Do the Watts Per Pound Rules Apply to Large Airplanes?

[FlyingW]

All,

I observed that my smallest Speed 280 powered planes fly with authority on 20-25 watts per pound.

My mid size planes perform well at 50-100 watts per pound.

My largest plane with Astro 60 at 16 pounds can hardly climb at 50 watts per pound.

Is there some scaling factor that must be taken into account as planes get larger?

Thanks for your opinions,

Paul

[jrb]

Watts per pound don't fly a plane; its the prop.

A better prop (matched to the plane's mission) will make the most from the available watts.

Tell us more about you Astro 60 plane and set-up.

[tIANcI]

Watts per pound don't fly a plane; its the prop.

A better prop (matched to the plane's mission) will make the most from the available watts.

Tell us more about you Astro 60 plane and set-up.

JRB - that is something I have ALWAYS asked on RC Groups but no one wants to answer it and its always ignored. This is even more absurd when you talk about EDF, how can you use Watts/pound for something that is relatively inefficient compared with a prop. In the glow scene people still talk about pounds of thrust to pounds of plane.

FlyingW - for hanging and uplines you cannot get away from the 1.5:1 power to weight ratio but for general flying I suspect that you do not need as much as the plane gets bigger, the big planes actually float better than the smaller ones. Seen many 150 cc planes come in on a dead stick ... they float much better than 40 sized planes.

[R/C Dallas]

IMHO...the watts per pound formula as a general rule seems to work well in the larger planes. I have noticed a trend of more extreme W/Lb in the giant scale planes but I think it's more of a "Top Dawg" thing than a need to have thing. Some folks are going all out to show the gas guys that e-power is the way of the future.

I am looking to get 150-200watts per pound for my 31% Extra 300L that's under construction. I want to learn IMAC basic/sportsman routines and fly 3D as well. As long as I can match the right prop to the airframe and mission it should be good to go. Actually, I'll probably have two props for the two types of flying I'll want to do and will swap when necessary. That way I can get optimum performance in each situation.

[Airboatflyingshp]

Ivan Pettigrews Mini Cat PBY flys scale and ROW on two GWS geared units thats as low as 34W per lb and an 80plus inch wing span but 40 W per lb will do nicely as long as its a low wing load and it can fly slowly.
Even his sealand 480 will do some aerobatics on a couple of lowly geared S480 Jamaras and fly for 30 plus mins on Nimhs -his 120 inch mars and 100" solent come in at about 10lb and use the same set up but times 4. They also fly scale plus.

Light AUW and low wing loading and adequate amounts of thrust means size isnt the problem. Too much wt and too little wing area is what eats the power....try to fly on the wing not the prop...........unlike a hellicopter.
In the UK Tony Nijhuis has a 14' span Spruce Goose.http://www.largemodelassociation.com/waterpla.htm and also at http://www.westfieldmodels.co.uk/index_files/page0006.htm
try this for giant electric and cheap power wing load is glider like.

More links for Ivans stuff and waterplanes here http://www.rcgroups.com/forums/showthread.php?t=448080&pp=15

[jrb]

The right prop for the mission (read as thrust vs. airspeed), and enough watts is what it takes to fly a plane.

A 30% Cub doen't need as many watts (per whatever) as compared to 30% Extra.

An Astro 60 is a great motor; even better when geared -- turns a much bigger prop.

I'm guessing its being used as a direct drive so the prop is too small for the mission/plane.

[Jocke]

Watts per pound is a good general rule to get started in the conversion, but to fine tune and get best performance out of the powersystem one needs to consider the prop and other things to like motor eff.

[FlyingW]

The plane is a high-winged sport Cub/Kadet-like shape with 124" span, Astro 6011t, 2.86:1 belt drive, 32 cells IB3600 nimh, 22x12 APC-E prop. All up weight is between 16 and 17 pounds. Wing loading is about 20 oz/ft2.

My next experiment is to try a Zinger 22x14 prop for more pitch speed.

