Electric plane to carry/launch 2M sailplanes

[aeajr]

I was reading about using a mount to carry 1-2M sailplanes on the back of a powered plane as a launching method. Here is what they are doing.
http://www.poboys.org/framepage.asp?frame=/tips/glider0.htm

Our field is restricted to non-fuel planes. We have permission to use and electric tug plane strictly in the context of aerotowing. The tug can handle up to about a 10 pound scale sailplane and up to about 4.5 meters. Beautiful to watch. I am about to put up my first; a Ventus 2C 3.6M

So I was thinking about the piggyback method for Spirits, Gentle Ladies, Aspires, and the like with a different electric "tug". This would be a new class of "towing" that we would need approval for but it might make a fun group project for the club.

Here are a few candidates for the carry plane. I would really appreciate any input you might have. These have suggested motors and battery combos. Would those be OK for this purpose?


J3 Cub - 72" wingspan, 690 sq. in. wing area, about 40" long and 7.3 lbs. flying weight
http://www.hobby-lobby.com/j3cub.htm

Taxi Cup II ARTF Trainer RC Model Airplane
63" wingspan, 47-1/2" long, 597 sq. in. wing area, 96 oz. flying weight
http://www.hobby-lobby.com/taxicup.htm


Kits Glow planes to be matched with brushless motors

Telemaster 40
73" wing span, 848 sq. in. wing area. 53-1/2" long fuselage
http://www.hobby-lobby.com/tel40gas.htm

Senior Telemaster RC Model Airplane Kit
95" span (almost 8 feet!), 1330 sq. in. wing area, plus 320 sq. in. lifting stab area. 63" long.
http://www.hobby-lobby.com/srtele.htm


Was thinking 12-16 NIMH 3300GB cells up to about 25 amps. These would be a lot cheaper than Lithium so I can buy cells and build 4-6 12-16 cell packs for around $150 rather than buying one or two lithiums of similar capacity for $200+ and a charging systems to feed them. Plus, I have two chargers that can handle 16 cell NIMH packs.

Would these motors be appropriate for the Telemasters, to carry up a 2M sailplane? I have no experience with brushless motors.

PJS 3D 1500 External Rotor Brushless
http://www.hobby-lobby.com/brushless_external.htm


AXI 2826 Outrunner Brushless Motors
http://www.hobby-lobby.com/brushless-axi2826.htm


Actro CL-8" External Rotor Brushless Motor
http://www.hobby-lobby.com/actro-cl.htm

Again, any advice or experience you might have would be appreciated. If you have done this kind of carry launch with an electric, I would love to know of your experiences.

[aeajr]

To expand on the power quesiton. I was thinking 16 NIMH 3300GB cells. This pack would weigh about 24 oz or about 1.5 pounds. I estimate draw at about 25 amps. NIMH would be a lot cheaper than Lithium so I can buy cells and build 4-5 16 cell packs for around $150. I have two chargers that can handle 16 cell NIMH packs.

Two lithiums of similar capacity (5S2P) for would be about $250+ and I would
need a charging system to feed them.

If I can get the carry plane down to 7 pounds and the sailplane at 2.2 pounds
that takes me to 9.2 pounds combined.

16 cells = 19.2V nominal X 25 ampls = 480 watts

On a 7 pound plane that would give me about 69 watts per pound =
sport/aerobatic plane

combined weight of 8.7 pounds = 52 watts per pound solid flyer - as I
understand it.

This is all theoretical at the moment, but I want to test my understanding.

At 52 watts per pound, would think that should give me enough power to make a
good carry. On reports I have read, I have seen good reports of easy flying
electrics at 35 watts per pound. Recommendations I have read have suggested
about 70 watts per pound for sport/aerobats and 100+ for 3D flight.

Combined wing loading will actually go down with the combined load. Drag will
go up.

More power is always better, but I am trying to get a solid balance between
power, performance and cost. Getting above 500 watts seems to drive up the
cost of the motor system by about 50%. I am not interested in aerobatic flight
performance for this plane. It will exist strictly as a carry plane.

Is my reasoning flawed? I really want your input. I have never done this
before, but wanted to share my reasoning.

I have only used brushed motors in my parkflyers. This is certainly no
parkflyer.

Would these motors be appropriate for the Taxi Cup II or the Telemasters
listed above, to carry up a 2M sailplane? Would you suggest a different type
of plane? I have no experience with brushless motors or planes of this size.

AXI 2826/10 Outrunner Brushless Motors
http://www.hobby-lobby.com/brushless-axi2826.htm
For 10 to 16 cells, 11x5.5 to 14x7 props. RPM/V: 870, Resistance: 42 mOhms, I Zero: 1.7A, Max. current allowable: 45A/30 sec., Max. efficiency current is 20 to 30A

AXI 2826/12 - Outrunner Brushless Motors
http://www.hobby-lobby.com/brushless-axi2826.htm
For 10 to 18 cells, 11x6 to 14x7 props. RPM/V: 730, Resistance: 62 mOhms, I Zero: 1.2A, Max. current allowable: 40A/30 sec., Max. efficiency current is 15 to 25A

Actro CL-8" External Rotor Brushless Motor
http://www.hobby-lobby.com/actro-cl.htm
For 8-16 cells in propeller models and 18-24 cells in helicopters. Propeller range is 10" to 16" in diameter. This 8 turn motor has an RPM/Volt of 600, resistance of 115 mOhm. Maximum efficiency of about 88% in the 25-35 amp draw range.

Jeti "Phasor" 45/3 Brushless
http://www.hobby-lobby.com/jetibrushless.htm
14 to 16 cells, 10x7 or 10.5x7 prop, 30 to 38 Amps 3 turn windings intended for direct drive of a prop. Motor resistance 0.044 Ohms. Maximum RPM 15,000.

Again, any advice or experience you might have would be appreciated. If you have done this kind of carry launch with an electric, I would love to know of your experiences.

[Dave Robelen]

Hello Aeajr,
I have had some experience lately with the "Piggyback" technique of glider launching. In my case, the whole project was smaller and less powerful (a highly modified Litestick). However, the trends are as you anticipate. Adding the clean glider at low angles of attack does increase the climb rate and reduce power requirements. To evaluate the motor choices, why not download the trial version of Motocalc www.motocalc.com This will give a fair idea of the increments between motors, and some feel for potential performance.
Regards, Dave

[aeajr]

Thanks for the suggestion on motorcalc. I did not consider it since I have two airplanes tied together.

Will take a look. Have never used it.

[RC-Collector]

I just bought a Multiplex Magister,

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXGZV2&P=ML

The CD that comes with it shows the trainer towing a sailplane (102"?, it certainly has a larger span than the Magister's 64") from a ground start, using the optional RC aerial towing kit.