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Nov 23, 2021, 01:39 PM
u2builder's Avatar
Thread OP

Effect of fake engine cylinders on Performance

I am getting ready to build a Twisted Hobbies Crack Camel. This little flat foam plane has a wingspan of 35" and weighs 9 oz or so ready to fly. The cowl is 5" diameter and has 8 fake cylinders, 4 of which align with the foam in the fuselage. This plane is usually set up per design recommendations with 600 mah 2S and 9 x 4.7 prop (2.2 thrust to weight, Ecalc) or 450 3S with 8 x 4.1 prop (3.1). One could also run a 9 x 3.8 prop (3.4) on 3S but throttle curve would need to reduced to keep amp under 20 A.

Ecalc of course does not account for such a dirty airframe or the fake cylinders. Most everyone seems happy enough, but a few say climb suffers and some have gone to 9" props or cut out 4 of the cylinders.

Efficiency calculated by Ecalc is similar for all the combinations at say a 2:1 thrust to weight ratio for the chosen 1620 kV motor.

Which option is likely to be most efficient in the real world:

Run a 9" prop on 2S (or on 3S but adjust the throttle curve to limit amp draw)

Run an 8" prop on 3S and remove half the cylinders that don't line up with the fuselage foam

How bad are the fake cylinders? How good is an extra inch of prop? If the goal is to maintain a 2:1 thrust at minimum amps.
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Nov 23, 2021, 01:42 PM
Registered User
richard hanson's Avatar
Put a fake prop on it and pretend you are flying it….
Nov 23, 2021, 02:33 PM
B for Bruce
BMatthews's Avatar
There's no doubt that there will be more drag. But that might prove to be an advantage in the down lines. After all, that which goes up.....

There's a whole lot of study and data on stuff like this where there's a cowl or other structure with lots of blockages. A few things you could do is remove the extra cylinders like you say. Plus you could round over the front edges. But in the end any air flow through that cowl is going to be very turbulent and cause a fair degree of drag simply due to the size of it and all the plywood bulkhead supporting it. Removing the parts not attached to any foam would help. But only a little. It'll still be very draggy any way you look at it.

You ask about the climb improving with the bigger prop. But of course it will since the option you listed will also pull more power. So of course it'll climb better with more watts to the motor. But then you toss in the idea of a 2S option. Now THAT is not a bad idea at all. But in that case the RPM will be lower so I think you'd want to look at some other 9" prop options up to a 9x5. Depending on the prop brand a 5 inch pitch is still a viable "hovering" prop since a lot of examples at that pitch can run with a clean sounding sizzle that suggest the prop blades are not stalled at static running conditions. Or maybe if the gear is long enough try a 10x4 or 3.5 or something on 2S? The issue being that you still need to load the motor to pull the same watts as the 8 inch prop on 3S. Otherwise you're comparing camels to kumquats.

The other feature suggesting that 2S and a bigger prop might work out well is that for the same watts of power to the motor a bigger prop is going to do better at static and climbing thrust. Prop disc size wins for these flight modes.
Nov 23, 2021, 06:25 PM
Registered User
Just for reference, the first 25% of a prop blade contributes virtually nothing to thrust. The outer 50% contributes about 80% of the total thrust.. So a larger prop will be less blocked by the cowl.
Nov 23, 2021, 06:31 PM
AndyKunz's Avatar
The Ecalc I use has an airframe Cd parameter.

I'm using the late Sid's version.

Nov 23, 2021, 07:04 PM
u2builder's Avatar
Thread OP
Thanks for the info on the prop. I was wondering about that.

************************************************** *****

I'll look at that parameter on Ecalc. Thanks.
Nov 23, 2021, 09:45 PM
springer's Avatar
No math, but I built a Mister Mulligan (actually two, one 30" span from dollarstore foam) and one 40" from MPF) . In both cases, the original intended prop was "inadequate" and I replaced with a larger diameter prop and got lots better performance. In the 30" one, I swapped a 7x6 apc style for a gws 8040dd. Odd swap, but it is what I had on hand, and worked just fine.
Nov 24, 2021, 01:48 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Originally Posted by u2builder
... Run a 9" prop on 2S (or on 3S but adjust the throttle curve to limit amp draw) ...
That won't cause problems for battery or motor.

