Feb 10, 2012, 10:06 PM
Gopher huntin' stick jockey
East Bethel, MN USA
Joined Jul 2009
12,468 Posts
Quote:
 Originally Posted by electricrc68 the thing is....\$70 is a lot to me since im still under the driving age.
OK. That makes sense!

Joel
Feb 11, 2012, 01:09 AM
Registered User
United States, CA, Ridgecrest
Joined Dec 2011
38 Posts
Quote:
 Originally Posted by turboparker Scale speed doesn't work that way...
I respectfully disagree: if you place a 1/2 scale plane 1/2 the distance from you as a full scale plane they'll look the same size but to subtend the same angle in the same time the 1/2 scale plane has 1/2 the distance to travel and must go 1/2 the speed to appear to match the full scale one.

However, I completely agree with you that if the power loading is insufficient to do the kind of aerobatics the real one can then that's a drawback. The full scale Spit has a power loading of 0.34 W/gm. The PZ micro Spit is 46.8gm so that's 16W or 4.3A which, with their 150mAh battery, would only be 2 minutes. So you're probably right.
Feb 11, 2012, 02:10 AM
Gopher huntin' stick jockey
East Bethel, MN USA
Joined Jul 2009
12,468 Posts
Quote:
 Originally Posted by TP715 I respectfully disagree: if you place a 1/2 scale plane 1/2 the distance from you as a full scale plane they'll look the same size but to subtend the same angle in the same time the 1/2 scale plane has 1/2 the distance to travel and must go 1/2 the speed to appear to match the full scale one. However, I completely agree with you that if the power loading is insufficient to do the kind of aerobatics the real one can then that's a drawback. The full scale Spit has a power loading of 0.34 W/gm. The PZ micro Spit is 46.8gm so that's 16W or 4.3A which, with their 150mAh battery, would only be 2 minutes. So you're probably right.
Actually, it's not a linear relationship. Scale speed works like this. From Understanding Scale Speed - by Bob Boucher:

'Suppose our full size cub flies at 60 miles per hour or one mile per minute. We want our quarter scale Cub to fly one quarter scale mile in one quarter scale minute. Since in quarter scale distance is divided by four and time is divided by two our model will fly at one half the speed of the full size Cub or 30 miles per hour. Like scale time scale speed varies as the square root of the scale factor. This brings us to rule 6.

Rule 6. To fly at scale speed our models need to fly slower than the real airplane by an amount equal to the square root of the scale factor.
'

So, I am right about the 1/4-scale plane flying at 1/2 scale speed, since the square-root of 0.25 = 0.5. But we are both way off regarding scale speed at 1/28 scale:

1/28 = 0.0357 Square-root of 0.0357 = 0.189

320 MPH x 0.189 = 60.48 MPH

To fly scale speed, the little Spit would have to be going 60 MPH on that WEP pass!! We'd need the Sbach's 2500Kv motor on 3s & a semi-symmetrical wing for that. And very good eyesight!

Interesting calculations on power loading. The 8.5mm motor & gearbox they're using tops out at about 1.5A or so @ WOT in the Sukhoi XP, P-51, F4U, etc. That works out to be a bit under 6 watts,which works out to be 50-60W/pound.

Electric flight performance guidelines:

• 50-70 watts per pound; Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models
• 70-90 watts per pound; Trainer and slow flying scale models
• 90-110 watts per pound; Sport aerobatic and fast flying scale models
• 110-130 watts per pound; Advanced aerobatic and high-speed models
• 130-150 watts per pound; Lightly loaded 3D models and ducted fans
• 150-200+ watts per pound; Unlimited performance 3D models

It will be interesting to see how the four-blade prop works out. As a rule, scale multi-blade props are less efficient than correctly-sized two-blade props. I hope they've found the magic formula for a four-blade prop that is a good match for the 8.5mm motor.

