How to choose a power system - Page 5 - RC Groups
Thread Tools
This thread is privately moderated by Chris F, who may elect to delete unwanted replies.
Jan 29, 2008, 06:04 AM
<- Balsa flies better ->
wolw's Avatar
Convert the idea to an E-glider, don't need no 500W for a 5.4lb 3.5m thermal e-glider.
More lift, less power

This is a good starting point

Wing Loading
Loading Type
10 oz/sq.ft Glider
15 oz/sq.ft Trainers
20 oz/sq.ft Sport Plane
25 oz/sq.ft Fighters

Here's an very good wing loading calculator
And a very useful Glow to Electric Conversion Power System Calculator based on Jim Bourkes article in the E zone.

Last edited by wolw; Jan 29, 2008 at 06:21 AM.
Sign up now
to remove ads between posts
Jan 29, 2008, 06:19 AM
Proud member of LISF and ESL

You were saying you optimize toward wing loading rather than just power. Got it.

Yes that would follow the guidelines that suggest about 50 watts/pound for an easy flying or slow flyer. Although I have to say, my Easy Glider Electric has a 180 watt brushless running on two lipos and comes in a 32 ounces. At 90 watts per pound, on the bench, She climbs steep and fast. I like that. Might be able to lose 1.5 ounces with a 140 watt motor and lighter battery. Take it to about 72 watts per pound. However, mine is still around the weight of the brushed 400 set-up + 7 cell Nimh with about twice the power.

Love those brushless motors and Lipo batteries.


I have been considering convering my 3.4 M Ventus 2C scale glider to self launch, since we lost our tow plane. The plane weighs 6 pounds as a pure glider. Figure I would add 1 pound with a 450 watt brushless motor and battery taking it to 7 pounds giving me around 64 watt/pound loading. That should be hand launchable, perhaps with a little help from a 10' bungee cord.

Here is an example, So people can see the process.

Was thinking a Himax 3528 and 60 amp ESC - $190
450 watts

Add a 14X7 folder - all up about $40

3 Cell 4800 mAh lipo - $140

Probably need a separate BEC - $30

$400 is a lot less than the cost of a tow plane.

Should get me 4-7 climbs on a pack. Won't be a rocket but should get me up to soaring height in a minute or less.

Put a nice 2.4 GHz radio system in it and no concerns about channel conflict.

Sounds good!

If I wanted to spend a bit more for more power and carry a bit more weight, this package would probably take me to 8 pounds, but 800 watts and 100 watts per pound. I would have to invest another $150 in motor+ battery, but then I would need a new charger, so add another $180 to that.

Much better climb but higher wing loading too. Not sure I want to go there.

Himax .46 combo - $213

5 cell 4800 pack - $250

Unfortunately that would require me to buy a new charger, so add another $190. I would buy this one:

So many options. the things of dreams.
Last edited by aeajr; Jan 29, 2008 at 09:14 AM.
Jan 29, 2008, 07:11 AM
<- Balsa flies better ->
wolw's Avatar
The PC-9 flew extremely well, suitable to be a first low wing.
Today if I would have bought another one I would have propped it for some more speed.
The same motor rests now in a .40 Tucano, an APC-E 12x8 instead of a 12x6 and better LiPos made power jump to close to 700W, which will come handy as it's heavier and have smaller wing area.

Right now I'm quite excited to convert the Sky Runner to brushless, I know it's gonna be a blast

Jan 29, 2008, 07:19 AM
<- Balsa flies better ->
wolw's Avatar

Keep us informed
Feb 02, 2008, 03:04 PM
Registered User
wolw> You mentioned :
1- The discussion here is about static thrust, its not the same once airborne.
When choosing a prop I only give thrust a quick glance to see if it looks resonable, then I mock around until I find a nice prop with the right pitch speed compared to thrust suitable to the models weight and the specifications of the motor.

2- Playing with numbers can be fun but they're no that exact.
I would say that the 132W/lb power/weight recommendation is "too much" for a regular sports model, personally I like lots of power but to say that it's a "rule" and "needed" I can't comply with. Going for higher W/lb ratio makes your plane heavier too.

My experience tells me that the 100W/lb is a quite correct recommendation for a sports model, more is fun .

My answer is simple:

1- Yes, you are absolutely right. However, in order for us to make an objective analysis of the "static thrust", we also have to refer to the diameter of the airstream when we do the bench test. Then, we can select the right prop for our model. We use objective rather than subjective method, "I only give thrust a quick glance to see if it looks reasonable" to select the right prop.

