Plan build 80" Corby Starlet

[Grahamd]

I've been absent from modeling for over 2 years, and the best way I know of getting back into something is to throw yourself into it big time, and the Starlet suits that object fine.

 First lets see what a Starlet looks like, yep its a small homebuild plane that is built the same way we build our models.

 

 The original plans (Traplet) were drawn by Dennis Tapsfield (now deceased), and are a bit aged in there design, so hope to make a few small changes as we go, the biggest being the use of a Large Brushless Motor, instead of the suggested 20cc petrol engine (or 1.20 4s), but more on that latter.

 

The first thing I like to do with plans is either get a copy, or in my case take them to pront-a-print and get them to scan them to uncompressed .tif on a cdrom. This has two advantages, one you can view the plans on your pc (at home or WORK) and second using software such as IsIplot you can print all or some of the area to standard A4 pages.

 

The Tailplane on the plan only shows one side (saving paper) but using IsIplot you can print both sides (I do this using the "mirror" optin on my printer) and stick them together using tape.

 

I also print out the parts I want to cut and using baking paper (from Wilkos) I trace and cut the parts I need.

 

Then its of we go with the tailplane construction (using lite balsa where possible to keep the weight down).

 

Then I sheeted the surface with 1/16" using small pvc bags (I got her indoors to make these) with lead shot inside to hold the balsa in place.

 

This is a photo of the full size, which looks like a large model....

 

So I looked at the above and made a few changes to the elevators, 1/ I only used the 4 ribs as per the full size, and 2/ I added some 1/2" strips front and back that I then scalped to shape.

 

Then using G10 I made some elevator horns (still got to drill them), one on each elevator.

 

If your still reading, next post I'll describe the electrics I'm going to use.

 

 

 

 

 

[Biggles' Elder Brother - Moderator]

Looks an interesting build Graham, I'll be sure to follow it.

 

BEB

[Reno Racer]

I'm following too, looks good..... right where's that Traplet site..

[Grahamd]

Had a few hours to kill whilst it rained , so I completed the second elevator. Now the whole thing is taped together so I can sand it as a complete item.

 

 

[Grahamd]

Ok, if anyones still around? I said I would talk about my electric setup;

 

I've spent many weeks browsing the  net and reading and asking to try and understand what seems to be a black art of understanding what electrics are about, and the following is what I have gleamed and understood, and I hope understandable to anyone else in a similar position.

 

POWER = WATTS
Volts = abbreviated as V.
Amps = abbreviated as A.
Watts = V x A (This is a measure of power ie the work of turning a propeller to move the plane. (746 watts equals 1 horsepower))

I found the following guideline on-line and is an indication of watts per pound for different types of flying

25 W/lb = minimum for level flight
50 W/lb = Trainer/Casual/scale flying
75 W/lb = Sport flying and sport aerobatics
100 W/lb = aggressive aerobatics and mild 3D, effortless loops from level flight.
150 W/lb = all out performance (or reasonable 3D).

200 W/lb = Unlimited highspeed vertical flight.

 

Assuming the Starlet comes out at 12lbs (original design weight was 11lbs, the then LMA limit) so 100 watts x 12lb = 1200 watts required.

 

Now working out the current required, if we use a 3 cell lipo, we have 11.1volts, so 1200 watts divided by 11.1v (Current = power divided by volts)ending up with 108.1 Amps !!!! so I increased the voltage to 4 cells (14.8v) now calculate 1200watts / 14.8v = 81 Amps, still too many, so 5 cells1200watts / 18.5volts = 64.8 Amps, a more realistic amount.

