Scratchbuild of an Extra

[RC Fan Chris]

So I got a little ambitious the last few months and I started to build an electric sport plane based on the Extra. Its made of blue Dow Styrofoam and some Elmers' foam board. So far, I built most of the fuselage including the tail, and hotwired the wings out. For the cowl and canopy, I tried to do the pop bottle deal, but I ended up making both out of blue foam. I got the motor (950 kV brushless) and some hitec HS 55 servos. And by the way, I was wondering if these servos are okay or if I'll have to get bigger ones... I already have a receiver and transmitter for this, so I'll just need a suitable ESC and battery. I have a 30 amp ESC and a pretty hefty Li Po, which will probably need a bigger ESC, but since my dad and brother are working on another plane they could use em if I cant. So, I'm just a little iffy on what kind of battery, ESC, and servos I need. The plane is about 36 inches long with a 40-42 inch wingspan (subject to change). How heavy should this plane be? I saw that the PZ Extra 300 is about the same size, while weighing in at like 36 oz or so, I think.
LOL, thats a lot of questions!
Pics:

[Larry3215]

Awesome workmanship Chris!

Lighter is always - well, almost always - better, so keep it as lite as you possibly can.

If you can keep it under 2 pounds you will have a very nice flyer

Id recommend servos one step up from the HS-55's for that size bird. The HS-55's will work - as long as you keep the model lite - but you will be pushing them to the limits.

HS-65's are more $$ but will serve you much better I think.

Did you get those nice curves on the fuse by carving/hot wire or did you do some bending too?

Good luck!

[RC Fan Chris]

thanks for the compliments! About the fuse and stuff, that was the result of hours of sanding after I cut the basic shape out. I hotwired the wings, which simplified things a little. Was wondering if the 950 KV motor would be okay, I heard that overpowering a plane is safer than underpowering...

[Larry3215]

The motors kV alone doesnt give me enough information to answer that question.

There are 950 kV motors that weigh about 1 ounce and can handle 100 watts max and there are 950 kV motors that weigh 30 ounces or more and can handle 5000+ watts.

The other thing is - you need to have the motor and the batteries and the prop and the esc ALL be of the correct specs so that they work well together for your particular application.

A motor with a kV of 950 might work fine with a 3S pack and a 12x6 prop producing 300 watts while it wont work well at all with a 4S pack and a 6x4 prop. On the other hand a different 950 kV motor wight work best on a 12S Pack with a 18x8 prop producing 5000 watts.

If you list the details of all the power system components, one of us can help you decide if they will work well together or make suggestions for better choices.

Details are important

For battery packs - voltage (cell count) and weight and 'C' rating.
Motor - a link to the mfg's spec sheet is best but we need to know kV, weight, continuous and max current handling and/or continuous and max power handling. Many motor mfg's will list recomended props to use with various battery voltages and the expected power output.
Prop = diameter and pitch and style - We need to know if its a slo-fly type or a standard E prop.

You also need to know what the prop clearance is on your plane. In many cases, prop clearance dictates the largest diameter prop you can use - which may not be the optimum prop for your motor/battery combo.

It might seem like a minor change to go from a 12" prop down to a 10" prop but thats actually a major change for any power system. A motor that might produce plenty of power on a 12" prop might not be able to take off with a 10" prop. The opposite is true as well. A power system thats perfectly happy running with a 10" prop may burn up if you go to a 12" prop.

[RC Fan Chris]

I have a 2S 25 2200mAh battery (LiPo) with a 30 amp ESC. I have a feeling that the ESC amp should be higher... The prop is 11x7, not slo-fly type. It says 'electirc only.' I'll get back to you on the motor.

[Bill G]

Nice to see a scratch build of a plane that is almost always ARFed these days. That's the only way I'd want to have one, as it is an original.

I've run a number of similar motors in similar sized planes on 3s lipo with around 9x5 prop on 30A ESCs, and haven't burned one out yet. A good example is an Electrifly 28-30-950 in my Eflite SeaFury, with retracts and all. Not as powerful as some like it, but plenty of power for enjoyable flying. Come to think of it, I believe it has the stock prop which is 10x7, and has numerous flights on 3s with a 30A Eflite Pro ESC. I think you'll be fine. I've never experimented with the 2s setups on these motors, but I would imagine you could run an 11x7 prop on 2s and be fine with a 30A ESC. I'm assuming you want the thrust for this kind of plane.

On the ESCs, the amp ratings are often a joke, and therefore I combine them with brand, to get a realistic rating. The Castle TBirds, Pentiums (under 1 million names) and Turnigys have served me well, in terms of living up to their ratings. Generally the TBirds are as much as I care to spend, or even have.

