P68C Advice

[phild]

If you use these 1800 20c batteries you will probably be better off getting  four of a similar size that you can use in other aircraft and double them up as Gareth suggested. I recently purchased 4 flight max  2200ma 3s 20c from hobby city for about $11 each with postal cost of $26 total of $70 or about £45 at current exchange rates. We were talking of  a 40a max requirement, but in reality you will probably fly your twin for most of the time on less than 20a draw, so they will be ideal.  They are also a usefull size for other small to medium size single motor aircraft.

Phil

[Mike S]

Thanks Phil, Gareth suggested the same.

Something along the lines of a 3s 2200mah 25c that can be used in my blade 400 and my 3d 400.

Found some good prices on raj's website think it was around 19.99 per lipo and on on the hobbyking site.

May just buy a couple to start with and work from there.

[Mike S]

Well things have come together now and have almost finished the nacelles they are on and in place but need a little modding for the esc rx wires.

Now thinking about the rx and rather than using a Ar7000 I am going to use a Ar500 to save a little on weight?.

[Gareth]

Mike,

The AR500 will work find for this job, it wont allow you to do flaps but i dont think you would need then anyway.

I look forward to seeing this fly.

Gareth

[Mike S]

Well its nearly there just need a y lead, ar500 and the lipos.

I weighed the model at home before I started this and it come out approx 2.5lbs on standard cheapo scales without rx,  lipo and escs and now after adding the new motors, esc, rx and lipo (for weighing) it came out out at 2.76lbs or 1256g thanks to Gareth for weighing it with proper scales.

If I have done the math right on the wing loading using the website russ suggested in his post it comes out as follows:

One wing measured from wing tip to fuse is 24"/610mm so total wing span is 48"/1220mm

Wing cord at fuse is   7.5"/190mm

wing cord at wing tip 7.5"/190mm

Lift Coefficient I have not idea what this is?


Results are:

Lift Coefficient default value 1.0

Wing loading oz/sq.ft 17.72 / 54 g/sq.dm

Cubic loading 11.21 oz/cu.ft

Stall speed is 20.8mph / 33.5 km.h

[Russ]

Mike,

You need to recalculate your figures accounting for the area of the wing buried in the fuselage. Estimate/measure the root cord at the fuz centre line and the increase in wingspan. This should improve things a little.

Your current figures for cubic wing loading suggest performance bordering on sport/scale, but you'll need to factor in your watts-in/pound when you get a battery. You may also need to experiment with flaperons for landing  (assume this model has independent aileron servos) given your estimated stall speed.

The proof as always will be in the test flight  , but your estimates should not be far out.

[Mike S]

Thanks Russ....

Wing cord at the centre is 7.5" /190mm and the wing span at the fuse centre is 4" making my wing span 52"/1320mm

Based on these calcs it comes out at:


Wing loading oz/sq.ft 16.36 / 50 g/sq.dm

Cubic loading 9.94 oz/cu.ft

Stall speed is 20.0mph / 32.2 km.h


Each aileron is independent so yes I can us flaperons

Russ I'm not sure how you work out the watts in/pound.

My motors are both BRC A2212-10T 1400kv 140watts with 25amp speed contollers

Based on the leaflet it suggests

Prop 7x5
Amps 14
Volts 3s
watts 170

If you heard the sound of wind then that was when all this went over my head

[Gareth]

Mike,

The figures you have there are:

2.76lb's
2* 170w

So that gives you 126w/lb

At that figure you will have enough to prophang and go vertical.

Gareth

[phild]

With gentle use of the throttle you will have a good trainer performance, with a little more spirited use of the throttle you will have good sports performance, and with switch use of the throttle you will have 3d performance, depending on the aerofoil section.
The motors and props were bought on my advice, so I hope they will give you what you want, its just a matter of using your left stick to give as much or as little power as you want. The aircraft all up as I understand it is less weight than with the 400 can set up, so you will have a much better all rounder than before. Just be carefull you don't rip the wings off in full power climb outs from dives or loops. I am sure that your aircraft will be a joy to fly, so get it down the patch and lets see it before the winter sets in.

Phil

[Mike S]

I forgot to add props to the list of things needed,

So that makes it:

props x2
Lipos x2
Y-lead
Ar500 to replace Ar7000 (nicked from the Raptor)

I have reconnect the control surfaces back to the airframe and rehinged them now so thats done.

Next thing is to setup the flaperons on the tx as Russ suggested, something new to learn there.

[Russ]

Not sure if you will be able to set up flaps with the AR500 - depends which TX you are using. It is possible with a DSX9 - see this post, but I'm not sure if you can program it with a Spektrum TX.

Mind you, with your revised figures for cubic wing loading and stall speed, you'll probably not require flaps.

[Russ]

Russ I'm not sure how you work out the watts in/pound.

My motors are both BRC A2212-10T 1400kv 140watts with 25amp speed contollers

Based on the leaflet it suggests

Prop 7x5
Amps 14
Volts 3s
watts 170

If you heard the sound of wind then that was when all this went over my head

Mike,

You should check out the following article - 'Everything You Wanted To Know About Electric Flight'

It's all worth a read, but in particular page 39 - 'Sizing Power Systems for Electric Airplanes'
& page 27 - 'A comment on voltage versus amperage'

should start to give you some answers.

[Mike S]

Thanks Russ. A great read and have learnt some stuff about motor Kv and prop to motors matches. But I have only touched the surface but very good.

[Mike S]

Well Its 99.9 done. New props are on and seem balanced. Flaperons set up with two postitions at the moment 30% and 50% Is this any good?.

Russ I did some research and it can be set up on a Ar500 but you use the Gear channel instead of the second ail port and some nifty setup on the TX.

Picked a Overlander lipo 3s 2200 25c (up to 35c) today for £18.00 so need to work out the flight time?.

So some more practice then flying the 3D 400

[Russ]

Good to hear you have the flaperons programmed - they may come in useful. 

When setting up any electric model, especially TX mixes, I strongly recommend having the prop(s) removed.

Next step is to weigh the complete model with everything (including the battery) installed = total flying weight.

Assuming you have a watt meter, you then need to fully charge the battery and static test your setup. This should give you a peak watt figure and peak current draw. You can then work out your watts/per pound.

As for flight time, working on the 80% (of total battery capacity) rule  for good lipo management, you have 2200 * 0.8 = 1760mAh of juice.

If you divide this figure by your peak current draw (both in amps) you'll have an estimate for flight time at WOT (wide open throttle).

Lets say your peak amps are 40,

1.76/40 = 0.044 hours or 0.044* 60 = 2.64 mins.

Alternatively if you aiming for a flight time of 7 mins you would need to manage the throttle so that your average current draw

= (1.76*60)/7 = 15A