Seahag721

Hey gang, I need some help putting together a power system for my House of balsa 2 x 6. I want to go with a nose mounted outrunner and a folding prop. I am looking for a system that will get me up with the buzzards with reserve left for a few additional climb outs. Just leisurely gliding, no acrobatics etc. Since I live in south Texas near the coast, where the winds have a tendency to come up suddenly, it has been suggested that I get a system that can get me back to the flying field if necessary. I've been looking at e-flite, hacker and scorpion motors but am still a little confused[sm=confused.gif] on a sailplanes power requirements. Seems that the widely accepted watts per pound calculations don't really apply to sailplanes and along with that the final flying weight will be different. So any recommendations would be greatly apprecated. Thanks

BMatthews

A lot of folks will say you need 100 watts per lb even for sailplanes. But while that will guarantee a hot climb it's really not needed.

I've flown a couple of the older school brushed style models for years and they have a nice adequite climb that resembles a full sized lightplane for climb angle which gets the model to around 500 feet in around 70 to 90 seconds depending on the charge condition of the pack. I'm doing this at just 39 watts per lb. A 56 oz model using an old Robbe motor on a gear drive that draws 16 to 17 amps off an 8.4 volt pack. So based on that you only need around 40 to 50 watts if all you want is to get two or three 80 to 90 second climbs up to thermal country.

Penetration comes as much or more from the plane itself as the motor and prop. Do NOT depend on power to get you back upwind. At the first signs of the wind picking up head back upwind with the nose pushed down a few degrees to a speed that generates your best lift to drag ratio to cover the ground as best you can. If that's not making headway then you'll need to add power and possibly increase the nose down angle by another couple of degrees to attain a shallow dive. Be wary of increasing the angle to too steep though. If you nose down too far the speed will be impressive but the model will actually attain LESS ground coverage depending on how greedy you get. The ideal dive angle is actually quite shallow even for a "gasbag" like the 2x6.

No need to go really high brow on the motor and ESC either. For something basic like this the cheaper import motors and ESC's will work just fine. You're only really looking for a highstart replacement after all. Not a motor that's going to run for the whole flight.

This MAY be a little extreme but if your model could come out at the low 30 oz overall weight this would fly it nicely on a 10x4 or 11x4 folder at 14ish amps and it's dead cheap and light. Read the reviews on the motor....
http://www.hobbycity.com/hobbycity/s...runner__1200kv

This would be overkill given the reviews at the bottom of the listing but it's light enough for this size of model and the shaft is rugged enough to withstand some bad nose landings. You'd want to run a bigger 2 cell pack rather than a smaller 3 cell to "hold it back" and prop it for around 12 to 16 amps depending on model weight...
http://www.hobbycity.com/hobbycity/s...trunner_1140Kv

This would be a nice match to the 2x6 running with a 10x4 folder on 3x1000mah cells along with a 20 to 25Amp ESC....
http://www.hobbycity.com/hobbycity/s...utrunner_Motor

EDIT- Sorry about the *'s. I'd forgotten that even though this outfit is an advertiser here they have some weird tracking software running that fouls up RCU. Management is working on that.... Just sub in hobby city dot com with the usual no spaces for the *'s.

Tinkrerpilot

My Feedback: (2)

Bruce has a few good ideas there. Here are a few more for ya! Just a do a search here, and look for Gentle Lady Conversion, or Electric Gentle Lady.

http://www.rcgroups.com/forums/showt...ight=electric+

http://www.rcuniverse.com/forum/m_48...tm.htm#4849221

http://www.rcuniverse.com/forum/m_67...tm.htm#6706044

Keep us posted as to what you come up with. Good luck.

tink

Seahag721

Bruce & Tink Thanks for pointing me in the right direction guys. I'm still learning the basics of electric power but there is so much equipment and information, it's like a kid in a candy store, which one do I pick. Some of the ideas are off the chart (one person suggested a electric hairdryer motor[sm=bananahead.gif]). I had to giggle, but after seeing how serious he was, I thought to myself this guy could make a brick fly. Locally we have a hobby shop whose owner seems to be on a buy it/try it attitude. Being a skydiver, I'm glad this guy didn't take up the sport. But I need to keep him in business, (he owns the club field) so I'm determined to learn as much as possible before I buy. Anyways, let me digest your suggestions and I'll keep you posted on which way I go. Thanks :-)

BMatthews

With all the options there's certainly no doubt at all that there's LOTS of room for confusion.

Here's a couple of bits that may help.

First and foremost you need "X" amount of power per pound of model weight to fly the way you wish. On gliders and other styles that "fly on the wing" rather than "on the prop" you can get away with a lot less power loading. However I would not suggest going much at all under 40 watts per lb. It'll fly but the climb will be so lazy that you won't like it by the time you get down under 35 watts/lb. 40 gives a nice Cessna 150 like climb with 500 feet coming within around that 70 to 90 seconds I mentioned. Actually it may come sooner since my 38 watts/lb is still currently on old brushless motors with gear boxes. Toss out the loses in the gear box and put that directly to the prop and 38 isn't anything to sneeze at if you're not after rocket like performance.

Next is the motor specs. First is the max continuous power. You don't want to exceed that obviously but for sport flying I would also suggest you only plan on using around 80 to 90% of it so the motor stays cooler than its max temperature. It'll just last longer. Next is the Kv value. This is how many RPM it'll turn per volt of battery power. For a glider that'll be running a large folding prop you want to stick with a lower value for the motor size range. Something under 1200 for a glider of this sort is the way to go. Around 1000 would be better. Avoid motors with high Kv values for glider use. Those two are the key ones. The first will avoid you burning up the motor and the second will allow you to turn a larger and better climbing prop on a glider.

