FF kit being stubborn...

bjt4888
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Re: FF kit being stubborn...

Post by bjt4888 »

Chuck, Jeff and Josh,

PumptatoCat has been communicating with me using the Scioly private message system and I have given her similar recommendations to what you all have noted. Cat has also shared video with me and the airplane is climbing steeply, turning tight and slightly stalling. I also gave her the advice to move the CG forward initially, till she later shared all of her trim measurements. Knowing all her trim measurements now, my advice is different (see below).

I asked her to shift our discussion to the public message board so that other students could learn from the back and forth regarding trimming, so that is the source of Cat's most recent request. Her airplane's trim measurements are as follows:

Rudder Offset (actually "cocked" stabilizer as rudders are on stab tips): 5.76 degrees
Wing Incidence: 4.32 degrees (stab is at zero)
Stabilizer Tilt: 4.71 degrees
CG Location: Approximately 2.0" from TE
Left Wing Washing: about 3/4"
All of the airplanes clay ballast is on the nose (per kit recommendations) and the airplane built to about 7.6 grams without ballast.
Design has zero wing offset (right wing and left wing are the same length)

As I have one of my 14 high school students flying the stock Freedom Flight kit, I have direct experience trimming the airplane and I have asked that she first reset all trim measurements to the kit recommended settings, which are:

Rudder offset: 1-3 degrees (we are using 4.0 degrees currently)
Wing Incidence: 1.15 degrees (really, the rec setting is this low); we are using about 2.0 degrees
Stabilizer Tilt: no recommended setting, just "slight"; we are using 2.0 degrees
CG Location: 2.125" from TE (we are using 1.875")
Left Wing Washin: about 3/4"

From the initial trim measurements we got good results from both moving the CG back 1/8" (small change; big difference in flight character; as is true of Open Penny Plane wide chord short tail moment airplanes; they go from slightly diving to slightly stalling with tiny changes in CG). We also increased wing incidence to 2.0 degrees (front wing post out 3/16" more than rear wing post). And we added a little turn using the rudder (used rudder as my student had glued the stab in place so vigorously that breaking this joint to readjust tilt would have been more difficult; but, probably would have been better to increase tilt instead of rudder)

Also needed relatively low prop pitch (almost stock Ikara symmetrical pitch) to give the horsepower for reasonable climb. In one quick test session and without matching rubber well at all (dead sticking from about 4 ft) we're getting 2:45 at 23 ft climb height (the other design we're flying; with better prop and rubber matching is flying considerably longer flights).

So, hopefully this catches us up on Cat's excellent job of contributing questions to the forum.

Brian T
Last edited by bjt4888 on November 23rd, 2022, 6:39 am, edited 3 times in total.
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Re: FF kit being stubborn...

Post by bjt4888 »

Sorry, I see that I wasn't very clear in one paragraph. When I wrote, "From the initial trim measurements we got good results from...", I am referring to my student with the Freedom Flight kit that I directly coach. This is not what Cat has done with her trim changes as of yet.

Brian T
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Re: FF kit being stubborn...

Post by pumptato-cat »

Sorry, I've had more technical issues and I thought I posted this but I guess something went wrong?
bjt4888, thanks for posting my trim measurements. I will reset to FF default and do some test flights this Friday(am unable to fly today or tomorrow :c)
I would send flight times and updates but the entire stab attachment is messed up, so I need to cut some new pieces from scrap balsa and redo the TB.
I'll PM you a video of my plane if that helps, coachchuckaahs.
Thanks for the replies, and sorry for the late response... I should have a new computer soon, but the current one refuses to turn on occasionally and is unresponsive half the time :(

EDIT: here is a picture of my stabilizer. I'm not sure what I did to get 5 degrees 😅
calculations were: 5.5mm for distance from ruler to left edge of stab, and 4mm from center of stab to ruler, 13.8cm for the horizontal distance between tip and center. calculation was : sin^-1(1.5/138) = 0.6227 (*10 for mm to cm)
I think I somehow confused myself... not sure now whether this tilt is enough or should be increased.
How do you all adjust rudder offset if your stab is glued to the plane? I've tried using DUCO Cement and it refuses to dissolve(I had to tear it off in the end...)
Last edited by pumptato-cat on November 23rd, 2022, 2:16 pm, edited 2 times in total.
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Re: FF kit being stubborn...

