Question Electric power to weight ratio

[adam_one]

Hi,

Is there any rule of thumb for how many Watts per weight is recommended for:

1. sports plane
2. aerobatic plane

thanks

[steve lewin]

See Electric Flight FAQ http://www.ezonemag.com/pages/efaq.htm#design1

Steve

[adam_one]

Thank you Steve for your quick reply

[vintage1]

For can motors, 35W/lb will get you in the air - just. 50W/lb gets you flying with decent climb rate. 75W/lb will either net you a very respectable rate of climb, or warbird speed, but not both.

100W /lb will get you to near vertical climb rates or exteremly fast, but not both.

125W/lb gets you into hovering, very high climb rates, OR very high top speed.

150W per pound is 'ballistic'; and you can get decent top speed and full vertical, or trade one for the other. and get massive top speed but without unlimited vertical.

Around 200W/lb takes you into glo territory


My calcs suggest that 200W/lb is achievebale for a pack and motor using Nicads, without the airframe, but with latest generation LIPOS, you can do even better so an overall 200W/lb in terms of AUW is achievable with carefully matched LIPOS and motors.

Slower models - parkflyers - don't need so much power to stay in level flight so these will actually fly on as little as 25W/lb for the slow stick types.

[tim hooper]

Originally Posted by vintage1

My calcs suggest that 200W/lb is achievebale for a pack and motor using Nicads, without the airframe...........

The F5B glider fliers bust that barrier with regular ease. In fact my own Bandit hotliner comes in 195 watt/lb, and that's not even remotely competitive! The downside though is that the motor duration suffers at these levels - about 2 mins burn per charge.

tim

[adam_one]

Thank you guys for your detailed information.
I guess that 200W during short time may easily be achieved with the right motor as I've heard that the Nicads can deliver up to 100C during very short time.
But of course, for longer flight times one has to choose a higher battery capacity, which means increased weight.

[plane_jon]

I have a Graupner 7.2 FG3 speed 600 motor with a gear ratio of 3:1and I am using 6 'C' size NiMh cells with a capacity of 2800mAh. If I want to use this combination in a trainer model ,what overall weight would be suitable for the full aero model to have a good decent trainer flight?? Thanks.

[Virgil Kee]

Whoa! So far, no replies on this thread have mentioned that these rules of thumb should be understood as WATTS OUT! if you can't find out the efficiency rating of your battery pack, it's best to assume no more than 60% for this rate. Example: assume a pack of 6-NiCds drawing 7A ..... 6V x 7A = 42 Watts, right? Yes, BUT, that's 42 Watts in!

To include the efficiency rating, multiply the 42W by the suggested 60%, and you arrive at only 26.2 WATTS OUT! This represents the actual wattage available to the motor!

This may appear a bit disturbing, but 60% is about the lowest eff. rating I've seen. Note that (Super High Discharge) Li-Polys from Kokam claim 90% efficiency!

Do you agree, Vintage 1? Your replies always seem the most trustworthy.

Virg.

[TheAeronut]

Really?? Battery efficiency?? When I read 6V and 7A at the leads from the battery, that IS 42 WATTS - there is NO such thing as battery efficiency involved at this point in the power cycle. Now, if you take out 1100 mAh, and go to charge it, you will probably have to put in more than 1100 mAh to achieve a fully charged battery - there is an efficiency factor there, but it does not affect the electrons that are flowing through the leads to the motor in flight.

J.P.

[adam_one]

plane_jon wrote:

I have a Graupner 7.2 FG3 speed 600 motor with a gear ratio of 3:1and I am using 6 'C' size NiMh cells with a capacity of 2800mAh. If I want to use this combination in a trainer model ,what overall weight would be suitable for the full aero model to have a good decent trainer flight?? Thanks

Since the normal NiMHs are not able to deliver as high instantaneous and continuous current as NiCads (I think the max recommended in order to not shorten their life is 2C continuous) you should choose a prop that wouldn't draw more than 6 Amps continuous in flight, which means about 42 watts.
So, my roughly weight estimation for your model should not be greater than 1.0 to 1.8 pounds dependig on the wing's airfoil and size.

virgilkee wrote:

Whoa! So far, no replies on this thread have mentioned that these rules of thumb should be understood as WATTS OUT! if you can't find out the efficiency rating of your battery pack, it's best to assume no more than 60% for this rate. Example: assume a pack of 6-NiCds drawing 7A ..... 6V x 7A = 42 Watts, right? Yes, BUT, that's 42 Watts in!

To include the efficiency rating, multiply the 42W by the suggested 60%, and you arrive at only 26.2 WATTS OUT! This represents the actual wattage available to the motor!

I guess that the efficiency you're referring to is the motor's efficiency, which should be at least 70% for the rule of thumb to be valid.
The batteries efficiency refers to how much energy you have to put in (charge them) and how much energy they put out (discharge) ratio.
For example, a NiCad that has a capacity of say 1000 mAh (able to deliver 1000mA during one hour) needs about 1h 25min to be completed charged from flat level at a constant charging current of 1C.

[adam_one]

Hi,
Another factor is the thrust that depends on the motor - prop combination and gives the weight to thrust ratio.
In level flight and at constant airspeed, lift is approximately equal to weight and thrust is approximately equal to drag.
During take-off and climb, the thrust required will be greater than in level flight but less than the weight.
The extra thrust required depends on the type of aircraft, type of airfoil and how fast one wants to climb.
But I still wonder if there's any recommended value for weight to thrust ratio of a sports plane. Any ideas?

Thanks

[steve lewin]

In fact the Watts/pound rules of thumb have always been clearly understood to be watts IN as very clearly stated in the FAQ. You can only measure watts IN and there's absolutely no point quoting figures that no-one can measure.

It is true that if you use inefficient cheap can motors you probably need a bit more power than the formula would suggest and if you use high quality brushless at around their optimum efficiency point you could get away with less.

Battery efficiency has absolutely nothing to do with it. We are already directly measuring the battery output. What you can't easily measure is the motor output. BTW can motors can easily run at well under 50% efficiency. That's why they get so hot when abused .

Steve

[tim hooper]

If we look back to Vintage1's original answer, he does start it with the phrase "For can motors....."

tim

[Ender]

Since he quoted only "Can Motors" it should be noted that Brushless motors are fully areobatic at a much lower rating.
An Axi 2208/34 drawing 60 watts on a 12 oz X3D is fully areobatic and capable of hovering as well. This would equate to 80 watts/lb.

Ender

[Virgil Kee]

I apologize for my previous (screwball) posting which mentioned "battery efficiency". I was duly shot down, BUT, as I cited in the case of Kokam LiPos, I've seen efficiency percentages included in vendors' copy describing their batteries.

Consider this from FMA Direct http://www.fmadirect.com/:

Kokam Super High Discharge LiPo Electric Pack 2000 mAh, 2 cell series (7.4V)........(spec......, spec.....,.Ratings: 15C / 90%). Just what is Tim Marx telling us?

Steve, thanks for pointing out the guidance on this subject in the FAQ. I now feel that I can rely on the 50 watts/lb. rule-of-thumb to achieve reasonable performance for general usage, only needing to determine watts in.

Virg.