Joel
Last edited by turboparker; Feb 11, 2012 at 02:29 AM.
Feb 11, 2012, 05:48 AM
Kit Manufacturer
United States, MA, Southbridge
Joined Feb 2010
3,577 Posts
Quote:
 Originally Posted by turboparker Exaggerating things just a bit, I see. Flying a 2s UMX plane surely does not require a large bank account. The difference in price between the Spit & the AS3X-equipped, 2s brushless Gee Bee or UMX Mig 15 EDF is \$70. That's like what - just over a tank of gas for the car? An evening out with the better-half? Surely, those are not things that require a large bank account. I simply stated my opinions as others have done. I am not fond of using cheaply-made, intermittent-duty pager motors with inconsistent output as powerplants. And, I prefer warbirds that fly like warbirds - not like sport-planes. One of the main reasons I fly warbirds is to sharpen my flying skills. I find no challenge in flying a warbird that behaves like an aileron trainer or sport-plane. Also, when I'm flying a fighter, I want to be able to fly it like a fighter. Which means having the power & speed of a fighter (in scale terms, of course). I'm more of a function over form guy with most things in life. First, get the function part right - then work on the look. To me, having the look without the function is pointless. Joel
Another thing to consider if that is your desire is that you might not be the primary target audience for the plane. You might be happier with the larger 3s size warbirds. my feeling is that these are set up for the intermediate pilots stepping up from Champs and Embers to something a bit better and faster. Not every plane is for every pilot and so there are plenty of different types, UM versus UMX. YMMV
Latest blog entry: And into the winter season...
 Feb 11, 2012, 08:14 PM Registered User Ft Lauderdale, FL. Joined Jan 2007 2,254 Posts Man you guys are pulling out all the stops
Feb 11, 2012, 08:33 PM
Kit Manufacturer
United States, MA, Southbridge
Joined Feb 2010
3,577 Posts
Quote:
 Originally Posted by Machy2k Man you guys are pulling out all the stops
slow news day
Latest blog entry: And into the winter season...
Feb 11, 2012, 08:43 PM
Gopher huntin' stick jockey
East Bethel, MN USA
Joined Jul 2009
12,468 Posts
Quote:
 Originally Posted by coreman Another thing to consider if that is your desire is that you might not be the primary target audience for the plane. You might be happier with the larger 3s size warbirds. my feeling is that these are set up for the intermediate pilots stepping up from Champs and Embers to something a bit better and faster. Not every plane is for every pilot and so there are plenty of different types, UM versus UMX. YMMV
I'm not interested in a 3s warbird that I have to go to the club field to fly. I already have 60-size glow-powered warbirds to fly field. I am, however, very interested in a 2s UMX warbird with true warbird performance that I can fly in the yard.

At any rate - I was simply stating my opinion - no different than others have done.

Joel
 Feb 11, 2012, 10:04 PM Horizon Hobby Employee Mahomet, Illinois Joined Aug 2007 2,297 Posts Joel, Bless you for championing the "scale speed = square root of the scale factor" argument! David
 Feb 12, 2012, 08:21 AM Registered User yorkshire UK Joined Oct 2007 4,685 Posts I'm glad I decided that I just like powered gliders best now and don't feel I need to have every new plane that comes out, but a nice spity that is and I might be tempted yet. I used to fly my little planes at the park while my daughter played on swings but she is older now and I can go down fields and fly my gliders, but with another kiddy on the way now I think the micros will be coming back again and a spitfire would suit me just fine, shame it's not brushless though----come on parkzone who wants a crappy brushed motor in their planes these days?
Feb 12, 2012, 09:06 AM
Gopher huntin' stick jockey
East Bethel, MN USA
Joined Jul 2009
12,468 Posts
Quote:
 Originally Posted by Dave Eichstedt Joel, Bless you for championing the "scale speed = square root of the scale factor" argument! David
David,

Thanks for noticing!! I was beginning to wonder if anyone else cared. Scale speed seems to be one of the least-understood concepts in the hobby.

BTW - with the flurry of recent release announcements, you guys must be burning the midnight oil! I look forward to your next creation! PS: Hope you do a 2s brushless UMX P-47 at some point. A semi-symmetrical airfoil & flaps would be very welcome additions!

Thanks for your dedication to our hobby!

Joel
Feb 12, 2012, 09:07 AM
The Sweet Aroma of 92 Octane
United States, PA, Downingtown
Joined May 2011
4,411 Posts
Quote:
 Originally Posted by nigelsheffield I'm glad I decided that I just like powered gliders best now and don't feel I need to have every new plane that comes out, but a nice spity that is and I might be tempted yet. I used to fly my little planes at the park while my daughter played on swings but she is older now and I can go down fields and fly my gliders, but with another kiddy on the way now I think the micros will be coming back again and a spitfire would suit me just fine, shame it's not brushless though----come on parkzone who wants a crappy brushed motor in their planes these days?
i dont really know much about motors, so what is the issue with using a brushed motor?
Feb 12, 2012, 10:18 AM
Way to many airplanes!
Joined Oct 2009
5,489 Posts
Quote:
 Originally Posted by electricrc68 i dont really know much about motors, so what is the issue with using a brushed motor?
Short life! The friction on those little brushes kill them relatively quickly. On average, you get 100 fligths. Sometime up to 300 flights, but also sometime less than a 100. So you replace them and keep going. Brusless motors last a VERY long time unless you drive them too hard (too much current).

