, some drooping/flapping in the mylar is normal. In flight, it will balloon upward to more closely match the curve of your ribs. And either way, it isn't real critical, more important is to have it wrinkled and loose enough that it doesn't distort the wing frame.

Jeff Anderson
Livonia, MI
Nice, thanks for informing. I agree that testing out the rubber is the best way to go; is there a best way to test a loop of rubber properly, say stretching out to 4x or even more, but do you walk in because it's unknown how many total winds the rubber can take before testing it first, or do you stand stretched at the same length for the whole time?

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Re: Stalling and Bad Stability

Post by BalsaFerret » March 21st, 2019, 8:28 pm

bjt4888 wrote:BalsaFerret,

Good job supplying details!

Coach Chuck and Jeff Anderson have given excellent advice for determining your airplanes issue and resolving.

One additional question. Although very low pitch propeller will fly with lots of power, you may have noticed from other comments on this wiki that the recommended pitch/diameter ratio for best duration is probably in the range of 1.4 to 1.7. The 30 degrees you mention, if measured at the 2.25” radius that the FF tool is set to, would be a ratio of 0.87. Very, very low. Best for your airplane would need to be determined by testing. Also, of course, change in pitch would mean testing different rubber densities and lengths to match.

Sorry for the late entry into this discussion. Finished our Regional on Saturday and have four WS teams and two ELG teams that are prepping for States in five weeks.

Brian T
I haven't heard of this idea of pitch/diameter ratio before, but it seems like something I could test into and find optiumums out of it. Thanks for introducing it to me.

Sorry to continue along with my trim, but today after resolving the dutch roll and testing a few trims, some of them were very inconsistent. For those flights, right after I released it, the plane flew for 1 sec and just stalled and as it's angle of attack approached vertical. This prevented any lift from being generated, and the plane basically fell tail straight down. With the same 45cm .109", 0.55 torque, 1/8" wing and -1/32" stab, non-flaring 30 pitch prop, I'm not competely sure what's causing this to happen. If it does recover at all, it will fly somewhat fine but will stall occasionally. Is it due to the great amount of energy being released in the first few seconds of the flight, or is something else causing this major stalling to occur? Otherwise, if this doesn't happen, it flies decently with a moderate lift and descent.

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Re: Stalling and Bad Stability

Post by coachchuckaahs » March 21st, 2019, 9:43 pm

While these tandem planes do not seem to exhibit normal stall on letdown as the cg is moved back, we have seen early on a tail slide or two. We moved the cg forward a bit (cm or so) and it resolved. There really was not any other indication. Often this would occur after a ceiling hit, but sometimes more spontaneous. Moving CG fixed it for us, and we have not explored further. Could still be a combination of CG and incidence.

Try moving CG more or adjust incidence a little.

When we had Dutch roll, we resolved with a little scotch tape. Our design for regional and state had plenty of fin area and has no tendency to Dutch roll

Coach Chuck
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Nationals Results:
2016 C WS 8th place
2018 B WS 2nd place
2018 C Heli Champion
2019 B ELG 3rd place
2019 C WS Champion
AMA Results: 3 AAHS members qualify for US Jr Team in F1D, 4 new youth senior records

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Re: Stalling and Bad Stability

Post by fifty_missions » March 24th, 2019, 7:48 pm

...and testing a few trims, some of them were very inconsistent. For those flights, right after I released it, the plane flew for 1 sec and just stalled and as it's angle of attack approached vertical. This prevented any lift from being generated, and the plane basically fell tail straight down. With the same 45cm .109", 0.55 torque, 1/8" wing and -1/32" stab, non-flaring 30 pitch prop, I'm not completely sure what's causing this to happen. If it does recover at all, it will fly somewhat fine but will stall occasionally. Is it due to the great amount of energy being released in the first few seconds of the flight, or is something else causing this major stalling to occur?
I suspect you have the greatest hurdle to overcome... The infinitely changing variable

In Wright Stuff almost one hundred percent of this is from using torsionally weak motorstick. This means the motorstick bows or twists under torque load. These will both change the aerodynamics of the airplane while in flight-
1) If the motor stick bows, it will point the nose down and force the stabilizer into positive incidence (trailing edge low). This will cause a dive until the tension of the rubber motor relaxes.

2) If the motor stick twists, then the angle of incidence on both wings is constantly changing through the flight hence the infinitely changing variable.

Build a very strong motorstick. Save mass elsewhere. BUT never use basswood- BOWS EXCESSIVELY

BTW, your CG change during the flight.... most likely due to having the motorhook too far aft. Motors unwind from the front while the heavier knots move aft. This can change the CG if too close to the tail. Move your hook about 1" forward.

Good Luck,
50 Missions

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