[jrb]

Don’t do Zinger!

At least consider my experience 1st; the worst prop I ever tried on my 80” 9+# Cub – almost lost her on take-off. Never flew that prop another time. Normally, I can’t tell too much difference when making a 1” change between APCE’s; but that Zinger 15x10 ate the amps w/o giving me the thrust (had been flying a 15x10E).


I was actually looking for slower pitch speed.

I couldn’t do any slow fly bys with my Cub; but could nearly hover – go vertical anyways.

I finally got the slow put/put approach speed I wanted by going with a 15x8E, and made it look like wood with a Sharpie!


Here’s the APCE that are in the range of you current prop – is it about 27amps @ WOT?
LP20015E
LP21012WE
LP21013WE
LP21014E
LP22010E
LP22012E
LP22012WE
LP24012E

These are not very deeply pitched prop; so the loose thrust with air speed a lot quicker than a square prop – 1:1.


My calcs say you’ll go near 34amps with the Zinger; my thinking would be to go with an 21x12E – few more revs & velocity for the same 27(-)amps.

A 20x15E gives some interesting calcs; still near 10# of thrust w/60mph on a couple of fewer amps! And its P/D = .75

[FlyingW]

Jrb,

Thanks for the advice on props. I like the 21x14E as an alternative. I really wish APC made a 22x14 or 23x14 - I believe that those would be perfect.

Thanks,

Paul

[peterangus]

The "watts per pound" numbers do not change with the size of the model, but they do change with the wing loading.

Look at http://www.rcgroups.com/forums/showthread.php?t=627924

[huckinstix]

what size plane you talking baout? my buddy has about 3700 watts in his 30% extra, so i guess it does matter.

[R/C Dallas]

what size plane you talking baout? my buddy has about 3700 watts in his 30% extra, so i guess it does matter.

What motor is your buddy using?

[Dennis C]

Check out post 16 in the thread linked by peterangus. it is as close as you are going to get for a rule of thumb.

It links weight, wing loading and the type of flying you intend to do with the WATTS per lb.

by the way it says that 50 watts per lb is enough to take off, about what you have

Dennis

[FlyingW]

I am unable to locate a suitable brushless equivalent for my Astro 60 and belt drive. I need at least 22 ounces for nose weight, and a maximum of about 2" wide to fit into the drive. Even the larger outrunners cannot spin a 24" prop without pulling 70 amps.

What ever happened to Astro's proposed "60" size brushless?

[Jocke]

What size of outrunner have you been looking at for 24" prop?

[Airboatflyingshp]

Have you checked the geared outrunner combos offered by MEC........ they will swing those sort of sizes?

[FlyingW]

I looked at the Hacker A60-18L. About 50 amps with a 24" prop and 10s A123 cells.

The large MaxCim has the right performance, but weighs in at 45 ounces.

I'll look at the MEC geared outrunners - sounds interersting.

[airorv]

FlyingW,

I feel your pain. I too have an Astro 60 11T 2.75 SuperBox and I cannot get the setup right either. Last year I flew it with a 20x15 APCe on 36 3300mah nimhs and then I tried with a 12s 3100mah lipo. Of course the lipo was better but in my case I feel like I don't have enough pitch speed. I have to fly at close to full throttle all the time. My plane is a 1/4 Scale Sig Clipped Wing Cub. I am looking to increase RPM. With the 36 nimhs I am only pulling 27 amps so there is room for a higher voltage if possible, or a lower number gear ratio, like 1.5 to 2.0

If I cannot get the power with a 20 inch or smaller prop I think I will try an AXI 5330 on a 10s pack of A123 cells.

Hope this helps

[Airboatflyingshp]

The other option is to use a varable pitch prop set up........................... just thinking?

[tIANcI]

I think this next question will be interesting as it suits the topic. Many people talk about W/Lb. 100W/Lb is nice for sport flying and 150W/Lb is a good start for 3D ... some use this for EDF ... how can that hold true when a DF by nature is not as efficient as a prop. Am I mistaken?