I think Larry is from the same school of thought as I am. I've ran setups that folks would say should be over pushed, but hold up fine. As he was stating, all motors perform differently with different prop sizes, even when they are supposedly the same kv. You just have to experiment with the meter and see what you get.

[kyleservicetech]

Originally Posted by RC Fan Chris

So I got a little ambitious the last few months and I started to build an electric sport plane based on the Extra. Its made of blue Dow Styrofoam and some Elmers' foam board. So far, I built most of the fuselage including the tail, and hotwired the wings out. For the cowl and canopy, I tried to do the pop bottle deal, but I ended up making both out of blue foam. I got the motor (950 kV brushless) and some hitec HS 55 servos. And by the way, I was wondering if these servos are okay or if I'll have to get bigger ones... I already have a receiver and transmitter for this, so I'll just need a suitable ESC and battery. I have a 30 amp ESC and a pretty hefty Li Po, which will probably need a bigger ESC, but since my dad and brother are working on another plane they could use em if I cant. So, I'm just a little iffy on what kind of battery, ESC, and servos I need. The plane is about 36 inches long with a 40-42 inch wingspan (subject to change). How heavy should this plane be? I saw that the PZ Extra 300 is about the same size, while weighing in at like 36 oz or so, I think.
LOL, thats a lot of questions!
Pics:

What can help here is one of those PC spreadsheets that help with your decisions. I've been using www.motocalc.com for several years with good success.

Only problem with motocalc (as well as the other model spreadsheets) is it can have accuracy problems if those inexpensive (Cheap!) motors are used where the vendor has no idea of what the motor specs should be, so they just put in anything.

But, you can always modify the kv specs in the motor to match what the motor actually does, prop size and RPM to get you into the ballpark.

[RC Fan Chris]

Cool. Well I double checked the battery specs, and this is what it is: Tenergy 25C 2200 mAh 3 cell 11.1 volt LiPo. (whew.) Motor is a Suppo BL-2217/9 --> http://www.bphobbies.com/view.asp?id...7&pid=B2632605
btw, I don't even know where I got the 2S part from, lol.

[kyleservicetech]

Originally Posted by RC Fan Chris

Cool. Well I double checked the battery specs, and this is what it is: Tenergy 25C 2200 mAh 3 cell 11.1 volt LiPo. (whew.) Motor is a Suppo BL-2217/9 --> http://www.bphobbies.com/view.asp?id...7&pid=B2632605
btw, I don't even know where I got the 2S part from, lol.

Just noticed that I'd had a 3 blade prop in the mix. Updated numbers with a 10-7 TWO bladed prop, KV=950, No Load Amps=0.9, Resistance=0.095, Weight = 2.59 ounces are per below:

Plugging in these numbers into Motocalc, I got 18.9 Amps, 10.5 volts, 199 watts, 69% motor efficiency, 7330 RPM, and 27 ounces of thrust.

[RC Fan Chris]

620?

[kyleservicetech]

Originally Posted by RC Fan Chris

620?

That would be in KV, read my updated remarks in thread #9

[Larry3215]

The motor performance chart on the BP web site doesnt asgree very well with those motocalc predictions. Not surprising, Id suspect the motor specs are a wild guess at best

If the chart on BP's web site is accurate, you will be ok with that 11x7 prop and those packs and your esc.

They are shoing that you will be puling about 17.9 amps at around 212 watts with that setup.

That will work fine as long as the model weight comes in under 32 ounces. That will give you around 100 watts per pound for decent sport performance - no 3D though.

Dont run the motor at full throttle for long though or it will over heat.

Good luck!

[Bill G]

Originally Posted by Larry3215

The motor performance chart on the BP web site doesnt asgree very well with those motocalc predictions. Not surprising, Id suspect the motor specs are a wild guess at best

If the chart on BP's web site is accurate, you will be ok with that 11x7 prop and those packs and your esc.

They are shoing that you will be puling about 17.9 amps at around 212 watts with that setup.

That will work fine as long as the model weight comes in under 32 ounces. That will give you around 100 watts per pound for decent sport performance - no 3D though.

Dont run the motor at full throttle for long though or it will over heat.

Good luck!

Yep, no 3D at 32oz. My Rake M1c is close to that, with an Elecrifly 28-30-950 on 3s driving a massive 12x6 vintage scale scimitar prop. Seems to be holding up fine on near full throttle flying. The plane is around 36oz and 40" span. At 32oz I would imagine it would pick up a good bit of performance. Amazing how well some of these bl motors hold up to being worked hard.

[RC Fan Chris]

Would 3D weight be attainable with this plane? What weight should I keep it at? I guess its not absolutely important for me to get that kind of performance, but extra reserve power would be nice...
on a different note, what do you guys usually do for landing gear on these kinds of foam planes?