 

So if I could get a lipo that could supply 18.5volts and 65Amps for an hour I could fly full wack for an hour! But I can't, however I can get a 5 cell lipo (18.5volts) rated at 5000mah (5Amps) with a "c" rating (amount that can be obtained) of 20, that means 5amps x 20c = 100Amps but only for 3 minutes (60 minutes / 20). Now if I double up on the lipos (series) then I have 37volts (18.5v x 2), now 1200watts / 37volts = 32.4 amps, so 2 X 5 cell packs of lipos rated at 5000mah and a capacity of 20c will provide 37volts and 100amps still for only 3 minutes, BUT I now only need 32.4amps so 33amps/100amps = 3 times as long, ie 9 minutes of flying at full wack. The reality is the model should be lighter not heavier and no one flys at full throttle for the whole flight (do they?). So I need to order two 5000mAh 5S1P 20C (where 5000mah is 5amps, 5S = 5 cells (18.5volts), and 20C is the amount (AxC)).

 

After some help I found an XYH63-64 230KV 55A Outrunner motor

 

Kv : 230 rpm/V (the k or kv figure on a motor represents the speed of the motor)
efficiency current : 42A
Current capacity : 55A/60s
Input Voltage : 4-10 Lipo (or 14.8volts to 37volt)
Recommended prop size : 20x10 / 20x13 / 22x8

 

Because I could potentialy have 100amps I need to use a speedcontroller (esc) rated at a minimum of 100amps.

 

Hobbywing Pentium-100A-HV Speed controller esc
1.1 Output: Continuous 100A, Burst 120A up to 10 Secs.
1.2 Input Voltage: 2-12 cells lithium battery or 5-36 cells NiCd/NIMh battery.
1.3 BEC: None. (no bec means I will still need a battery for the reciever)

Well thats the theory, now I have the motor and the esc

 

Hope I haven't driven anyone away?

 

[Tim Mackey]

Hi Graham...only thing I would mention is that your powertrian "as designed" will only be pulling around 35 - 40A maximum, therefore you dont actually need such a large ESC.

Of course, you still need to find one that can handle the high input voltage, and having a large current headroom is no problem, other than the cost and small extra weight over say a 60A unit.

You may also like to consider a decent UBEC unit instead of the separate radio battery?

I take it you have done a few rough and ready estimates of pitch speed using that  230 Kv motor on 10s? I I reckon you will probably see around 35V not 37, and therefore with no losses the theoretical RPM would be around 8000. In practice, I usually get about 10- 20% less than spec, so this puts you at about 6500 - 7200 RPM.

Nice going so far

 

[Reno Racer]

With this size electric conversion, I'm defiantly watching this one. Good luck, easy sanding and hopefully not too many balsa knife finger cuts

[Grahamd]

Whahay fin and rudder now almost complete (just need to add some small sheeting to the front of the rudder, ala full size rudder), the fin was sheeted in 1/16" and the rudder open (will be covering in Solartex).

 

The rudder is over 18" long

 

 

 

[Ross Clarkson]

Wow, looking good Graham.

 

Following with interest.

[Grahamd]

I now have all the wing ribs, braces etc cut and ready to go, but I'm off on me hols for a week, so building the wing will follow;

[Klippy]

Nice post Graham, I'll join the lurkers!

[Ton van Munsteren]

Hi Graham,

 

I will be following your every step of the build.

 

Love those Corby's and have designed a model myselve for a lipo 4S pack and a wingspan of 1,3 mtr and am working on a plan for the magazine.

 

Here some pics

 

 

Ton

[Sheldon Holy]

Oh, probably a bit late, but the hole on the control horn should be over the hinge line, you get a much less sloppy linkage.

Nice build though!

Sheldon

[Sheldon Holy]

Another double post...

Edited By Sheldon Holy on 16/05/2010 15:07:42

[Grahamd]

Very interesting Ton, any more details ?

 

Sheldon, they are not yet fitted, it was just an fyi of what they will look like when completed.

[Dusty]

Nice work guys! I like the look of this aircraft

[bouncebounce crunch]

I see David Ashby has a starlet build how did you lot get on I am Very interested.

cheers, bbc.

[bouncebounce crunch]

I had a better look at David Ashby's build photos and noticed the text at the bottom, so we will see how Mr Ashby and Ton Van Munsteren, sorted things out This September; Can't wait !

bbc.