[flieslikeabeagle]

I observed that my smallest Speed 280 powered planes fly with authority on 20-25 watts per pound.

My mid size planes perform well at 50-100 watts per pound.

My largest plane with Astro 60 at 16 pounds can hardly climb at 50 watts per pound.

Is there some scaling factor that must be taken into account as planes get larger?
Paul
YES! There is! The scaling factor is, in a nutshell, the stall speed of the model. Bigger and heavier models will usually stall at higher speeds.

Here's the essential physics: power = force x velocity. In the case of our model aeroplanes, the force is the thrust available, the velocity is the flight speed of the model, and the power is the power delivered to the air, after allowing for propeller and motor efficiency.

You can rearrange that equation in this way: thrust = power/velocity. Now, here's where the model size comes in: a well matched power system will typically deliver a pitch velocity of two to three times the stall speed of the model. Raise the stall speed of the model in any way ( i.e bigger/ heavier/higher wing loading), and you have to increase the pitch speed. Raise the pitch speed, and you lose thrust. So you need more power for the same performance.

I wrote a (free) motor/prop/gearing estimator called WebOCalc that shows this effect quite well. You can find WebOCalc at my RC Groups home page, http://flbeagle.rchomepage.com . Click on Software, then WebOCalc.

Here is a previous post of mine on this same subject, which includes a couple of screenshots of WebOCalc showing the effect of pitch speed on flight performance. The screenshots compare two models of equal weight and power, but one with a higher stall speed; it necessarily has weaker performance.
http://www.rcgroups.com/forums/showpost.php?p=7441003&postcount=612

-Flieslikeabeagle

[dusty IV]

Check out www.members.cox.net/moorman1/apr.htm for a cubic wing loading rather then standard wing loading. This compensates for different airplane size. Reynolds in action.

Multiply the motor displacement by 1.75 x 1000 as a ball park figure for determining electric power. ie .60 x 1.75 x 1000 = 1050 watts or vice versa for your needs. Some claim 1.5 is the magic number others claim 2 is the multiplier to I split it.

Never have figured props out so I buy a bunch and play around.

For big electrics the new 123 is the way to go.

[flieslikeabeagle]

Check out www.members.cox.net/moorman1/apr.htm for a cubic wing loading rather then standard wing loading. This compensates for different airplane size. Reynolds in action.
You may be interested to know that WebOCalc includes the effects of cubic wing loading. :)

Early versions of WebOCalc used plain old wing loading (not cubic wing loading), which worked fine for predicting the performance of my parkflyers. Then someone used that early version of WebOCalc to predict the performance of a gigantic 25-foot wingspan pink foam airplane built by a creative and crazy engineering student from Wisconsin. The WebOCalc prediction was so far off that it was just mortifying!

I found out about cubic wing loading while searching for the reason why WebOCalc had failed so badly on the big model. After I incorporated its effects, WebOCalc now does a very acceptable predicting performance of models from 1 oz indoor models to the 25-foot pink foam floater.

-Flieslikeabeagle

[flieslikeabeagle]

Never have figured props out so I buy a bunch and play around.

This is surprisingly hard. So many factors come into play - the battery voltage, gear ratio if any, motor Kv, and the desired pitch speed, for starters. Beyond that, motor resistance and idle current, speed control resistance, battery internal resistance, temperature coefficient of all these resistances - it gets very complex very quickly if you want very accurate results!

WebOCalc also calculates propeller sizes for you, and can get you pretty close very quickly, though it does not attempt to get the highest accuracy possible in order to minimise complexity. Final fine-tuning in the real world by flight testing and static current measurements is always a good idea.

Here's a WebOCalc screenshot (parameters are for a North East Sailplanes AcroPhat).

-Flieslikeabeagle

[peterangus]

If you want to scale-up [or down] in a way which results in the same appearance in flight [ie. speeds in proportion to linear size] , this link offers some guidance.

http://www.peterangus.pwp.blueyonder.co.uk/model_to_model.htm