Watts per pound for Giant Scale Models?

[Splummer]

Watts per pound on Giant Scale models seems to be a great topic to discuss.

I believe the old school electric Dr. Keith Shaw method was:

50W/Lb = Scale or slow flight
75W/Lb = Sport and mild Aerobatic
100W/Lb = High performance and Fully aerobatic

What is the MODERN DAY rule of thumb for Giant Scale Electrics? I have my own opinions but want to get some other thoughts on the matter....

-Sean

[portablevcb]

Sean,

My only input would be that 50W/lb is high for slow flight. And, for a lot of scale models, slow flight is almost too fast :D

The good part is that there is a thing called a throttle that let's us fly in a scale manner even when we have too much power :)

charlie

[twest]

Any plane 20 onces or larger might be like this:

100 watts/lb: minimum for sport flying

150 watts/lb: solid conventional aerobatics, scale warbird (true scale P51 is 100 watts/lb output, most scale flyers like to have ample reserve power). Ok for ducted fan if hand launched.

200 watts/lb: No-limit FAI or 3-d flying, minimum for solid ducted fan performance (including ROG).

> 200 watts/lb: high speed, i.e. pylon racing, f3b, f5D, crazy folks.

[staggerwing]

This is an interesting subject to me as I try to define systems for other large aircraft I want to power & as well try to explain some of these basic parameters to non elec flyers in our club.
I've more or less gone by the basics Splummer has mentioned & will mention my latest endeavor, a 20.8 lb 96" Rearwin Speedster which is somewhat aerobatic & thoroughly enjoyable.
This power sys @ WOT pulls 1650 W @ 56 A @ 5100 rpm w/ a 22 X12 APCE wide blade using (2) 8C 6600 MAH 4s EP batts in series.
Fully charged @ 33.24 v, yesterdays (3) approx 10 min flights left 31.78 v remaining.
Elec power is not my forte & I'm still learning so this may or may not be the type of input you're looking for but the thrust from this set up certainly has me surprized as It will cruise scale like @ 1/4 throttle or will hover for a short time @ WOT.

Bob

[blucor basher]

I think we have to factor this for size in terms of acceleration. A certain number of watts per pound will get us a certian amount of acceleration. Acceleration is change is velocity per unit of time. Since we tend to perceive velocity relative to the size of an airplane (a vip racer doing 120 looks a lot faster than a full-scale plane doing 120 b/c the VIP is covering more fuselage-lengths per second) we find that we naturally think a shockflyer w/ 200W/lb. accelerates harder than a GS plane w/ 200W/lb. In reality, they are accelerating at approximately the same rate in feet/second^2, but the shockflyer reaches a greater number of "fuselage lengths per second" in the same amount of time. The apparent velocity of the smaller airplane is higher.

I thik this is why there is some discrepancy in what 3D pilots consider appropriate power/weight levels for GS airplanes. What we actually want is a certain number of fuselage lengths/second^2, and so if the fuselage is 2X as long on the bigger airplane, it will take 2X the watts/lb. to achieve the same *apparent* acceleration rate.

Looking forward to discussing this.

[BoneDoc]

Interesting thoughs. I would add, differing people have differing ideas of what a "rocket" is also. So in all fairness, we should really post videos of upward acceleration (if we're talking 3D here, which is what I mostly do :D), or top end speed (officially radar gunned), and then we'll be talking apples to apples, not apples to oranges.

[GWRIGHT]

Watts per pound? Why is that important? I've hovered a 48 ounce plane on 200 watts (66 watts/lb), and I've made it almost unflyable (not enough power) on 450 watts (150 watts/lb). 18X10 versus 5X5 prop geared highly and direct drive, on same motor. Niether was anywhere near optimal but it proved a point. Watts per pound doesn't tell the whole story.
Another example is a largish plane of mine, that originally flew on 900 watts and a 24X12 prop (9 lbs weight). Hover, and full 3D capable. Now I fly them on 2000 watts and a 22X10 (different motor and cellcount, can't use the larger prop on this motor). I thought it was fine on the original setup, but like it more on the higher power setup now. However, with more cells and watts, at 2600 watts, but a smaller prop (18X10),.. it's noticably worse than the 2000 watt and 22X10 setup.

Synopsis,.. you can't make a blanket statement about watts per pound being "good","bad","optimal". Watts don't fly planes, thrust and speed does.