However ...

Should current at wide open throttle be too high for the controller then reducing throttle, using the stick or using end-point-adjustment, to get current below ESC max.current is not a good idea. Better to prop down.
Throttling down if current is too high for motor causes no problems. But here too it is wiser to prop down, that way current can never get too high. Unless motor is blocked of course, the motor will act as a massive short.

In fact, reducing throttle is even harder on ESC than to keep overamping at full throttle.
Much safer to prop down a bit to reduce current.
See:edit, w.r.t. to Piece's post #20 further down below:
The problem above does not apply to newer ESCs with 'active freewheeling' a.k.a. active/synchronous rectification in EE geek-speak.
See this post:
Active freewheeling - RCG

Vriendelijke groeten Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC
Last edited by Ron van Sommeren; Nov 26, 2021 at 06:26 PM.
Nov 24, 2021, 07:26 AM
u2builder's Avatar
Thread OP
I read the links but don't fully understand since the throttle is almost always reduced using the stick when flying.

I agree that it generally makes sense not to deliberately use too large a prop and then limit the throttle, but in this case a larger prop would seem to be more efficient since more of the blade is in clean air, but also may provide too much thrust for comfortable modulation while flying.

I'd lower the prop pitch, but am already at the lowest pitch possible.

Switching to 2S would be the best approach, but I don't have any 2S batteries of this size and dislike having special batteries for each of my planes.
Nov 24, 2021, 08:57 AM
Registered User

Is there a way to make Sid's Electricalc work on Windows10? and hopefully on Windows 11?
Nov 24, 2021, 09:47 AM
Registered User
richard hanson's Avatar
The largest dia prop.. with enough pitch to fly the model
The higher voltage setup is more efficient
So go for a 10” prop ..thinned And narrowed

If possible.. use a prop hub extension ,even a short one
Get the prop forward the cowl face as much as practical
Forget the calcs
Sorry guys….
Get a Whatt meter and actually measure the current pulled with each prop
Theory is fine..except on these very small props and models …
The a actual results can be waaay off
Measure actual thrust,,, static
Measure Actual battery draw..static
If you enjoy fling little electrics, you needa combination watt, ohm volt meter
Learn to read what is a tually happening
The rest is just guessing n hoping
This one is$22.00
Last edited by richard hanson; Nov 24, 2021 at 10:37 AM.
Nov 24, 2021, 10:07 AM
AndyKunz's Avatar
I don't know, Al. I'm running Win7 Pro.

Nov 24, 2021, 11:09 AM
"Is there a way to make Sid's Electricalc work on Windows10? and hopefully on Windows 11?"
I'd like to know that too !!
Nov 24, 2021, 11:17 AM
u2builder's Avatar
Thread OP
Thanks Richard. I do have a power meter, a Dr, Watson maybe, and will test amp draw once I get this plane and get it built. This little plane will clearly fly with the 3S lipo and 8 x 4 prop, but it did seem like longer would be more efficient and give me a little more flight time on a small battery. I usually have success with Ecalc to get in the ballpark, but knew that cowl threw a wrench in the calcs.
Nov 24, 2021, 11:30 AM
Registered User
richard hanson's Avatar
I hd a number of tiny electrics ,including a 2/3 cell model .which was cute but a poor flyer
At these sizes it practically impossible to make a very scale like model, light enough
The power/ prop setup which works at sea level is not good at 4500 ft elevation
That is easily proven
Bottom line
To get best thrust to weight…….. you have to measure .. not calculate
The battery suppliers are sometimes wildly optimistic or just plain old bs ers
The proximity of prop to cowl is tricky
A long extension looks bad and can wobble but get as much as you can without wibble or just plain old ugly
If You dont want it to end up as a ceiling display . It might take a few expiriments.
Last edited by richard hanson; Nov 24, 2021 at 11:37 AM.

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