dz06

Engine running strong with no problems in the air. I'm setting high end to 8500 rpm (max less 300 rpm). There are no transition, mid range or flame out problems. Seems to run cool with visible smoke trail above 1/4 throttle. Engine mounted inverted on UCANDO with cowl removed until I'm happy with setup.
The only problem is that I can not seem to get engine to idle below 2000 rpm. At 2000 rpm with 16x4 prop the UCANDO will taxi at a fast pace unless restrained. Some other facts: Engine idles rpm drops 200-300 rpm when glow plug starter removed. With standard tank I fly for 15 minutes and have at least 1/3 tank of fuel left. Fuel used is Wildcat 15% heli mix (18% full synthetic oil). Easy starts if you do not overprime. Engine has at least 1 gallon of fuel through it with maybe 15 flights.
I've tried adjusting low speed idle but there is very little effect on low idle with even with large changes (+- full turn).

Help!

Gry101

My Feedback: (8)

DZ06,

A couple of things. A 16x4 is too much prop for this engine. You will not get a good reliable idle due to this. Your rpm will always drop a bit after the glow plug is removed. Try a 14x6 or a 15x4 and look for an rpm around 9700 to 10k for flying. You will also probably need to run another gallon through it before you can really adjust for a good idle and get max wot rpm. Both of my OS91s idle at between 1800 and 2000 rpm and max-300 at between 9700 and 9900 with a 14x6 apc.

Andrew

dz06

Thanks for the suggestions Andrew. I ran the FS91 with a 15x4 APC prop and had the same idle problem. Although the max rpm for 15x4 was greater at 9500 rpm (max less 300 rpm) the UCANDO vertical performance was just barely adequate. The 16x4 at 8500 rpms is quite good, but not outstanding. In the air there are no problems with engine, prop or performance.

The OS FS91S2 manual lists props up to 16x6 as suitable for this engine.

FS-91S2/91S2-P
Stunt planes: 11x11-12, 12x10-12, 13x9
Scale models: 13x9, 14x7, 15x6, <<< 16x6 >>>, 12x8(3-blade)

Thanks,
Dave

Gry101

My Feedback: (8)

I know that it says it can turn a 16x6 but for scale applications, not stunt applications. I have tried the 16x4 on a 3d plane with the 91. This engine likes to run between 9k and 10k. 8500 may be lugging it and causing extra wear and heat, no matter how much of a smoke trail you see. Try leaving it alone until another gallon is run through the engine and then try to tweak it for best idle and top rpm. You might also check the tach forum and eninge manf forum to find some answers and what guys are running on this engine and plane combo.

Andrew

Richard L.

My Feedback: (24)

Is this the standard version or one with a pump?

If your idle rpm drops when you remove the glow plug igniter, then your low end is slightly too rich. However, a 200-300 rpm drop might still be OK. If you turn your low speed screw too far one way or the other, then it might have gotten out of range and can no longer affect the idle. As said earlier, you want your ground rpm to be above 9K, preferably around 9.5K.

Sport_Pilot

Suggest using a hotter plug, Thunderbolt 4S is the hottest I have found. That may be too hot for the high end though. You may have to use an onboard glow ignitor on this engine. I think most people are useing .90 two strokes on this plane. It will provide more thust with a smaller prop.

Richard L.

My Feedback: (24)

Most people are using either a YS .91 or YS 1.10 in this plane. A search in the ARF forum will reveal this. You do not want to use a two stroke, which has inferior throttle response and torque, in a 3D plane.

Bax

My Feedback: (11)

If you have a reliable idle of 2000 RPM with your particular prop, then you have a good idle. We accept anything below 2200-2300 RPM as good. If your model tends to roll at idle, and your idle is as low as possible with good transition, then it's something you have to live with, or do something to the model to keep it from rolling so much (tighten the wheel collars on the tailwheel to act as a brake?)

Lower idle RPM can be obtained, depending upon the particular prop, fuel, plug, and needle settings, but you may compromise acceleration. The goal is to get as low as idle as possible, but have good throttle transition, too.

That said, we'd also state that you have your engine over-propped. We prop the engine to turn in the high-9,000's on the ground at full throttle when leaned rich of peak RPM. If you have the pumped version, lean it as far as possible without RPM drop or detonation and leave it there...don't richen.

Target the high-9,000's and then play with different diameter/pitch combinations until the model flies the way you like it. For 3D use, you want lower pitch propellers, but going too low will require too much diameter to keep from over-revving the engine, and will also result in low airspeeds.

See the 3D forum for the engine/prop combinations used on the U-Can-Do-3D. The thread is long, but will give you a wealth of information on the setup of this fine model.

dz06

Thanks to you all for your help.
1. I see the consensus is that I am running too big a propeller. I will switch back from 16x4 to the 15x4 and recheck performance this weekend. I thought the 16x4 prop was probably to much for FS91, but the plane really performs well in the air with the 16x4.
2. Although many are using engines bigger than FS91ii, Great Planes has a special addendum warning not to use engines greater than 91 four stroke due to the significant possibility of flutter and catastrophic failure at high speeds. They did not design the plane for the stress of larger engines.
3. I want the best thrust and am happy to sacrafice speed for thrust. The UCANDO is designed for low speed flight and manuvers.

Sport_Pilot

While a good choice, most people elect not to spend that much. I see about 80% of them with two stroke engines.

Sport_Pilot

That would be false, two strokes have just as good of a throttle response as any four stroke. Except for the YS they have more torque than the same size four stroke.

Richard L.

My Feedback: (24)

I don't know about that statement. I tested OS .40LA, .46FX, .50SX, .61FX, .91FX, and a couple of Magnum 2-strokes at the field. All of them took a while to go from idle to max rpm when the throttle stick was jammed forward. On the other hand, my OS .91 four stroke pumper took significantly less time (maybe a fraction of a second), and all my YS four strokes went to max rpm from idle in a flash.

Sport_Pilot

It takes less time to accelerate from 2,000 to 9,500 than from 2,000 to 13,000. Two strokes that are proped to run at similar RPM's have the same response, especially two strokes designed to run at those speeds.

dz06

OK, I admit it! The APC 15x4w works just as well in the air as the 16x4w and is a better overall choice for the OS FS91ii. At the same idle I could taxi the UCANDO with the 15x4w and not have to hold it to prevent it from racing away. I made no changes to low speed adjustment, but did lean out the high speed (maybe 1/2 turn) for 9500 rpm (max 9800 less 300).
Thanks again for everyones help.

Dave

dz06

Switching from Wildcat 15% heli fuel to Magnum 30% Heli fuel eliminated 300 rpm drop at idle when igniter was removed. Now I am leaning out idle mix to get lower steady idle and adjusting for higher top end rpm.

Thanks
DZ06