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1/4 scale Pitt Special

Old 08-30-2011, 08:42 PM
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woodfit
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Default 1/4 scale Pitt Special

Hi All,
First post so here goes, I have at last come over from the dark side onto electric and have just finished converting my 1/4 scale AM Pitts Special. Dont know if she flies yet as I am awaiting batteries (5000MaH 6S 40c) and hopefully my watt meter should arrive tomorrow although I do have a borrowed 4000MaH 6s 25c battery (brand new) to possibly try the Pitts but I need some figures first, the set up is:
4000Mah 6s 25c
100 amp ESC (no bec)
Eflite 90 motor (wattage unknown)
16 x 10 prop

As yet no rpm figures but any comments welcome, I hace attatched a couple of pics of the battery compartment !!
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Old 08-30-2011, 08:56 PM
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Great spot for the hatch.
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Old 08-30-2011, 09:46 PM
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Hey Woodfit, welcome to Wattflyer. Nice plane ya got there. I've got a 27% Pitts M12.

What wet fuel engine did you have in that? What is the wingspan and weight on it?
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Old 08-30-2011, 09:53 PM
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Originally Posted by woodfit View Post
Hi All,
First post so here goes, I have at last come over from the dark side onto electric and have just finished converting my 1/4 scale AM Pitts Special. Dont know if she flies yet as I am awaiting batteries (5000MaH 6S 40c) and hopefully my watt meter should arrive tomorrow although I do have a borrowed 4000MaH 6s 25c battery (brand new) to possibly try the Pitts but I need some figures first, the set up is:
4000Mah 6s 25c
100 amp ESC (no bec)
Eflite 90 motor (wattage unknown)
16 x 10 prop

Hey, welcome to wattflyer!!!

Below are the specs on your Eflite motor:
http://www.horizonhobby.com/Products...90A#quickSpecs

Just ran the numbers on your motor through www.motocalc.com (free for 30 days, then $39). Motocalc predicts this motor will turn a 16X10 prop at about 6300 RPM, pulling about 46 Amps out of your 6S LiPo battery. That's only about 950 watts.

Problem is, even though the prop will put out about 150 ounces of thrust, that is only at a prop blade pitch of about 60 MPH. Don't know the wingspan, area, or weight of your Bipe to get any idea if that pitch speed of 60 MPH is safe or not.

The Eflite motor is rated at 50 Amps on an 8S Lipo. If your Lipo puts out about 3.5 Volts per cell at 50 Amps, that's 1400 Watts on a suitable 8S1P Lipo battery.

You might want to consider going to the 8s Lipo battery. Motocalc predicts that your motor will turn a 15X10 prop at 8200 RPM, putting out 200 ounces of thrust with a prop pitch speed of 78 MPH.

When playing with these higher powered electrics, the cost of the equipment begins to be a little important. Electric motors are DUMB. They will happily turn over a grossly oversized propeller. Until the smoke starts pouring out of the motor windings.

To prevent this from happening, you have used good sense to purchase one of those wattmeters such as Astroflights wattmeter. That way you can check your motor current, voltage and watts to make certain you're running a safe setup.

And you can determine how accurate the E-Flite 90 motor published specifications are, against the real world.

As for electrifying a "Mid power" electric model, take a look:

Thread on 70 size glow engine conversion to electric
http://www.wattflyer.com/forums/showthread.php?t=45222

One of my models is a giant scale 78 inch wingspan, 1200 square inch Extra 330. Power system is a Hacker A60-16M motor, 12S2P A123 battery pack and a 19X12 APC-E wide blade prop. That power system turns the prop at 7000 RPM and hauls the 19 pound model up at 45 degrees, til its out of sight.
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Old 08-30-2011, 10:19 PM
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woodfit
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Hi Guys
Many many thanks for the replies, it is appreciated,
The previous engine was a Evolution 26cc gasser which happily flew the Pitts at half throttle, the wingspan is 61" and I am oild the loading is approx 20 ozs per sq in (i think?) I might have got that wrong

