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Aug 25, 2021, 03:48 PM
A man with too many toys
Thread OP
That's because 3-blade are less efficient.
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Aug 25, 2021, 06:40 PM
Registered User
scirocco's Avatar
More likely pitch is not sufficient
Aug 26, 2021, 06:30 PM
I just want to go fly!
walter3rd's Avatar
Yeah agree. I wish there were more 3 blade apc options. I need the ground clearance on this model.
Aug 27, 2021, 02:55 AM
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scirocco's Avatar
Quote:
Originally Posted by walter3rd
Yeah agree. I wish there were more 3 blade apc options. I need the ground clearance on this model.
Still one of the coolest looking models I've ever seen! Have you tried near square or even square 2 blade props in the same diameter as your 3 blade?

You might lose a bit of static watts/lb, but if the pitch gets your pitch speed up above 3 times stall speed the higher thrust at flying speed might well offset any 'on-paper' reduced power.

Not a lot of flights yet, but my Deuces Wild on 4S Power 32 and 11x10 put a smile on my face and quietened the peanut gallery, despite having a lot less static power than the recommended 13x6.5.
Aug 27, 2021, 09:03 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by RC Man View Post
Is there a table that gives the watts equivalent to various sizes of glow engines. ...
E-motors are more versatile and can deliver their power at lower rpm, cranking a more efficient prop, making a comparison fairly difficult.
Better don't do ic-engine ↔ e-motor equivalencies, plane type and mass are leading.

Below an excellent quote about motor selection.
From
brushless motors Kv?.
Quote:
Originally Posted by scirocco View Post
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it:
    Magic numbers for modellers.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.

The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS
Prettig weekend Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC
Aug 28, 2021, 09:54 AM
Registered User
Watts/lb. Ray, I thought your comment quite rational and insightful. Ever since abandoning liquid fuel some years ago, I have continually down-graded estimates for what it will take to fly a model the way I want. I see others' Rules of Thumb about W/lb, and consider them grossly over-estimated. My approach is akin to yours: use a scale-diameter prop, and fiddle with the pitch and voltage until it flies as desired. Not uncommon that I end up with half or less of the power that others are quick to recommend.
JGO
Aug 30, 2021, 07:29 PM
I just want to go fly!
walter3rd's Avatar
once you realize that flying on the wing is super fun you can scale back on those numbers i agree. my duallist is not the rocket i see on YouTube but still puts a smile on my face every time she's airborne.
Sep 17, 2021, 12:17 PM
A man with too many toys
Thread OP
What kind of performance can I expect with 100 W/lb on a 10 lb sport airplane?

Anyone done a large airplane conversion. That would be using a 14" prop.

.
Sep 17, 2021, 08:04 PM
Registered User
dan culv's Avatar
Quote:
Originally Posted by eflightray
P.S. I also no longer do conversions of models built for IC engines, that is just away of carrying extra weight around.

.
Bud valid if you want similar flight times,..
Sep 22, 2021, 11:20 AM
Registered User
E-Challenged's Avatar
Scale models with large ring cowls/dummy radial engines may need to use a larger diameter prop, etc., for adequate thrust. Cub-like models, built light, may fly well with a fairly wide range of motor/prop/battery pack options. I prefer scale-like flight speeds using light weight, but adequate, motor etc. options and locating battery packs and RC gear to minimize need for nose ballast. Using a lighter battery pack can save weight, and improve flight, with some sacrifice in flight time duration. Hate to watch a heavy Cub fly that has to fly fast.
Sep 22, 2021, 08:40 PM
Balsa Flies Better!
To RC Man

If you're talking 1000 watts on a 14" prop- you may want a somewhat larger prop. 100 watts/lb can give you very nice performance- if you don't want glow style speed. To get glow style performance- you need about 150 watts/lb or so.

I have flown 7-8 lb airplanes- but generally with more like 1200-1400 watts using 12-13" props. These were converted pattern aircraft. For a sport ship with a 1000 watts- you probably should be looking at a 16-17" prop. Won't be all that quick, but should have decent vertical.

If you can't go larger than a 14" prop- then your setup will fly the airplane fine- but it'll be a bit sedate.

Sam
Sep 23, 2021, 04:51 AM
A man with too many toys
Thread OP
Quote:
Originally Posted by Megowcoupe
To RC Man

If you're talking 1000 watts on a 14" prop- you may want a somewhat larger prop. 100 watts/lb can give you very nice performance- if you don't want glow style speed. To get glow style performance- you need about 150 watts/lb or so.

I have flown 7-8 lb airplanes- but generally with more like 1200-1400 watts using 12-13" props. These were converted pattern aircraft. For a sport ship with a 1000 watts- you probably should be looking at a 16-17" prop. Won't be all that quick, but should have decent vertical.

If you can't go larger than a 14" prop- then your setup will fly the airplane fine- but it'll be a bit sedate.

Sam
I already have the power system and am looking for an airplane. Looks like a 10lb airplane is too large. Maybe something in the 6 to 8 lb range would be good. Anyone know of a PT-17 in that weight range. I have always wanted a PT-17. Don't need crazy power just enough for sport aerobatics.


.
Sep 23, 2021, 07:05 PM
Balsa Flies Better!
There's the Sterling kit of the PT-17- 65" should be in that size range.
Sep 23, 2021, 07:52 PM
Registered User
scirocco's Avatar
Quote:
Originally Posted by RC Man
I already have the power system and am looking for an airplane. Looks like a 10lb airplane is too large. Maybe something in the 6 to 8 lb range would be good. Anyone know of a PT-17 in that weight range. I have always wanted a PT-17. Don't need crazy power just enough for sport aerobatics.


.
1000W into a 14" prop will fly pretty most 10lb models and allow sport aerobatics. At 8lb naturally you get stronger longer verticals and more rapid acceleration. The only caveat is for other than slow 3D flying to ensure that pitch speed is at least 2.5 times stall speed. If 1000W has the system maxed out with insufficient pitch speed, it is usually worth reducing diameter and increasing pitch
The Hangar 9 Ultra Stick 10cc ARF is near 9lb and flies just fine on under 700W (I was targeting 900W but old batteries ).
What is the existing power system?
Sep 25, 2021, 09:28 PM
Registered User
dan culv's Avatar
Quote:
Originally Posted by scirocco
1000W into a 14" prop will fly pretty most 10lb models and allow sport aerobatics. At 8lb naturally you get stronger longer verticals and more rapid acceleration. The only caveat is for other than slow 3D flying to ensure that pitch speed is at least 2.5 times stall speed. If 1000W has the system maxed out with insufficient pitch speed, it is usually worth reducing diameter and increasing pitch
The Hangar 9 Ultra Stick 10cc ARF is near 9lb and flies just fine on under 700W (I was targeting 900W but old batteries ).
What is the existing power system?
Not enough power for my tastes.


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