FF kit being stubborn...

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pumptato-cat
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Re: FF kit being stubborn...

Post by pumptato-cat »

Sorry for another post-the board will not let me edit posts so here:
neutral point is in front of CG-just realized that... not sure how to adjust this but 99% sure that this causes the instability
anything'll fly if you throw it hard enough
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Re: FF kit being stubborn...

Post by bjt4888 »

pumptato-cat wrote: November 26th, 2022, 9:55 am I will post more details later but currently, my plane stubbornly refuses to fly stably... The flight circle is TINY(like 8-10 feet, sometimes less) and it's sometimes stable, sometimes shaky. I have tried decreasing stab tilt by 2-3deg to help the circle widen out, but no change at all. The plane still stalls and drops frequently- I have set everything except incidence to FF kit defaults..(initially tried default incidence but it was stalling badly) moved CG forward by adding ballast on the nose, experimented with pretty much every cg+incidence+washin combo and nothing's helping. Any ideas? 😞
Cat,

It still sounds like it’s tail heavy still. How far forward did you move the balance point when testing. Specific measures would be good. What was the gram weight of the ballast you added to the nose. How long is the nose at this point (measure from LE to end of motor stick). Take pictures of you balancing the airplane with the motor installed and take pictures of the wing incidence with a ruler next to the wing post so that we can see your measurements.

Now that you have trim settings at kit recommendations, here are the steps you could take.

1. With the kit recommended wing incidence of 0.10”, remove the propeller and replace it with clay ballast on the nose of equal weight (wrap clay over the propeller “hanger” to protect it). Also add 1.98 grams of clay ballast to the top of the motor stick at the exact center point between the front hook and the rear hook (this ballasts to mimic the motor. Test glide the airplane gently releasing it very slightly nose angled down so that it glides smoothly from your hand. It’s trickier than it sounds to test glide accurately. If you release with any nose up attitude, it will stall immediately. This is not an airplane trim stall, it’s a bad glide launch stall. Once you’re sure that your test glide launch is smooth and accurate, add clay to the nose till the any airplane trim stall goes away and the airplane glides nice and flat.

Now put the propeller back on and remove the nose ballast that was taking the place of the propeller. Also remove the ballast that was taking the place of the 1.98 gram motor. Leave the nose ballast on that your placed to remove the stall and fly t low power (maybe 50 winder turns total and 0.30 in oz torque.

Brian T
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Re: FF kit being stubborn...

Post by bjt4888 »

Cat,

So, what I’m recommending based on having trimmed a FF kit with my student is to trim CG first.

Is it still rolling with the left wingtip down quite a bit? Is it climbing poorly? Also, very important, is the moveable wing saddle rubberbanded tightly to the motor stick and do the wing posts fit snugly in the red wingpost tubes?

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Re: FF kit being stubborn...

Post by bjt4888 »

pumptato-cat wrote: November 26th, 2022, 10:59 am Sorry for another post-the board will not let me edit posts so here:
neutral point is in front of CG-just realized that... not sure how to adjust this but 99% sure that this causes the instability
Cat,

Because the rules favor the extremely wide chord short span wing and because the fuselage (and therefor the tailmoment arm) are very short, this design will trim with the neutral point in front of the CG. This is not typical, but the airplane configuration is not typical. Our design and the FF kit design we are flying are both flying well with negative 3.3% static stability margin (this means with neutral point 0.165” in front of the CG).

Pitch stability is fine with this strange trim and our airplanes recover quickly from ceiling hits.

Brian T
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Re: FF kit being stubborn...

Post by pumptato-cat »

Yes, the plane is still rolling-I will PM pictures and a video later. (bad lighting right now...) I'm sure the wing saddle fits snugly, along with the red straw tubes, and I really can't think of another reason why the circle might shift. Climb seems all right-- it is quite inconsistent though...
I couldn't really move the balance point forward--no matter how much ballast on the nose I added or how far back I moved the wing, I didn't see any measurable change(except maybe a couple of mm?). The nose is currently 4.2cm from the front of the wing post for the LE.
I'll do some CG adjustment on Tuesday! (can't fly tomorrow)
What should I do with the width of the circle? It's way too small... What other factors adjust circle besides rudder offset and stab tilt? (I can't find anything about this...)
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Re: FF kit being stubborn...

Post by bjt4888 »

pumptato-cat wrote: November 27th, 2022, 6:28 pm Yes, the plane is still rolling-I will PM pictures and a video later. (bad lighting right now...) I'm sure the wing saddle fits snugly, along with the red straw tubes, and I really can't think of another reason why the circle might shift. Climb seems all right-- it is quite inconsistent though...
I couldn't really move the balance point forward--no matter how much ballast on the nose I added or how far back I moved the wing, I didn't see any measurable change(except maybe a couple of mm?). The nose is currently 4.2cm from the front of the wing post for the LE.
I'll do some CG adjustment on Tuesday! (can't fly tomorrow)
What should I do with the width of the circle? It's way too small... What other factors adjust circle besides rudder offset and stab tilt? (I can't find anything about this...)
Cat,

Now that you have stabilizer tilt, wing incidence, rudder offset and left wing washin all set to the kit recommendations, there are only two remaining trim settings to work on and CG is next.