[lakeside1000]

hi, im quite new to this buiding models, ive buit a low wing from scratch but am having problems finding out what motor thrust and wing incidence angles to install, how did you decide about the wing tailplane angle, ie in line or slight leading edge up, my high wing flies well with 3 degrees incidence on the wing but i dont think the aerobatic wing will perform in the same way, any help would be much appreciated, great model by the way, love the fuselage lines , regards alan in spain.

[RC Fan Chris]

I just had both at (more or less) zero incidence. At least I hope it turns out that way, lol. Good luck to your plane as well! Did you post a thread on it?

[lakeside1000]

hi, no Ive only just joined the wattflyer forum but I will try to do a few items as i get going with the planes, 4 so far all scratch built , two have flown with some trim problems hence the enquiry, but only one minor crash so far, hot glue gun to the rescue , thanks for your time, regards alan in spain

[RC Fan Chris]

I also have a question with the wing spar. Right now I'm just using a wooden dowel, about 3/8 inch diameter. Would i need the carbon fiber? I'm just cautious on spending the money on CF if its not really necessary...

[kyleservicetech]

Originally Posted by RC Fan Chris

I also have a question with the wing spar. Right now I'm just using a wooden dowel, about 3/8 inch diameter. Would i need the carbon fiber? I'm just cautious on spending the money on CF if its not really necessary...

Don't know about the wooden dowel for a spar. You've got to watch the grain of the dowel.

Depending on the weight and wingspan of the model, a plain old balsa spar of suitable size, or a piece of good quality hobby shop aircraft spruce will do the job. And spruce is easy to glue to.

[RC Fan Chris]

I'm not sure exactly what type of wood it is. it's definitely not balsa. It seems strong enough, granted, its not as stiff as CF but it should do.

[kyleservicetech]

Originally Posted by RC Fan Chris

I'm not sure exactly what type of wood it is. it's definitely not balsa. It seems strong enough, granted, its not as stiff as CF but it should do.

Are you using a single piece of wood for a spar? Or two pieces located at the top and bottom surface of the wing?

The usual two piece spar is much much stronger than a single piece of wood for a spar. These two pieces are normally glued with vertical fillers to keep these spars from bending loads. The two piece "I" beam style spar will be much lighter for the same bending strength.

[RC Fan Chris]

Hmmm I've never even considered 2 piece spars, lol. Right now, I'm having the wings removable with holes drilled in them for the spar. How would I do a 2 spar design with removable wings? I'd be very interested in doing it this way...

[kyleservicetech]

Originally Posted by RC Fan Chris

Hmmm I've never even considered 2 piece spars, lol. Right now, I'm having the wings removable with holes drilled in them for the spar. How would I do a 2 spar design with removable wings? I'd be very interested in doing it this way...

Question:
What is the wingspan, weight, and estimated flying speed of your model? Plus, the projected watts input to your motor? And, have you done any work on the wing structure yet?

I've built a LOT of models over the past 45 years, including a number of sailplanes with two piece wings. They include a pair of 10 foot, 8 pound sailplanes that used two 1/4 inch diameter, 18 inch long pieces of music wire for wing joiners.

Not a full blooded model designer by any means, but I've never had a two piece wing fail, even when the sailplanes were launched with a high powered winch.

[RC Fan Chris]

40 inch wing all foam, 25-30 oz plane, maybe 40-50 mph or thereabouts. Dont know about the watts. All I've done on the wings are just drilled the hole for the center spar.

[kyleservicetech]

Originally Posted by RC Fan Chris

40 inch wing all foam, 25-30 oz plane, maybe 40-50 mph or thereabouts. Dont know about the watts. All I've done on the wings are just drilled the hole for the center spar.

Oh, OK.

For a foam model with that weight and wingspan, consider using carbon fiber or fiberglass arrow shafts from your local outdoors supply house.

Imbed the arrow shafts inside your drilled hole. You need to glue the shafts in with epoxy. You will need epoxy only for perhaps 5 - 6 inches from the entry point of the drill rod into the wing. Other types of air dry adhesives will never cure. Watch the weight! Then locate a piece of music wire or aluminum shafting that will fit tightly inside the arrow tube.

You would need high grade aluminum for the wing rod. Stuff from Ace hardware would bend to easily.

Making a two piece spar for a foam wing would be a real mess, when you need to drill the center hole for the wing rod that passes through the fuselage. A metal supplier in Wisconsin is http://www.speedymetals.com/. Take a look. They also give a lot of information on the characteristics of every type of metal they stock.

Several years ago, I purchased two pieces of rectangular tubing from them. The pieces were 3 by 8 inches by 20 inches long, with a 3/8 inch wall. They sell just about everything.