I did think of putting on a 16 x 12 but will need some figures when I get my meter tomorrow to see what the 16 x 10 is producing (watts) I have also ordered a tacho to find the rpm then I can calculate the airspeed.
I am told the stall speed is 32mph and the 16 x 10 will happily take it to 50mph so some room for error.
Currently I have on order 6s batteries but may well order some 2s batteries(identical) and connect in series to give me 8s.
Obviously funds are crucial so hopefully for now the 6s batteries will fly her safely.
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Old 08-30-2011, 10:25 PM
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Sorry .....the all up weight is just under 10lbs
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Old 08-30-2011, 10:34 PM
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At 10lb she should be a 'floater'.. Motocalc predicts 23mph stall speed so your pitch speed should be adequate on the 16x10

A 16 x11 might work even better if you can get one but the 10" pitch should work ok.

Steve
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Old 08-30-2011, 10:40 PM
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woodfit
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Hi Steve
So if the calculations are correct at 950 watts the its literally 95 per lb which is adequate. I know she wont prop hang but it should be safe to fly her on those calculations, also when I get the tacho that should nail it Also do you think a 16 x 12 would be better??
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Old 08-30-2011, 10:56 PM
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A 16 x12 would probably be blade stalling so may not be as quick to pick up from rest when taking off... But it may be better in flight.. Probably some experimentation would be in order. I'm sure the 16x10 will get you in the air ok though and it wont be taxing your motor too hard.

It will be interesting to see what the wattmeter says.. i bet with a fresh battery you will be over 1kW which means over 100W per lb which is tons of power for a lightly loaded slow flying model like this.

Steve
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Old 08-31-2011, 01:32 AM
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Originally Posted by woodfit View Post
I did think of putting on a 16 x 12 but will need some figures when I get my meter tomorrow to see what the 16 x 10 is producing (watts) I have also ordered a tacho to find the rpm then I can calculate the airspeed.
I am told the stall speed is 32mph and the 16 x 10 will happily take it to 50mph so some room for error.
Currently I have on order 6s batteries but may well order some 2s batteries(identical) and connect in series to give me 8s.
Obviously funds are crucial so hopefully for now the 6s batteries will fly her safely.
One thing to keep in mind on these electric powered models, those motors are sort of a "Constant RPM" type of power plant. As the model gains speed in flight, the motor won't increase in RPM more than perhaps 10-15%. A gasser or a glow engine will wind out as the model gains air speed.

So, the bottom line, the electric models generally put out their maximum thrust, and horses at zero air speed. If your models stalling speed is 32 MPH, and the prop pitch airspeed is 50 MPH, that could be cutting it a little close.

I'd go the 16X12 pitch prop for first flights. I've got a 16X12 APC-E (Electric prop) on my Hacker A50-16S motor running on 7S2P A123 cells. It's pulling about 65 Amps at full throttle, a little over a Kilowatt. You can hear the prop stalling at full power during take off, but once the model starts rolling, that 16X12 prop really bites in and goes.

Do you have the total wing area??
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Old 08-31-2011, 06:13 AM
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Originally Posted by woodfit View Post
Hi All,
First post so here goes, I have at last come over from the dark side onto electric and have just finished converting my 1/4 scale AM Pitts Special. Dont know if she flies yet as I am awaiting batteries (5000MaH 6S 40c) and hopefully my watt meter should arrive tomorrow although I do have a borrowed 4000MaH 6s 25c battery (brand new) to possibly try the Pitts but I need some figures first, the set up is:
4000Mah 6s 25c
100 amp ESC (no bec)
Eflite 90 motor (wattage unknown)
16 x 10 prop

As yet no rpm figures but any comments welcome, I hace attatched a couple of pics of the battery compartment !!
Hi Woodfit and Welcome to Wattflyers, use a big seperate Receiver Battery Pack for the electronics on that plane, a UBEC will work, but a big battery pack IMHO will be better and maybe cheaper too Take care and have fun, Chellie