Post by bjt4888 »

Cat,

Too bad about the computer issues. Remember to detail how you are measuring your trim adjustments as the picture of your stabilizer seems to indicate a 2.6 degree rudder offset (inverse sine of offset measure of 1/8” divided by the stab chord dimension of 2.75”) and not the 5.76 degrees mentioned in a previous PM.

Brian T
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Re: FF kit being stubborn...

Post by pumptato-cat »

Sorry-somehow forgot to link the picture...
https://drive.google.com/file/d/1p6vSnq ... sp=sharing
The board won't let me edit any messages? Weird..
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Re: FF kit being stubborn...

Post by bjt4888 »

pumptato-cat wrote: November 23rd, 2022, 2:21 pm Sorry-somehow forgot to link the picture...
https://drive.google.com/file/d/1p6vSnq ... sp=sharing
The board won't let me edit any messages? Weird..
Cat,

Very good taking a picture of your rudder offset measurement. Although it seems like it should be possible to measure rudder offset using a long straight edge held up to the stabilizer TE, it's actually pretty difficult to be sure that this straight edge is perpendicular to the fuselage for accurate measure. And, as the angles we are measuring are very small (1 to 3 degrees).

The method I would suggest you use to measure is to use the center rib. This is still not perfectly accurate, but slight changes are easily accomplished and verified. The motor stick is 3/16" thick and the stabilizer center rib is 1/16" thick. So, if the right edge of the front of the center rib is even with one side of the fuselage and the left edge of the back (by the TE) of the center rib is even with the other side of the fuselage, you know that the rib is angled pretty much exactly 1/8" (3/16" minus 1/16"). Think of the 1/8" as the vertical of a triangle and the rib length (2.75") as the hypotenuse. This would give you an angle whose Sin is equal to the opposite over the hypotenuse (1/8" over 2.75" = .04545). This angle can be determined as the inverse Sin of .04545, which is 2.605 degrees. This rudder offset is within the kit recommended setting range of 1 to 3 degrees, so it does not need to be changed. So, based upon the picture you shared of your stabilizer that showed the center rib angle on the fuselage pretty well, I don't believe that your rudder offset is the 5.76 degrees. I believe that it is fine and is 2.6 degrees.

Now, the wing incidence angle can be measured the same way. The recommended setting from the kit instructions is to have the leading edge of the wing be 0.10" "taller" than the trailing edge. So, as the wing is 5" in width (chord), the angle would be the inverse Sin of 0.10 divided by 5, or the inverse Sin of 0.02. Which is a 1.15 degree angle.

As I noted above, my student that is flying the stock Freedom Flight kit needed to pull out his front wingpost slightly and now has a wing incidence setting of 3/16" (LE higher than TE; measured distance of LE from bottom of motor stick and TE from bottom of motor stick and take the difference between these two measures).

As I believe you have increased your left wing washin since your earlier testing. Remember that washin behaves a little like wing incidence. For an airplane that is close to being in trim, if you increase washin, you can decrease wing incidence.

Based upon the pictures you sent earlier, your stabilizer tilt looks to be almost double what you need (you indicated 4.71 degrees and we have found 2.0 degrees to be fine). On this airplane, a large amount of excessive stabilizer tilt will cause the airplane to roll and dive (the roll man not be visible, you might just see a poor climb rate; not climbing because of stabilizer tilt). This may be why you need so much wing incidence (to try to get climb rate back that is lost by excessive stabilizer tilt). Once you correct the stabilizer tilt to 2.0 degrees, you will probably be able to reduce wing incidence to a more normal 2 or 2.5 degrees (depending upon the CG you select) and your climb rate will be fine.

At this point, you should be in good basic trim and can slightly adjust the CG back (from the kit recommended 2.125"in front of the TE) to fine tune the climb rate. If you go too far in this rearward shift of CG, and if you're just a tiny bit too tail heavy, you'll mostly see this at the end of a full flight. Stalls will be most prevalent during the last 20-30 seconds of the flight.