2- A sport model is defined as a model that is able to perform some basic aerobatics; 3D maneuvers are not included. Some examples of basic maneuvers are inside and outside loops, rolls, stall turn, hammer head, and inverted half-cuban eight maneuvers. Any sport model that has 100 watts/lb of power can perform all these maneuvers ONLY IF the motor is EXTREMELY EFFICIENT. That's why 130 watts /lb is better. Only rarely can the model perform a regular-half cuban eight if it has only 100 watts/ lb :-). It is true that the weight of the model will increase when we go for a higher W/lb ratio; however, such an increase is often smaller than the increase in watts (Delta Weights < Delta Watts). The only issue may be the slight increase in wing loading, which may cause a higher sinkrate, stall speed, and landing speed. Then, we may be tempted to increase the wing area to compensate that slight increase in wing loading. But, doing that is a time consuming. Therefore, we don't have to be more concerned about the increase in wing loading (delta WL) when we increase the Watts/lb ratio because brushless motors are efficient.
Last edited by Hasina75; Feb 15, 2008 at 11:44 PM.
Feb 27, 2008, 03:51 AM
Proud member of LISF and ESL
I have collected much of what I have published into an e-book that you can read here:

I think seeing the info collected in one spot helps make it more understandable.
Feb 27, 2008, 04:09 AM
Proud member of LISF and ESL
Originally Posted by wolw
Convert the idea to an E-glider, don't need no 500W for a 5.4lb 3.5m thermal e-glider.
One thing to keep in mind with an e-glider used for thermal duration flight is that the use of the motor here is not for general flight but specifically to gain altitude quickly. The motor is used as an alternative to a hi-start, a winch or an aerotow.

My climb time to 500 feet from a hi-start or winch is around 10 seconds, or about 3000 fpm. If I want to use a motor as an alternate launch system I want to size that motor to get me to 500 feet in around 30 seconds and would prefer 20 seconds. That means I am looking for a climb rate of 1000 fpm to 1500 fpm.

Does your calcualtor help me predict something like this?

Experience and the watts/pound guidelines I use tells me I should look for a ratio of about 80 watts/pound or higher, and a prop setup optimized to thrust over speed to achieve this climb rate.

For this 5.4 pound plane that would be around 430 watts. So a 450 to 500 watts would be a good target.

I expect the motor to be on less than 5 percent of the total flight time.
Feb 27, 2008, 04:11 PM
<- Balsa flies better ->
wolw's Avatar
Hi Ed,

I only use the calculators to verify to some degree what I've calculated in my mind based on known (used) setups.

I've got no altitude meter but I believe motocalc predicts this.

When I choose a setup I go by numbers I already know. i.e I know my Bandit with a RG14 airfoil specks out in about 7sec with 222W/lb, my Svist reach ~250m in about 10sec on 122W/lb, both use slick/fast/penetrating airfoils.

The airfoil is one more thing to consider, slicker airfoils (that I have on my planes) like RG14, RG15 mod, MH32 have more to gain on having a resonable amount of pitch (more square > 13x10) as they don't need as much thrust to overcome the drag of a thicker wing and therefore will gain speed more quickly.

As it's rarely dead calm where I fly I prefer a wing with a more penetrating airfoil, if I were to choose a plane without regarding cost the choice is very simple Vladimir's Graphite 2e, thermal, hotliner, it'll do all A bit on the pricy side though

Calculators is good, experience is better, but more subjective and not as easy to put down in writing. Going with a popular plane makes it a lot more easy as there's a lot of proven setups to start from to get a setup of your own choice.

Feb 27, 2008, 06:19 PM
Proud member of LISF and ESL

Excellent examples. Helpful.

I don't know the airfoil on the Ventus 2C that I have. Very high aspect ratio.

I will probably stay in the 350 to 500 watt range because I want to stay within a 4S lipo as that is all my chargers can handle.

So, using 13V as average under load, for a 500 watt motor I can use a 4S pack at 38 amps. If the finished plane is 7 pounds, then that is 71 watts per pound

That is quite workable for me and shoud give me about a 60 degree climb to soaring height of about 600 feet or so in 20-30 seconds. That would be a good package for my needs.

On a 3800 mah 4 cell pack, that is 6 minutes run for about 8-10 climbs.

I also have a Mantis with an MH32 that will likely also be electrified. That will probably be about 4.5 pounds when done. That would proably get a 350 watt motor for similar performance at 77 W/lb. On a 3 cell lipo pack that would be about 35 amps again, a very workable number.

Using a 3 cell 3500 mah pack, that would give me about 6 minutes at full throttle, or about 8-10 climbs. Right where I would want to be.
Last edited by aeajr; Feb 27, 2008 at 06:26 PM.
May 09, 2008, 01:23 AM
Registered User
"When a prop cavitates, it loses bite. A vacuum forms around the prop and thrust drops off rapidly."