PS-- kyleservicetech AKA Dennis has a Married UBEC/Battery pack plans that you might be interested in, Get together with him on that setup, its neet.
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Old 08-31-2011, 07:46 AM
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Originally Posted by kyleservicetech View Post
If your models stalling speed is 32 MPH, and the prop pitch airspeed is 50 MPH, that could be cutting it a little close.
Yeah it would be if the stall speed really was 32mph, but wing loading indicates a stall speed of 24mph, which looks fine compared to a pitch speed of 60mph
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Old 08-31-2011, 05:42 PM
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Originally Posted by JetPlaneFlyer View Post
Yeah it would be if the stall speed really was 32mph, but wing loading indicates a stall speed of 24mph, which looks fine compared to a pitch speed of 60mph
Agreed, 32 MPH sounded a little high for a big Bipe with fairly light wing loading. That's higher than my giant scale Extra 330 with a wing loading of 38 ounces per square foot!
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Old 08-31-2011, 10:03 PM
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woodfit
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It definately has a wing loading of 20ozs per sq ft and the satall speed sounds like it should be about 26mph.....

Another point thats baffling me (just the one??) to calculate mph this is the way I figure it with a 16 x 10 prop..

6500rpm x 10 (prop pitch) = 65,000 ins per minute
65000 x 60 (mins in an hour) = 3900,000 ins per mile / 5280 ins in a mile =61.55 mph. (I think)

So my question is this .. those are the figures I calculated with a 16 x 10 prop at a speed of 6500 rpm. So if you put on a 15 x 10 prop which according to the above figures should in theory be better because the rpm will be higher and it should go faster but it wont!! so were are my calcs falling down.
I guess the watts and amps comes into play here.. Hopefully my watt meter will arrive tomorrow so I can get some figures.
Tom
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Old 08-31-2011, 10:44 PM
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Woodfit,

Nothing wrong with your calcs.. In theory a 15x10 would have a slightly higher pitch speed because it would rev a little higher. But the thing with brushless electric motors is that even with no prop at all the motor will only rev at its kv rating multiplied by the voltage, so electric motors are very much rev limited. Reducing the prop size will therefore only result in a very small increase in RPM.

The down side of the smaller prop is that thrust will decrease. Any plane will stop getting faster when the thrust it produces is equal to drag.. If thrust (at flying speed) is reduced then it is matched by drag at a lower speed, so despite the pitch speed being slightly higher the model will probably be slower with the 15x10. For 100% sure it's climb and acceleration will be much poorer.
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Old 09-01-2011, 12:21 AM
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Originally Posted by woodfit View Post
It definately has a wing loading of 20ozs per sq ft and the satall speed sounds like it should be about 26mph.....

Another point thats baffling me (just the one??) to calculate mph this is the way I figure it with a 16 x 10 prop..

6500rpm x 10 (prop pitch) = 65,000 ins per minute
65000 x 60 (mins in an hour) = 3900,000 ins per mile / 5280 ins in a mile =61.55 mph. (I think)

So my question is this .. those are the figures I calculated with a 16 x 10 prop at a speed of 6500 rpm. So if you put on a 15 x 10 prop which according to the above figures should in theory be better because the rpm will be higher and it should go faster but it wont!! so were are my calcs falling down.
I guess the watts and amps comes into play here.. Hopefully my watt meter will arrive tomorrow so I can get some figures.
Tom

Nope, your calculations are reasonable. But the 15X10 prop suggestion was based on using an 8S Lipo battery.

As always, check out those computer spreadsheets such as www.motocalc.com, a program that does all the number crunching for you. And also allows for what happens to propeller thrust as the model gains speed while in the air.
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Old 09-02-2011, 09:41 PM
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Ok the figures are in, the watt meter and the rev counter arrived, these figures are based on a 6s 4000Mah battery (Im still waiting for my 6s and 2s batteries to arrive) but based on the figures for the 6s whats your opinion??
Would she fly safely??

5600 rpm
35 amps
680 watts

The watts at initial start up were over 700 but they settled down to about 680 ..

