Maximizing Flight Performances at Unideal Flight Conditions

[Sapphire]

Pitch is equal to tan(angle) x circumference. The radius for the FFM pitch gauge should be 2.25 inches so your pitch is tan(38 degrees) x 2pi x 2.25 inches = 11" which is really really low. I don't believe Ikara should ever be pitched down, you can certainly try pitching it up though. In 22', you will probably see better times with a flaring propeller. Like Coach Chuck said, try a denser rubber if you're landing with a "few hundred winds left" (this is such a vague number though). You'll probably need a denser rubber with the higher pitch anyway.

However, it might not be time to trim the propeller and rubber yet. In div B, I think you should be able to get at least 2:20 consistently in 22ft assuming you're using a freedom flight kit without any prop adjustments.

[bosscrusher667]

Pitch is equal to tan(angle) x circumference. The radius for the FFM pitch gauge should be 2.25 inches so your pitch is tan(38 degrees) x 2pi x 2.25 inches = 11" which is really really low. I don't believe Ikara should ever be pitched down, you can certainly try pitching it up though. In 22', you will probably see better times with a flaring propeller. Like Coach Chuck said, try a denser rubber if you're landing with a "few hundred winds left" (this is such a vague number though). You'll probably need a denser rubber with the higher pitch anyway.

However, it might not be time to trim the propeller and rubber yet. In div B, I think you should be able to get at least 2:20 consistently in 22ft assuming you're using a freedom flight kit without any prop adjustments.

I would echo what Sapphire said here. I'm not entirely sure about division B, but at least for div C, the pitch should be around 15-19 inches. It shouldn't be that much different for div B, though.

[coachchuckaahs]

Thanks for the further information.

It appears your prop pitch is at a reasonable start point (17", if the FFM gage is at 3.5").

It appears you could get a better rubber to prop match, as leaving "several hundred" winds on the plane is not good. Going to slightly heavier rubber will help use this up.

By your observation the plane appears to be trimmed well. It may be good if you could share a video, either as a link here of if you consider it proprietary DM me a link, just to see if I can see something in the flight.

Low ceiling flying is very hard. I like low ceiling flying because it separates the skills from the lucky. It sounds like you are doing the right things in terms of winding, dewinds, etc. Low ceiling will generally mean needing a slightly heavier motor than high ceilings.

All that said, you should be getting more like 3 minutes when doing all of these things, even with a stock Ikara symmetric prop. I think there is a noticeable gain to be had on rubber selection, but not a 50% increase.

So, time to review your basic trim. Carefully review the plane for warps, especially in the stab. At the low launch torques you must have for this ceiling, you likely need little to no wing wash, and that is drag, so take out any wing wash unless it was explicitly needed to solve a problem.

After checking for drag-inducing problems, review the basic decalage/CG location balance. If the CG is forward, you may carry excess decalage. This makes for a stable plane, but at the expense of extra drag. So if you are carrying more than say 3mm wing incidence (and 0 on the stab), try moving the cg back until 3mm works. (3mm is a convenient starting point and not an end goal. But if you have 6mm or more, you are overly stable likely). These planes seem to be quite sensitive to CG location.

This is all going back to basics to start finding those 5 and 10 second improvements. A draggy plane can still look very nice in the air but just not have the duration that a clean plane will have.

Coach Chuck

[coachchuckaahs]

Pitch is equal to tan(angle) x circumference. The radius for the FFM pitch gauge should be 2.25 inches so your pitch is tan(38 degrees) x 2pi x 2.25 inches = 11" which is really really low. I don't believe Ikara should ever be pitched down, you can certainly try pitching it up though. In 22', you will probably see better times with a flaring propeller. Like Coach Chuck said, try a denser rubber if you're landing with a "few hundred winds left" (this is such a vague number though). You'll probably need a denser rubber with the higher pitch anyway.

However, it might not be time to trim the propeller and rubber yet. In div B, I think you should be able to get at least 2:20 consistently in 22ft assuming you're using a freedom flight kit without any prop adjustments.

