Adjusting prop pitch

[BalsaFerret]

Hi all, I've built a ffm plane that flies for ~2:00 with 90cm .102" rubber. It flies relatively fast (6 mph?) with ~7 in oz torque with a stock (untouched pitch) ikara 24 cm prop, and ends with 40% winds at landing. I figure that I should increase the pitch as I think that my pitch prop combination is not the most efficient.

However, I'm still stuck on how to effectively and accurately change the prop pitch without screwing anything up. Is it just bending the prop hub with tweezers coplanar with the hook (sry if that's confusing) so that there's a slight bend or is it more complicated? On a side note, how can I measure pitch easily without a pitch gauge, say a protractor?

[CrayolaCrayon]

2:00 is pretty good. How much does all that rubber weigh? 90cm sounds like a lot. Idk another way to measure pitch aside from a pitch gauge, like the one Zeigler has at FF models. I just slightly warm the hub of the spar, and bend it with pliars.

[bjt4888]

Balsaferret,

I agree with Crayola, this sounds like a lot of rubber. Is the loop length 90 cm?

40% turns remaining is quite a bit and the usual next step with that particular propeller and pitch would be to go to a shorter or thicker motor. If you increase pitch with the same motor, you will have even more turns remaining. A moderately large number of turns remaining, like 20 or 25% is not inherently bad, just a possible indicator to try shorter or thicker rubber motors. Of course, whatever gives the longest duration is the key.

If you could supply additional info, we could give more specific assistance (ex. Motor weight, max turns, max torque, backoff turns, launch torque, prop pitch, all airplane specs, ceiling height).

To adjust pitch, twist the prop spar with needle nose pliers per the picture in the FF kit instructions. You can use a little heat, but we don’t. If you hold the hub with your fingers, this takes a little finger strength. You need to twist pretty hard to get the spar started twisting and then quickly reduce force so as to not twist too much.

Brian T

[BalsaFerret]

Thanks guys for your replies, my whole rubber loop is 90 cm, so I meant 45 cm lol. It weighs 2.29 g with 2 o-rings attached.

The rubber I'm using right now is the thickest I can go, as .102 is the thickest in ffm kits. I ordered thicker rubber but it comes in a few days so I'm stuck with this for now. From my knowledge, increasing the pitch makes the propeller turn slower, which should in turn cause the plane to fly relatively slower, but I didn't know that it would also increase remaining turns. I've seen many previous top planes fly at a slow speed, ascent and descent. Also, regarding turns/winds, do the best freedom flight planes always finish their winds or very close, like 95%?

My current trim is as follows:
Motor - 2.29g/ 90cm/ 0.102"
Max turns (I'm assuming how many I can wind before breaking) - ~110 on 15:1 winder
Max torque - ~6.9-7.4 in oz on new-partially new rubber
Backoff turns - 0 currently, not very sure to how to approach
Launch torque (110 winds) - ~7 in oz
Prop pitch - untouched on non-flaring ikara 24 cm (I'm still not sure how to measure it )
Ceiling height: around 25-30 ft
Wing warp is how it is in Zeigler's directions, ~1/8" warp, for stab it's around 3/16 as well
CG is 2.5" behind rear post
Motor stick is ~14'
Flight diameter - ~20 ft
Wing "angle" is 5/32" (LE-TE) and stab "angle" is -1/32" (LE-TE) (the nose bearing is tilted downwards)
Rudder is tilted left 1/4"

If any more plane info is needed I should be able to supply it. Also, sorry that I still don't get it, but changing pitch is coplanar with the hub and hook, right? I just don't want to mess this up :p Along with that, I'm still wondering how to accurately measure pitch without a pitch gauge, rather with a protractor. Thanks!

[CrayolaCrayon]

BJT will give you very good and detailed feedback. If I remember, those props are either 35 degrees or 40. Not sure. I think it's 40 by default?

[BalsaFerret]

BJT will give you very good and detailed feedback. If I remember, those props are either 35 degrees or 40. Not sure. I think it's 40 by default?

Yeah, bjt is actually amazing in his knowledge of flying
Thanks for the prop info, but I still don't know how to measure them or utilize them greatly, so knowing these numbers are kinda out of my reach and don't mean that much to me, ya get me? My understanding is the greater the pitch, the greater the distance per revolution it travels.

[bjt4888]

Balsa,

I can reply tomorrow.

Brian T

[CrayolaCrayon]

BJT will give you very good and detailed feedback. If I remember, those props are either 35 degrees or 40. Not sure. I think it's 40 by default?

Yeah, bjt is actually amazing in his knowledge of flying
Thanks for the prop info, but I still don't know how to measure them or utilize them greatly, so knowing these numbers are kinda out of my reach and don't mean that much to me, ya get me? My understanding is the greater the pitch, the greater the distance per revolution it travels.

I strongly urge you to get a pitch gauge. They're the best way to measure pitch for your props. The greater the pitch, typically, the more efficient the prop is. It just scoops up more air, and should be slower (I think). But yeah, get some sort of pitch gauge (Like one on freedom flight models, only 5 bucks I think) and start testing!

[Polar]

Balsa,

As far as I know, increasing pitch decreases the rpm because it faces more resistance (catches more air) when spinning. As a result, it will spin slower, causing less winds to be used up. It is recommended to increase the pitch if the airplane is running out of turns. Vice versa, as in your situation, I believe you should lower your pitch to gain rpm to use up the 40% turns you have remaining. Though I believe someone else will be able to offer a better explanation.

[BalsaFerret]

Balsa,

As far as I know, increasing pitch decreases the rpm because it faces more resistance (catches more air) when spinning. As a result, it will spin slower, causing less winds to be used up. It is recommended to increase the pitch if the airplane is running out of turns. Vice versa, as in your situation, I believe you should lower your pitch to gain rpm to use up the 40% turns you have remaining. Though I believe someone else will be able to offer a better explanation.

I agree that increasing pitch decreases the rpm, so as a result it would spin slower and allow the plane to fly slower, effectively making every variable slower. In the end, if I end up with more turns remaining but have a better flight time, I might be able to build from that. If that's not the case, I could just end up changing rubber length or another variable so that I can fly with more efficient times. Also, still talking about pitch, which direction would I bend the prop in order to increase/decrease the pitch? I'm still not the most confident about this topic