Brian T
Last edited by bjt4888 on November 24th, 2022, 5:14 pm, edited 2 times in total.
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Re: FF kit being stubborn...

Post by jander14indoor »

If you have glued your stab to the tail boom, their are two conventional ways to adjust it.
One, while gently bending the tail boom, hold it close to your mouth and breath on it. The heat and moisture from your breath will make the balsa pliable. Now, hold the bend on your boom and take it away from your mouth. Hold it for a few minutes to dry and cool. This allows SMALL changes. However, with this years div C short planes and short or no tail booms, this isn't as practical.
Second, with a sharp knife, cut halfway through the boom, crack it (don't break fully, you want fibers to be continuous) and reglue while holding the tail in the new desired position. Again, might be problematic with this years div C design.

And if worst comes to worst, as you've already said, cut it off and build a new one. This is why many good teams with top times may start with a kit, but eventually end up with a scratch built plane!

Jeff Anderson
Livonia, MI
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pumptato-cat (November 24th, 2022, 7:59 am)
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Re: FF kit being stubborn...

Post by jander14indoor »

bjt4888 wrote: November 23rd, 2022, 6:34 am Chuck, Jeff and Josh,

PumptatoCat has been communicating with me using the Scioly private message system and I have given her similar recommendations to what you all have noted. Cat has also shared video with me and the airplane is climbing steeply, turning tight and slightly stalling. I also gave her the advice to move the CG forward initially, till she later shared all of her trim measurements. Knowing all her trim measurements now, my advice is different (see below).

I asked her to shift our discussion to the public message board so that other students could learn from the back and forth regarding trimming, so that is the source of Cat's most recent request. Her airplane's trim measurements are as follows:
<SNIP>
Left Wing Washin: about 3/4"

<SNIP>

Brian T
First, thanks to PumptatoCat for being willing to share this detail!

Second, Brian, how are you measuring the washin? 3/4" seems like a lot based on my previous experience, but perhaps you are measuring differently than I do. I typically measure the left front tip up from a plane made by the left rear tip, and right two tips.

Jeff Anderson
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Re: FF kit being stubborn...

Post by pumptato-cat »

I somehow managed to tear the mylar on the stab... Might have to just rebuild at this point. It's a small v-shaped tear near the center rib.
Duco cement just refuses to dissolve with acetone, so I had to cut the stab off. ARGHH.
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Re: FF kit being stubborn...

Post by bjt4888 »

jander14indoor wrote: November 24th, 2022, 6:45 am
bjt4888 wrote: November 23rd, 2022, 6:34 am Chuck, Jeff and Josh,

PumptatoCat has been communicating with me using the Scioly private message system and I have given her similar recommendations to what you all have noted. Cat has also shared video with me and the airplane is climbing steeply, turning tight and slightly stalling. I also gave her the advice to move the CG forward initially, till she later shared all of her trim measurements. Knowing all her trim measurements now, my advice is different (see below).

I asked her to shift our discussion to the public message board so that other students could learn from the back and forth regarding trimming, so that is the source of Cat's most recent request. Her airplane's trim measurements are as follows:
<SNIP>
Left Wing Washin: about 3/4"

<SNIP>

Brian T
First, thanks to PumptatoCat for being willing to share this detail!

Second, Brian, how are you measuring the washin? 3/4" seems like a lot based on my previous experience, but perhaps you are measuring differently than I do. I typically measure the left front tip up from a plane made by the left rear tip, and right two tips.

Jeff Anderson
Livonia, MI
Jeff,

3/4” is a lot of washin. As this years rules created an airplane with marginal roll stability (and marginal pitch stability), this is how much washin is required. The FF kit recommendations are for this much and my teams have 14 Div C airplanes flying great and all have this much washin too. Without this washin amount (and we tested, of course) they attempt to roll and dive and barely climb.

Side note, this washin amount is for the winglet style wing. The tip dihedral prototype wing we tested takes about 1/4”, which is more typical.

Also, the Freedom Flight kit this year doesn’t have a tailboom. Fuselage/tailboom is a single piece of 3/16” x 3/8” x 15.375” wood with the rear rubber hook at the TE of the Stabilizer.

Brian T
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