A bit off topic here, but: isn`t this why the "shroud" is present in ducted fans, and why it has specific tolerances for "blade to "shroud" distance"??
I don`t really know, but I think I read it somewhere, and would like a confirmation, or a "denial"? (what`s the word? the opposite of confirmation? my english isn`t that good anymore)
May 09, 2008, 02:22 AM
Suspended Account
Originally Posted by jankelfrodel
...and would like a confirmation, or a "denial"? (what`s the word? the opposite of confirmation? my english isn`t that good anymore)
Hi J.C.F.,

I don't know the answer to your first question, but the opposite of "to confirm" (confirmation) is "to refute" (refutation).

May 12, 2008, 01:15 AM
Registered User
dbl8ts's Avatar

The Whattmeter

I am still in the embryo stages of understanding electric flight. Thanks for all the info in this thread that has helped me understand watts, volts, amps, etc. However, a piece of equipment I am told all electric flyers must have is a "good" watt meter. Toward that end, I purchased an Astro Super Whattmeter days after acquiring my first electric plane (I started with EDC jets when I entered electrics and am working on my first prop). Now I can look at a lot of values, however the recording of the data is confusing.

For instance, while letting the motor run at a given throttle setting, the numbers on the watt meter continually changed. While some are going up, others are going down. I am trying out different props for an E-flight 450 Outrunner, and it is very confusing when trying to find the continuous amp value (14) and the short burst value (16). The first prop (11 x 8) was easily ruled out, because the max draw was well above 16 amps during an eight second run. But even so, the AH readings kept going up and never stabilized, and the volts decreased (this part I understand). However, the suggested prop (10 x 8) also gives a 16 plus amp reading at full throttle. Last night, I tried a 10 x 7 and got 14+ amps that dropped to 13+ after about a ten second run and stayed in the 13 range as I allowed the motor to continue running. All of the other values mentioned changed during the run, even though I let the motor run at that one setting.

Question: Should the amps and AH readings continually change when the throttle is set and left in one position for more than twenty seconds or more? I am trying hard to understand all that is necessary for electrical flight to keep from damaging my equipment or causing a fire. After a year and a half, I am just now beginning to feel comfortable with my Li-Po charger, but I do not want to take that long learning what the watt meter is showing me.

Your assistance is appreciated.
May 12, 2008, 01:40 AM
Suspended Account
Originally Posted by dbl8ts
Question: Should the amps and AH readings continually change when the throttle is set and left in one position for more than twenty seconds or more? I am trying hard to understand all that is necessary for electrical flight to keep from damaging my equipment or causing a fire. After a year and a half, I am just now beginning to feel comfortable with my Li-Po charger, but I do not want to take that long learning what the watt meter is showing me.

Your assistance is appreciated.
Hi dbl8ts,

I have a couple different watt meters, and they do tend to wander around a little. It's the nature of the beast and not to be taken too seriously.

Second, it looks like you've found the perfect prop for your Park 450. Don't forget that while your motor and prop are tied down, they will pull a little more current than when they are allowed to haul your plane through the air. This phenomenon is referred to as 'prop unloading'. This would mean your 13 amps pulled during WOT on a static run would probably result in a current draw of 11-12 amps while in the air.

Third, don't feel it necessary to wring out every last bit of power from your motor. I use Park 450's myself, and on my planes, they run very nicely with a 9x4.5 prop, pulling about 8 amps.

If you find that the only way to fly your planes with the power they need is to fly the motor right at the edge of it's recommended current draw, then simply move up a motor size and let the motor run at 80% or so of it's constant current rating. This will ensure a LONG motor life, and a cool running motor...

May 12, 2008, 01:47 AM
<- Balsa flies better ->
wolw's Avatar
Hi dbl8ts

Lot's of numbers but it's fun

As you wrote, the voltage (V) will slowly decrease and the Amps (A) and the power (W) as well as these are depending on each other, Watt=Volts*Amps.

The continiously increasing number is Amp hours or milli Amp hours used or drawn from your LiPo. If you used 500mAh from your 2000mAh LiPo you've used about 25% of its capacity.

The wattmeter can be a useful tool to use when charging (to see how much you get back into the LiPo) too, just put it between charger and LiPo.

May 12, 2008, 05:58 AM
Proud member of LISF and ESL
NoFlyZone has given you an excellent response and good advice.

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Help me choose a power system for a pattern plane. JustFly Power Systems 1 Jul 08, 2006 04:59 PM
HOW and WHY do you choose a power system for an aircraft? (Antic e-conversion) steve crewdog Power Systems 17 Jun 20, 2005 01:54 PM
Poll: MP-AP Power System and how many flights? thinair7000 Aerial Photography 0 May 24, 2005 03:27 PM
Question Steve Nolan (eBird) -- Please help me choose a brushless power system Red 61 Aerial Photography 52 Jan 19, 2005 10:38 PM
How about a power system database? Mitch G Site Chat 21 Jan 24, 2003 08:55 AM