It doesn't mean brushed motors are a bad choice... Most UM airplanes do not last 100 flights and quite often get destroyed before the original motor die! This keep the cost down, and so far, there's not really any good brushless motor that work on 1S battery anyway (that doesn't cost 1/3rd of the price of the whole airplane)
Feb 12, 2012, 10:24 AM
Gopher huntin' stick jockey
East Bethel, MN USA
Joined Jul 2009
12,468 Posts
Quote:
 Originally Posted by electricrc68 i dont really know much about motors, so what is the issue with using a brushed motor?

Brushed motors are typically less efficient than well-designed brushless motors. A brushed motor with a given output power will be lot heavier than a brushless motor of similar output. Brushless motors usually last far longer than brushed motors, and their power output remains constant over their lifespan.

Here's the deal with these little motors. The 8.5mm brushed motors used in these UM planes are called 'pager' motors. They are not intended to be used as powerplants. Rather, they are designed for intermittent, very low duty-cycle applications - such as vibrating pagers & cellphones, operating DVD drawers, changer carousels, and the like. Their brushes are very delicate, and they used brass bushings instead of ball-bearings.

Using them as aircraft powerplants is about as close to the polar opposite of the manufacturer's intended use as one can get. Because of this, we have seen significant variability between samples. Reliability is all over the map. Some last for hundreds of flights, some die right away, while most fall somewhere in-between,. Also, we've seen ~20% variability in power output between samples. That's not a big deal when you're vibrating cellphones or operating DVD drawers. However, a 20% difference in power is a very big deal in an aircraft.

The small brushless motors are designed for use as powerplants. Since there are no brushes or commutator strips to wear out, their output power does not diminish over time. Power output between samples is also far more consistent than it is with the cheap pager motors. They also use ball-bearings instead of bushings - which contributes to their long life.

Joel
Feb 12, 2012, 01:01 PM
The Sweet Aroma of 92 Octane
United States, PA, Downingtown
Joined May 2011
4,411 Posts
Quote:
 Originally Posted by RealGambler Short life! The friction on those little brushes kill them relatively quickly. On average, you get 100 fligths. Sometime up to 300 flights, but also sometime less than a 100. So you replace them and keep going. Brusless motors last a VERY long time unless you drive them too hard (too much current). It doesn't mean brushed motors are a bad choice... Most UM airplanes do not last 100 flights and quite often get destroyed before the original motor die! This keep the cost down, and so far, there's not really any good brushless motor that work on 1S battery anyway (that doesn't cost 1/3rd of the price of the whole airplane)
Quote:
 Originally Posted by turboparker Adding to what RG posted.... Brushed motors are typically less efficient than well-designed brushless motors. A brushed motor with a given output power will be lot heavier than a brushless motor of similar output. Brushless motors usually last far longer than brushed motors, and their power output remains constant over their lifespan. Here's the deal with these little motors. The 8.5mm brushed motors used in these UM planes are called 'pager' motors. They are not intended to be used as powerplants. Rather, they are designed for intermittent, very low duty-cycle applications - such as vibrating pagers & cellphones, operating DVD drawers, changer carousels, and the like. Their brushes are very delicate, and they used brass bushings instead of ball-bearings. Using them as aircraft powerplants is about as close to the polar opposite of the manufacturer's intended use as one can get. Because of this, we have seen significant variability between samples. Reliability is all over the map. Some last for hundreds of flights, some die right away, while most fall somewhere in-between,. Also, we've seen ~20% variability in power output between samples. That's not a big deal when you're vibrating cellphones or operating DVD drawers. However, a 20% difference in power is a very big deal in an aircraft. The small brushless motors are designed for use as powerplants. Since there are no brushes or commutator strips to wear out, their output power does not diminish over time. Power output between samples is also far more consistent than it is with the cheap pager motors. They also use ball-bearings instead of bushings - which contributes to their long life. Joel
ah okay thanks. i understand now. i just never knew the advantage of brushless, except for the obvious power increase. probably since i haven't had a brushed motor go bad yet. but thanks for the info.
 Feb 12, 2012, 01:17 PM Have fun Joined May 2007 6,236 Posts Electric, youll know when you get one. A geared brushed motor sounds/feels dirty to me. So why do i keep buying them