Tom
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Old 09-02-2011, 10:17 PM
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Originally Posted by woodfit View Post
Ok the figures are in, the watt meter and the rev counter arrived, these figures are based on a 6s 4000Mah battery (Im still waiting for my 6s and 2s batteries to arrive) but based on the figures for the 6s whats your opinion??
Would she fly safely??

5600 rpm
35 amps
680 watts

The watts at initial start up were over 700 but they settled down to about 680 ..

Tom
IMHO, I'd not try it.

That's only 680 watts/10 pound model weight, or 68 watts per pound. And with the low RPM, your prop thrust is going to drop off quickly once the model gains speed.

Once you get the 8S battery pack installed, you will be good to go.

FYI, with these electric motors, going from a 6S to an 8S battery with the same prop will nearly double the watts input (and the horsepower) to the motor.

That's why it's critical to have access to a wattmeter to measure what's going on when trying different propellers and batteries.

It's interesting, www.motocalc.com suggests this motor with a 16X10 prop will turn over at 6300 RPM, pulling 950 Watts with a 6S Lipo. Makes me wonder if the motor specs on your motor are a little off.

(I ran into this previously with another Eflite 90 motor: http://www.wattflyer.com/forums/showthread.php?t=44686 Post #64)

One way to find out, is to take the prop off, put two pieces of white tape on the outside of your outrunner motor. Then run the motor at full throttle, and measure RPM with the pieces of white tape. Motocalc suggests it should turn over at 7200 RPM, pulling about 2.1 Amps.
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Old 09-02-2011, 10:25 PM
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I agree, thats way lower than predicted, very strange. The only solution I can see is to go to 8 cells. I'd guess that you will pull about 60A on 8 cells with the same 16x10 prop, which would give to plenty of power.

Good job you caught it, you would have struggled with that power level.
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Old 09-02-2011, 10:31 PM
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My sentiments entirely, I think its too close to call but I have had to get the figures, I have always been over cautious and dont rely on the fact that a little knowledge is dangerous....I need to know!!]

I think the white tape opn the motor is for the tacho??

Its a great little tacho only 10 off ebay!!

So is that whats the tapes for?
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Old 09-02-2011, 10:43 PM
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PS.. is your ESC and, if you are using one UBEC rated for 8 cells... Some are limited to 6
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Old 09-02-2011, 10:50 PM
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This is the spec for the esc, I dont think its suitable. what would you recommend??


<SPAN style="LINE-HEIGHT: 150%; FONT-FAMILY: 'Trebuchet MS','sans-serif'; FONT-SIZE: 14pt"><FONT face=Arial><FONT size=3>Weight: 85g
Size: 70x50x13mm
Cells: 2-7S Li Po
Max Current: 90A
Burst : 100A
No BEC
Timing: 1/7/15/30 programmable
PWM: 8/16K
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Old 09-02-2011, 11:00 PM
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The Castle Creations Phoenix ICE 75A would do the job nicely and handles 8 cells.. also has the advantage of data logging (RPM, Voltage, Amps, Watts etc).

There are no doubt cheaper options but Castle Creations are top quality ESC's and a nice model like this deserves decent gear.

Steve
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Old 09-02-2011, 11:09 PM
  #24  
woodfit
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Your not wrong there Steve .. thats the next buy.

Costing me a fortune at the moment, just got my wattmeter today and bought a Etronix 4 battery charger (still needs a psu) and of course all the 4 sets of batteries from HK.
But its all in a good cause, just cant wait to get her airborne but the time has to be right.
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Old 09-02-2011, 11:22 PM
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Originally Posted by woodfit View Post
My sentiments entirely, I think its too close to call but I have had to get the figures, I have always been over cautious and dont rely on the fact that a little knowledge is dangerous....I need to know!!]

I think the white tape opn the motor is for the tacho??

Its a great little tacho only 10 off ebay!!

So is that whats the tapes for?
Yup
Just put your model in bright outside light, turn up the motor, and aim the tacho at the little pieces of tape to get an RPM reading.
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