[blucor basher]

Gary - Certainly, you are correct. For the purposes of simplifying concepts for discussion, can we stipulate that watts/lb. is of some value in determining performance capability? Watts and RPM are the only easy-measured values for individual GS modelers out in the field, who have no simple way to accurately measure thrust.

[GWRIGHT]

You're right,.. and watts per pound is relevant,.. I was just making a point that there are other items to consider. Let's just say,..for "standard prop sizes",.. watts per pound actually is relevant <G>.
I fly a 16 lb plane with 4200 watts now myself (250+ watts/lb :D )

[Jocke]

I fly by my old rule, for 3D and powerfull aerobatic 350-400w/kg (w/Ib ? ;)

"Watts don't fly planes, thrust and speed does"

As Gary said, correct prop is very importand, done some tests on this, once flew a 40size CAP with a 3blade 18x9" prop, very effiicient but the plane flew like &%¤# a few more tests on other planes and i realise that to big prop is no good, hardest thing for me is to find the right balance.

[GWRIGHT]

I know what you mean. You can certainly go too large also. It's really strange to test fly something that has a lot more thrust than weight, so it hovers fine,.. but the speed is so slow you can't fly forward out of hover <G>.

I fly by my old rule, for 3D and powerfull aerobatic 350-400w/kg (w/Ib ? ;)

"Watts don't fly planes, thrust and speed does"

As Gary said, correct prop is very importand, done some tests on this, once flew a 40size CAP with a 3blade 18x9" prop, very effiicient but the plane flew like &%¤# a few more tests on other planes and i realise that to big prop is no good, hardest thing for me is to find the right balance.

[KatManDEW]

I agree that watts alone don't tell the whole story. It depends on how effectively those watts are converted to thrust and speed.

Seems that 200 watts/pound used to be the rule for 3D, but it also seems like more than that is needed on larger planes. Maybe because large aerobatic planes don't fly fast in general, and we think we need more power to make them go faster. None of the big gas powered 3D planes I've seen fly, seem to be real fast. It's almost like they're in slow motion. Which I think is part of the attraction, because we have more time to think while flying them, versus a small plane that flys fast, and furious,.

I also think of the many glow powerd planes I've seen with nowhere close to enough power to hover, but they go like stink - very fast.

The watts need converted to thrust, and speed, with a balance between the thrust and speed that suits you. Seems to me that if you want thrust and speed, then your wattage requirement will be greater.

[GWRIGHT]

Very Very easy to get lots of thrust from very few watts.You can gear to the moon and use a huge prop to your advantage.However, to get speed requires a lot of watts.

[KatManDEW]

Very Very easy to get lots of thrust from very few watts.You can gear to the moon and use a huge prop to your advantage. However, to get speed requires a lot of watts.

Translating "backwards" :) from horsepower to watts, it seems like some of the glow planes are getting real good speed, with not much hover type thrust, from not all that many watts/lb (HP/lb). If they want clawing power in addition to speed, they need to add horses :)

Translating backwards, I've seen lots of glow powered planes go like stink with 100-120 watt/s per pound. They don't jump off the runway, but they go fast in the air. Seems like the same should be possible with electric power, but if we want speed and 3D thrust, we would have to add horses :)

[F1 Rocket]

So how many watts/pound for one of these? :)

http://www.aero-news.net/news/genav.cfm?ContentBlockID=3ee41f10-02b3-4067-aace-fd579501c254&Dynamic=1

[R/C Dallas]

I fly by my old rule, for 3D and powerfull aerobatic 350-400w/kg (w/Ib ? ;)

That would be 156-178w/lb Jocke. Myself,I have been using 10W/oz as a starting point for 3D. Should be fine on GS as well.

100w/lb vs. 200+w/lb is kind of like comparing a 1967 Shelby 427 Cobra to a pinto...both will use the same amount of horse power to go 60mph but getting there is another story. The pinto will take about 13 seconds to make it to 60. The lucky guy with the cobra can do it in 4. But here's the nice part...he can also do it in 13 if he wants too. All he has to do is use throttle management.

That is the rational for the majority of pilots going for higher power setups.

[Jocke]

That would be 156-178w/lb Jocke. Myself,I have been using 10W/oz as a starting point for 3D. Should be fine on GS as well.

