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Mini Sportster Construction and Power Plant - Need Advice

#1
Hi, I'm building a Mini Sportster, but I want a nice docile 4-channel flyer that I can fly in a school gymnasium. I have already purchased the power plant you see in the picture, it's a Racerstar BR2205 2300KV motor, a BLHeli 12A ESC, and a 5 x 4.0 x 3 propeller. First I want to know, is this an acceptable prop/motor/ESC combination? Second, for this combination, and with the goal of long docile flights, what battery do you recommend? Keep in mind that I'm limited to what's available in the one hobby shop in my city in Taiwan, as it's completely impossible to have lithium batteries shipped here.

I'm also curious to hear thoughts on the suitability of this model for my intended goal. In Taiwan we have no DTFB, so I am using 5mm paperless board and covering with tape in the style of Experimental Airlines. This means I have to make some adjustments for the increased thickness of the foam, but I don't have the weight of the paper to contend with . In the construction video for the Mini Sportster, it looks like it is just burning up the sky. I need a plane that can either fly indoors in a gymnasium or/and be transported on a scooter, since that's life in Taiwan. I look forward to any advice you have!


Mini Sportster Small.jpg
 

d8veh

Well-known member
#4
I have the full size Sportster. It can burn up the sky, but it can also fly very slowly under total control. It loops and turns so tightly that I'd be happy flying it in an an average sized gymnasium. If the smaller one has similar flight characteristics, you won't have any problems.
 
#5
I have the full size Sportster. It can burn up the sky, but it can also fly very slowly under total control. It loops and turns so tightly that I'd be happy flying it in an an average sized gymnasium. If the smaller one has similar flight characteristics, you won't have any problems.
That's reassuring, thanks :)
 

mayan

Well-known member
#6
I built a 50% scale of the Tiny Trainer from 3mm foam boards it’s definitely strong enough to hold some crashes. I couldn’t make it fly like I wanted it to but was close. The whole goal behind my attempt to make a small version of any one of the FT Models was so that I can fly it in very small areas. Have a look at what I did here post #231 maybe it will help you out. One way or the other I am with you on this challenge, because I too want to create something that I can fly when it’s raining outside.
 
#7
OK crew, so let's say I do make a Mini Sportster out of 3mm foam, paperless, with packing tape for external skin. Is the power plant I have listed above still suitable for such a plane, or is it overpowered? And what battery recommendations for nice docile scale flight?
 
#8
I did go out and obtain some 3mm foam. This stuff seems SO flimsy... but I'm making a Mini Sportster wing from it right now. I think once I put the tape on both sides, it will seem much more rigid. I plan on using some 5mm foam for the spar, though, so that the wing thickness at the spar is closer to that of the original wing (11mm vs the original 13mm).

I'd still like some advice on the powerplant above. And also, this is one of the few posts I've made outside of the Newbie Welcome forum, can you tell me if I'm in the right place for these questions? And how do you tag another user? I am hoping Hai-Lee will see this post as he also works in 3mm taped foam, IIRC.
 

mayan

Well-known member
#9
I’d to answer the question of the power plant but don’t have a clue and don’t want to miss lead you. As for tagging it’s done like this @Hai-Lee, can you give us your feedback?
 

d8veh

Well-known member
#10
Your motor and ESC will work as long as you choose the right propeller. At full throttle, any motor takes a catastrophically high current at low rpm, then the current reduces linearly as it speeds up. The idea is to run it at a speed that gives a current high enough to produce enough power, and low enough not to damage anything. The only real proof that you got it right is to use a wattmeter/ammeter to measure the current at maximum throttle. The bigger the propeller, the greater the slowing down force on the motor, so the slower it'll go at full throttle. It's the same with the pitch on the blades. I would say that you'll be in the right ball-park with what you have, especially if it already worked on something else.

Another thing you can do is run at full-throttle for about 10 seconds, then stop and feel the motor and ESC to see if they're too hot. Repeat with a 20 second run, then 30 seconds. If it passes 30 seconds, you'll be OK. If anything gets too hot, you need a smaller propeller or a 2-blade one.

I'd be looking at a 3S battery because they're a little more versatile. Something like 650 to 900 mAh.

You won't need much power to fly that Mini-Sportster slowly, but high power will not only give you more options in what you can do with it, but it can save it in some situations, like when you stall close to the ground.
 

Hai-Lee

Old and Bold RC PILOT
#11
I did go out and obtain some 3mm foam. This stuff seems SO flimsy... but I'm making a Mini Sportster wing from it right now. I think once I put the tape on both sides, it will seem much more rigid. I plan on using some 5mm foam for the spar, though, so that the wing thickness at the spar is closer to that of the original wing (11mm vs the original 13mm).