So, move the wing to the kit recommended setting with the LE 6.5 cm back from the fuselage stick nose and go through the unpowered glide tests that I described in my last post. Don't move the wing for these tests, just keep adding clay ballast to the nose till the stall goes away (if it is stalling; and most likely it is). Have someone take video of you doing these glide tests if your parents are ok with this. Video will allow determination of whether a stall is caused by a nose up release or by actual trim issues.

When you do testing like this, take the time to make measurements and report them. For instance, "I have all trim settings per kit recommendations and the nose length is 6.5 cm and I added 0.25 grams of clay to the nose (this is 0.25 grams in addition to the clay already on the nose that is the weight of the propeller; and also, see above, clay on the stick at center of the hook to hook distance to simulate the weight of the rubber motor)" Then glide it and observe. If it's still stalling, add another 0.2 grams of clay to the nose and glide test again. You will definitely reach a point where there is enough ballast on the nose to stop the stalling.

An airplane that is too light in the nose will circle tighter. Once the CG is in the correct place, the circle size should get better. If it doesn't, then there is something possibly misaligned (wing cockeyed as its mounted on the posts, winglets cockeyed, etc.). But proceed with this CG glide testing first.

Also, you didn't mention the rubber winding specs. When the airplane was circling tight and banking what was the typical launch torque, what rubber thickness (density), what max turns, backoff turns, max torque? This also matters and overpowering the airplane to make up for trim that isn't finished will also cause tight turning and excessive banking and poor climb.

Brian T
Last edited by bjt4888 on November 27th, 2022, 7:48 pm, edited 1 time in total.
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Re: FF kit being stubborn...

Post by pumptato-cat »

Okay, I'll report back when I do some tests tomorrow. How much would CG affect the flight circle? Would it account for 15-20ft of circle shift? In general, we wind to around .45oz torque, on .094 rubber, but it doesn't seem to affect flight circles--even when winding to .9oz torque(before I figured out that washin was a more effective way to climb, which reduced torque needed to around .5 for 32') the circle didn't seem to tighten or shift more than usual.
I would PM a video of testing but I am testing myself(my partner is not going, and my parents can't either) I'll see if I can figure out some sort of camera mount, though.
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Re: FF kit being stubborn...

Post by bjt4888 »

pumptato-cat wrote: November 28th, 2022, 6:25 am Okay, I'll report back when I do some tests tomorrow. How much would CG affect the flight circle? Would it account for 15-20ft of circle shift? In general, we wind to around .45oz torque, on .094 rubber, but it doesn't seem to affect flight circles--even when winding to .9oz torque(before I figured out that washin was a more effective way to climb, which reduced torque needed to around .5 for 32') the circle didn't seem to tighten or shift more than usual.
I would PM a video of testing but I am testing myself(my partner is not going, and my parents can't either) I'll see if I can figure out some sort of camera mount, though.
Cat,

Moving the CG forward 1/8" can sometimes increase the circle size 25-50%. You'll notice that I have never commented on circle shift. This is pretty much always due to airflow in your flying site. Airflow is often in different directions depending upon the flying height or the flying location within the site and it's only a concern if you're getting shifted towards obstructions or being pushed down or up.

Keep using numbers in your reporting. I can help you with data analysis best if you give specifics. Instead of relating that you had no performance change regardless of how much you changed a trim, tell me that "I moved added 1 g of clay to the nose, with the wing st 6.5 cm back from the nose and all trim settings per the kit which shifter the CG from 2.125" in front of the TE to 2.1875" in front of the TE and this is what I observed during my glide test.

Be sure to do the glide tests per the method I mentioned earlier and add clay till the stall is almost completely gone.

Brian T
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Re: FF kit being stubborn...

Post by KingDelphinus »

Hello. I am having trouble understanding 6b in the rules. It states: ". A bonus of 10% of the Flight Time will be added to the Flight Score of an aircraft that has the entire surface of the wing between at least 2 ribs or at least one of the wingtip fences or a vertical stabilizer completely marked with black marker or black tissue. If no ribs are present, the whole surface must be black. Aircraft with no wings or vertical stabilizer must have at least one black-colored lifting surface". May someone further explain to me what this means and the purpose of this bonus?
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Re: FF kit being stubborn...

Post by poonda »

this might not be the best forum but uh
color one of your wingtips or "squares" (forgot words) black to add a bonus 10% to your flight time;
i'm honestly not sure why this bonus exists, but hey free points. i guess it makes the plane easier to see, which could be helpful in certain flying sites.
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