100w/lb vs. 200+w/lb is kind of like comparing a 1967 Shelby 427 Cobra to a pinto...both will use the same amount of horse power to go 60mph but getting there is another story. The pinto will take about 13 seconds to make it to 60. The lucky guy with the cobra can do it in 4. But here's the nice part...he can also do it in 13 if he wants too. All he has to do is use throttle management.

That is the rational for the majority of pilots going for higher power setups.

I dont have to bother much about the Shelby or Pinto, i drive AlfaRomeo ;)

I often start with 350-400w/kg, but i always tend to go higher to test the limit, 1kw/kg is not realy needed in a 3D plane! i think.

[Bodywerks]

You're right,.. and watts per pound is relevant,.. I was just making a point that there are other items to consider. Let's just say,..for "standard prop sizes",.. watts per pound actually is relevant <G>.
I fly a 16 lb plane with 4200 watts now myself (250+ watts/lb :D )
You are very correct. My 35% electric is only about 180 watts/pound, but hovers just above half throttle and has TONS of punch out of a hover, initially. I say, initailly, because it reaches it's terminal vertical velocity quite quickly, and only goes vertical out of a hover at about 10-15 mph. It has unlimited vertical from straight and level flight, but it is only going about 30 mph on the vertical line. This is because it is a geared motor turning a 30X20 prop at only 4,000 RPM. The blade area is significantly greater than a typical 27X10 prop you would use on a 100cc engine in this plane, which likely accounts for the quick acceleration. The lower RPM, however, accounts for the slow vertical speed.
I would try to get a motor that turns a similar size prop (talking about giant scale only here) as it's gas engine counterpart would turn, and at a similar RPM, and shoot for a minimum of 180 watts/lb-to the prop, not out of the batteries-in order to get similar flight performance to its gas engine counterpart. This would likely translate to about 220-240 watts/lb out of the batteries.
I like the idea of geared motors turning HUGE props to get the max efficiency out of the powerplant, and thus having unlimited vertical with low watts/lb numbers, but I have found that the reality is that such a setup is not always the best setup, at least when it comes to aerobatics on a giant scale plane.

[Eagleburger]

For 3D I aim for 2:1 thrust and a pitch speed dependant on the plane. Shockies , 25-30mph .60 size :rolleyes:, 50mph.

I am building a 100in edge, what top speed should I be aiming for? It will be my biggest plane.

[Tipover]

I'm enjoying this discusion, having only dabbled with a 2000 wattish setup on an 8 lb airframe.

How do you guys think the feel of a big outrunner vs a geared setup compare to a traditional large IC engine? I'm gathering that the outrunner is the better simulation of a large gasser? And maybe present themselves as more dynamic in performance, with the tradeoff of being less efficient?

[edit] Just realized I probably should have posted this in the geared vs outrunner discussion. But the two discussions sort of run together. :)

Kevin

[BoneDoc]

Properly set up, electric is superior to IC. It's reproducible, has a higher throttle response, and not affected by conditions in the air. IC is cheaper, and properly tuned, very reliable as well.

[blucor basher]

Airframes last longer on electric as opposed to IC due to lack of vibration. They can also be designed lighter for the same reason.

[Tipover]

I understand electric is more efficient than IC. Just wondering if a big outrunner more closely simulates the throttle response and power dynamics of a large gasser, than say a more efficient geared setup. For converting traditional gas engine fliers, what setup do you think they would be most comfortable with, all costs aside?

Kevin

[blucor basher]

Well, personally, I think you'd have to really screw up any electric system to simulate the response of most IC engines.

The worst throttle response on any of my systems is on my Hyperion 90A ESCs. It's noticeably more lethargic than the Castle (but they have been LOTS more reliable than the Castles). However, it's still better than my gas engines. (note - I have found one gasoline engine manufacturer, Brillelli, which is really intertested in providing excellent throttle response in their engines. I recently received a video from them of a new 46CC engine that, in the video at least, has brushless-type throttle response. You can bet I'll have one soon.)

[Tipover]

So you feel throttle response is mainly an ESC issue? And for practical purposes our electric motors are linear devices?

Kevin

[blucor basher]

Feather and I were discussing this on the phone today. Here are my thoughts:

Most of us have experienced an esc "losing timing" (I'll leave it to the ESC experts to explain what is actually happening) when spinning big props on high voltage. It has to be a difficult trick to get a relatively small diameter ungeared motor to accelerate a large diameter, heavy prop. So, it seems that different ESC manufacturers take different approaches. Castle gives users several programming options to find optimum throttle response, just below the level of "losing timing" (or whatever it actually is), whereas the particular Hyp esc I mentioned has a very conservative setting programmed in from the factory. I never have been able to get it to "lose timing" even with really big props. But, it is noticeably lazy on throttle response - but still better than any of my current gas engines (and tons better than any glow I have ever owned).