Next is the ESC. Because we house them inside the model it's standard practice to get an ESC that is around 25 to 30% more current capable than the motor. And to help this along do NOT tuck it into some foam rubber. Keep it out in the air inside the fuselage. Just let it rattle around or just secure the wires so it's in clear air.

Next is the battery packs. LiPo is common as dirt these days. But you need a special Lithium cell charger. Then there's balancing chargers. A good thing for your packs. Study it but get one. 2 or 3 cell pack? If it's a slightly high Kv motor then run 2 cells and a prop that brings the motor to 80% max. ((power) watts= current x voltage. If it's under 1000 for the Kv then you can run with 3 cells. Just watch the prop to current. You can over prop the motor but it'll try to turn at the correct RPM and it'll draw power to try to achieve that. I'll draw until it burns out if you over prop the motor. So pay attention to the suggested prop sizes and pack voltages to avoid this.

Tinkrerpilot

My Feedback: (2)

Spot on the key points. Excellent advice.

Seahag721,

I know Bruce said it. Study all the option and then make a informed decision about what you after. Once you have the idea of what you need, then go do your shopping. There are other places to go if your LHS does not have what you are looking for. I know I try to support mine, but when he is not willing to help, I go other places. Don't limit yourself. Any other questions, ask away.
Good luck.


tink

johnpcunningham

I fly the F5B Tiger whcih is about $350 for the plane alone. I've tried out different props and motors and found that the Turnigy motors over at untiedhobbies and their ESCs work great. I would suggest sticking around 900 to 1000kv and running somewhere between 11" to 14" props. Sailplanes are limited on the inside diameter so pick your motor so that you know it will fit in the plane. Every plane is different so having some spare folding props around is a good idea as you dial in your plane.

I ended up with a setup using the Turnigy 35_48 1000kv, 14x6 CAM folding prop, Hextronix 80 amp ESC and two Rhino 11.v 2250MHa batteries in parallel. 14.4 made too much current with this setup so I backed off to 11.1; which provides more than enough power.
I then tried the Turnigy SK 35_48 900kv 14x6 CAM folding prop, Hextronix 120 amp ESC and two Rhino 3000mha 7.2v packs in series yielding 14.4v@ 300mha. Much more power but the way that I fly, either the 11.1v or 14.4v will work fine. I fly for a long time and
never run out of battery power - Always have plenty left when I land.

AXI 2820/12 ran great as well, but $120 vs $29 Turnigy was a wash in terms of performance, but big difference in cost. 5mm vs 4mm shaft (Axi to Turnigy). Turnigy SK has a 5mm shaft

JC

aeajr

My Feedback: (2)

Everyone has provided some good ideas. I will put my twist on it.

1) - wider props rather than deeper props:

When you are looking at motors, look for motors that will take wider props. A winder prop give you more thrust. On a glider you are more interted in thrust than speed. ON that 2x6 you might want something like 9X6 or a 10X5 folding prop.

2) 60 -80 brushless watts per ponds -

For a casual climb, assumming a brushless outrunner, you want to look at about 60 watts per pound. That should get you at least a 30 degree climb. I would target 70-88 to get a good 45-60 degree climb. That will get you up there quickly without needing a big battery to go verticle. 100 watts per pound would take you near verticle.

I am putting a Cox Dust Devil together. The plane should finish no more than 3 pounds and hopefully less. So I am putting in a 265 watt motor, if I used a 12X6 prop. I am going to use an 11X8 folding prop which will probably give me about 220 watts or about 70 watts per pound. Should get me about a 45-60 degree climb, which is fine for me. If it finsihes lighter, so much the better. I will be using a 4S pack at about 17 amps.

3) With that kind of climb rate, I should be able to reach soaring height in about 30 seconds. I will be using a 2100 mah battery, so I should get about 6-7 minutes of max throttle run time. That will give me 12 climbs at full throttle. That is plenty. More likely I will climb at 3/4 throttle at 45 degrees and get maybe 15 climbs. If I get 10 minute per climb that gives me 150 minutes per pack or about 2.5 hours of air time on the 2100 pack. If I catch a few long flights I could get 3-4 hours of air time on one battery pack.

So I can fly all day on that 2100 mah pack.

Jsut another way to look at it.

Seahag721

Hey gang, sorry it took so long to reply. I've been out of commission for a few weeks and I'm now just getting back into the swing of things. Thanks for all the great advice. I'm starting to get a general understanding of how sailplane power plants differ from the general flying population. JC you were definately right on about making sure the motor fit within the confines of the airplane. Some of the motors I was looking at were too large. Ed I've been looking and talking with Lucien at Scorpion motors and he gave a couple of recommendations that are almost identical to what you stated. I can either go with a smaller setup which will save me a couple of ounces in weight, but will limit me in power, duration and performance or with a slightly larger setup which will get me up quickly and give me some additional climbouts while adding a couple of ounces. So, I think I know where I'm going with this for now. I'll keep you posted on how it shakes out. Thanks for help and I'm sure I'll be back for more.[sm=biggrin.gif]

aeajr

My Feedback: (2)

When looking at weight, think of this:

how much weight do you have in the nose of the glider now?

What weight will you remove when you take the nose of to mount the motor.

Add that up. Let's assume that adds to 4 ounces.

So, if your motor/ESC are less than 4 ounces you may have to add lead.

Now, where are you going to put the battery. how much does that weigh?

If you want to go for the lightest set-up possible, chop off the nose and start placing weight where the motor, ESC and battery will go. By moving around the weight of the ESC and battery you can determine how light you can go. Sometimes we go so light, we have to add lead back. No help!

Just a thought. Don't foget the weight of the motor mount and the wires. :-)