I'd still like some advice on the powerplant above. And also, this is one of the few posts I've made outside of the Newbie Welcome forum, can you tell me if I'm in the right place for these questions? And how do you tag another user? I am hoping Hai-Lee will see this post as he also works in 3mm taped foam, IIRC.
Sorry for the tardy response but I was napping, (getting old). Firstly it is wise to have the wing thickness as close to the original thickness as is possible. The thicker the wing the higher both the drag and the lift. A thin wing will mean that the plane will need to fly faster for the same weight to gain the same amount of lift.

As for rigidity my favorite trick is to paint the foam once cut and assembled in white wood working glue thinly and apply any coating, tape, paper or even just paint over the dried glue. Also consider that if you suspect that the wing spar might be too weak you can cover it in glue and newspaper or similar to make it far more rigid, (use the glue sparingly and you do not need to do a full wrap. Paper is strongest when done such that it will go from bottom to top of spar, (when spar fitted).

As for the motor power Simply measure the plane's weight and if the same or slightly less than use the same power motor. The only personal preference I have is that I like to swing larger propellers at slower speed, (Lower Kv motors). This actually gives me calmer take offs due to a marked reduction in "P" factor. So if a first or similar model to learn on motor selection can make a huge difference to your experience. If experienced or after learning to fly properly then you can exchange the Motor and prop for a higher Kv setup for a real turn of sport performance.

Finally if not already stated the weight of the build will determine who close to the original design it performs. Lighter is good but heavier is a disaster! If heavier a smaller battery can help but gives reduced airtime, (Ensure that the battery can handle the required current).

Hope this helps!

have fun!
 

Bricks

Well-known member
#13
Instead of foam board do you have Depron or EPP available at reasonable cost? It is much much lighter needs no coating but does need carbon rods to help stiffen.
 
#14
Instead of foam board do you have Depron or EPP available at reasonable cost? It is much much lighter needs no coating but does need carbon rods to help stiffen.
I haven't been able to find anything like that here in Taiwan. Since no one really has carpeting here, the market for underfloor foam is nonexistent. I am however having some luck with the posterboard style foam they sell in all the stationery stores here. In Taiwan you can find a stationery store in walking distance of just about every school. I've been working up a tape-covered 3mm foam wing for the Mini Sportster following the advice in this thread, but it's on hold for a spell while we're having midterm exams.

@Hai-Lee I'm going to try cutting the spar from the denser, papered, 5mm foam. I'm curious about your glue-coat technique above. You said you apply a thin coat of white wood glue to the foam after it's been cut and assembled. But how does that work for, say, the wing, which requires being folded majorly, and wouldn't survive the tension without already being taped?
 

Hai-Lee

Old and Bold RC PILOT
#16
I haven't been able to find anything like that here in Taiwan. Since no one really has carpeting here, the market for underfloor foam is nonexistent. I am however having some luck with the posterboard style foam they sell in all the stationery stores here. In Taiwan you can find a stationery store in walking distance of just about every school. I've been working up a tape-covered 3mm foam wing for the Mini Sportster following the advice in this thread, but it's on hold for a spell while we're having midterm exams.

@Hai-Lee I'm going to try cutting the spar from the denser, papered, 5mm foam. I'm curious about your glue-coat technique above. You said you apply a thin coat of white wood glue to the foam after it's been cut and assembled. But how does that work for, say, the wing, which requires being folded majorly, and wouldn't survive the tension without already being taped?
The spar if to be reinforced with added paper or the like should be cut shaped and have the paper applied before fitting into the wing as normal. The coating with the white glue I was referring was done after the wing is completed and if it is still too soft or flexible then paint in the white glue and allow to dry properly before painting or taping.

This was something I used when I tried to build a plane using only the foam from the FB and to DEPRON plans and it was too weak to support itself in the air. I covered the outside in white glue and then painted it and it was so, so rigid that it flew for months until I managed to find a Goal post. Sadly harder was also more brittle! I did manage to salvage the gear and most is still in use in other planes now!

As for bending without the paper I managed to do it with a cut/groove on the inside of the fold which reduced the thickness at the line of the bend. Being a lot thinner it was able to do small angle bends easier. LE folds ALWAYS require tape reinforcement even when still paper coated as the paper can and does split sometimes under the pressure of the foam in compression.

In my Depron only days I tried a few FT builds and had nightmares trying to get the LE folds, (in Depron), until I just cut separate pieces for the top and bottom panels and used a shaped balsa stick for the LEs. (It was part of my inspiration for the approach I used in the Balsa FB spitfire I did a build post of just over a year now!).

Just what worked for me!

have fun!
 
#17
Ah ok, it seems you guys are talking about techniques for floppy foam. The foam I'm using is more like DTFB without the paper on it. Actually maybe even a little stiffer than that... I've seen naked DTFB in the video about custom skins and this stuff is a little more rigid.
 
#18
I just built my first plane (a tiny trainer as well, though full size) from DTFB and I am very surprised at how rigid it is. But maybe that's because my first plane is a WLToys F949 which was made as cheap and light as possible lol. After watching those wings flex in even a light wind, I'm pretty sure DTFB is going to seem super rigid in comparison.