Can someone check the radius of the FFM pitch gage? I thought it was 3.5". This post says 2.25", in which case jgrischow1 the prop pitch is actually quite low. This is also a very small radius to be measuring the pitch. Ideally you want to measure between 2/3 and 3/4 of the prop tip radius. At 2.25, you are at less than 1/2 of the radius. This is really important on the ikaras, since the blade is not formed with a lot of twist, and so the pitch changes along the blade considerably.

Coach Chuck

[bjt4888]

Chuck,

I have a FFM pitch gauge. I can check it later tonight, but I'm pretty sure it's 2.25" radius as I decided to add another 0.75" piece of wood to make it 3".

At the 3" radius, we've found the Ikara 9.4" symmetrical prop is typically about 28-30 degrees. Best flights with this prop in Div C last year were after increasing to 34 degrees at 3"R.

We don't use this prop; just tested last year as a baseline.

Brian T

[jgrischow1]

Yeah, the FFM pitch gauge we have isn't even 2.25. It's between 2 and 2.125 from wire to protractor. So you guys would recommend "lengthening" our pitch gauge, and then perhaps going with a lower pitch prop isn't the solution after all?

[coachchuckaahs]

Yeah, the FFM pitch gauge we have isn't even 2.25. It's between 2 and 2.125 from wire to protractor. So you guys would recommend "lengthening" our pitch gauge, and then perhaps going with a lower pitch prop isn't the solution after all?

At 2" radius, 38 degrees would be around 10" pitch, or a P/D of 1. That is really low. I suspect that FFM pitch gage was set up for a few years ago when we had really tiny props.

However, 2" is measuring fairly far in on the blade, not the area doing most of the work. Since the Ikara is under-twisted, as you go to the tip the pitch angle does not decrease fast enough, and so the pitch goes up. This effectively adds wash-in to the prop tip, when a lot of props are designed with a little wash out to prevent stalling at the tips.

I would definitely suggest measuring at 3 inches or even 3.5 inches as a first step, and then setting the P/D to around 1.6 to start. You will find the useful range of P/D to be around 1.5 to 2.0. Some reports indicate that the P/D at the lower end of this range (1.5-1.7) is better, though others have had higher P/D. This would put pitch around 15/2" for the 9.5" prop as a starting point. This would be about 38 degrees at 3", 35 degrees at 3.5", or 50 degrees at 2" radius.

Note: As in the CG and decalage adjustments, this is a STARTING POINT ONLY. Actual performance will depend on details of your prop pitch distribution, rubber, winding, and trim. As you explore adjustments in trip, you may find different optimums for pitch and rubber width. Performance may prove to be fairly sensitive to prop pitch adjustments, so after getting to this starting point, make small adjustments (1 degree) at a time and re-optimize your rubber before comparing times.

Bottom line, I would say your pitch may be off substantially from optimum, and such a low P/D may be a big impact on duration.

Coach Chuck

[Sapphire]

As I said before, you should only ever increase the pitch of a stock ikara. Using the numbers you've provided, you have around a 10.3" pitch, which is really low.

Increasing pitch should help but echoing what Coach Chuck said, your basic trim does not seem to be dialed in yet. Personally I wouldn't touch the propeller until I'm happy with what I already have. It'll just add another variable, and you need to be able to trim your CG and decalage well anyway. And be sure that your plane doesn't have any issues.

[Sapphire]

As I said before, you should only ever increase the pitch of a stock ikara. Using the numbers you've provided, you have around a 10.3" pitch, which is really low.

Increasing pitch should help but echoing what Coach Chuck said, your basic trim does not seem to be dialed in yet. Personally I wouldn't touch the propeller until I'm happy with what I already have. It'll just add another variable, and you need to be able to trim your CG and decalage well anyway. And be sure that your plane doesn't have any issues.

[jgrischow1]

Good info. Yes, the FFM gauge is from years ago when the props were limited in size. I should have realized this.

Speaking of which, we have many old, small Ikara props that are in bad shape. I was thinking of having my kids scavenge all the old nose buttons and make all the props we have, whether they are Ikara or IFFS and whether they are made for an IFFS-style bearing, one of those little aluminum bearings, or an Ikara-style nose hook bearing, into Ikara nose hook-compatible props since those are the kind of bearings we are currently using on our JH kits. That way, the kids can (correctly this time) measure all the prop pitch angles and we can easily swap them out. Thoughts?