[BoneDoc]

Blucor is absolutely correct on this one. When I was spinning Xoar 24x10, I would never loose timing, but on APC 24x12, I can get it to loose timing if I bump the throttle too fast.

[feathermerchant]

Well, personally, I think you'd have to really screw up any electric system to simulate the response of most IC engines.

One of the classic quotes!!

I have had to slow down the esc throttle response with my 5330/28 turning a 20X10 on 12S. That said I am probably overpropped :)

Anyway my understanding of brushless controllers is that they connect the battery to 2 motor wires at a time and measure rotor position by measuring the voltage of the unused wire. I don't know how advance is calculated but I think if you use too much advance, the motor could suddenly change speed or accelleration and cause the controller to lose synch. Losing synch is rally bad. I have had one controller catch fire and melt all teh components of the boards. Even the battery and motor wires. Exciting.

[bruff]

I use WATTS/LB as a guideline to make sure I'm going the right direction with my power system. I'm finishing up a TF Cessna 310. RTF weight SB 20 lbs. I'm using two Hacker A50-12L on 6S 5000 ma LiPo batteries each. Shooting for 990 WATTS per motor setup. That is 1980 WATTs total for the airframe. My WATTs/LB is 99 WATTS/lb. This should fly the Cessna very scale like. I'll know more after I run the system up and dial the props in. I used Vario 3 blade, variable pitch props. This lets me match both sides. 99 WATTS/LB will fly this plane very nicely.

[feathermerchant]

You'll also find that twins don't seem to need as many W/lb as singles do.
Don't know why.

[Tipover]

Feather and I were discussing this on the phone today. Here are my thoughts:

Most of us have experienced an esc "losing timing" (I'll leave it to the ESC experts to explain what is actually happening) when spinning big props on high voltage. It has to be a difficult trick to get a relatively small diameter ungeared motor to accelerate a large diameter, heavy prop. So, it seems that different ESC manufacturers take different approaches. Castle gives users several programming options to find optimum throttle response, just below the level of "losing timing" (or whatever it actually is), whereas the particular Hyp esc I mentioned has a very conservative setting programmed in from the factory. I never have been able to get it to "lose timing" even with really big props. But, it is noticeably lazy on throttle response - but still better than any of my current gas engines (and tons better than any glow I have ever owned).

Well, It makes sense that an electric motor can only accelerate a given load so fast with a given power source. So I'd agree these lose of timing issues are related to, too big of prop for a giving power system/advance setting. I’ve never had an issue with my Jeti Advance controller running a APEe 20x10 on my Hacker A50L 16, and I think that’s considered a pretty big load for this motor to start. I've even run a Zinger 20x10 bat with no problems :eek: Throttle response and pitch speed seem very close to my YS 110/ Master Airscrew 16x6 setup on the same airframe. Of course the electric setup with the larger prop disk has a much more locked in feeling for low and slow 3D.

Kevin

[Thomas B]

You'll also find that twins don't seem to need as many W/lb as singles do.
Don't know why.

It is the good old twin efficiency effect. Was/is the same with glow twins

Bascially, it works to prop area and thrust...two 10" props have an area of 157 square inches...about the same as a single 14" prop.

[SD_Raptor]

There is an important factor that I have not seen mentioned - drag! Say your plane is a 30% aerobatic plane - mixed IMAC and 3D flying. The amoount of power required depends a lot on the airframe - to respond the same in vertical or horizontal flight, a Yak-54 needs a different power setup than an Extra 260 or Edge 540. Even the choice of prop size and pitch depends on the airframe. AMA or FAI pattern planes will require a different setup for optimum performance.

The best advice is read as many different postings as you can find, visit as many flying fields as you can get to, get and learn to use a good motor performance calculation program, get an Eagle Tree or other in flight data logger - the only real means of learning exactly what the motor is doing in the air, and be will to experiment with different props, batteries, ESCs, etc until you find a setup you really like. Or, get to know a couple of other pilots who enjoy doing these things for other people.

Randy

[Sluf7]

Isn't watts per pound + what one has in Wallet to get the per pound motivation. :0)