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Old Mar 02, 2015, 10:00 AM
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Build Log
Valmet Vinka L-70 foamie

Hey guys! I've been building this plane for a while now and I thought that I should share what I have done in this time.
The plane is probably not very familiar to most of you people since its only used in Finland. The plane is used in Finnish air force as a trainer.
You can look more specific info from the planes wikipedia page.
Usually I buy my planes as a kit, but since this plane can't be bought I decided to try build it. I'm not trying to make it perfect, this is my first plane that I build completely out of scratch.
I also would like to get some tips in choosing the right sized motor and prop for this plane. I was thinking of maybe putting hextronik DT750 for the motor but I'm not sure if its the right one.
Anyhow, heres some pictures from the progress.
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Old Mar 02, 2015, 04:00 PM
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Looks good so far. What's the wing span? Can't estimate power without at least that. You might make the tail feathers a bit larger than scale.
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Old Mar 03, 2015, 12:58 AM
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The wing span is 1100mm and lenght of the frame is 830mm
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Old Mar 03, 2015, 09:05 AM
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Interesting build, kind of combat model build technique. Do you plan to cover it with paper?

What batteries do you plan to use (3S, 4S, how many mAh, weight of the battery)? I think this should be the starting point when selecting motor & prop. Then I would consider if the model will use gears or just belly land. If you use gears, then prop clearance is one variable to consider.

After battery is selected and we have a clue on the motor requirements it would be good to calculate some kind of rough estimate on the target AUW of the model.

Based on my experiences DT-7xx series is a robust, but kind of heavy, motor. Still it's quite low kv and your plane might be better suited with little higher kv motor. Of course if you plan to run 4S then the story is different.

I'm building Ki-61 Tony in quite similar style & size currently (hot wired wing & fuse)...
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Old Mar 03, 2015, 09:55 AM
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Quote:
Originally Posted by Late99 View Post
Interesting build, kind of combat model build technique. Do you plan to cover it with paper?
No, I was thinking of painting it with thick layers of regular spray paint, I've seen some people do it.

Quote:
Originally Posted by Late99 View Post
What batteries do you plan to use (3S, 4S, how many mAh, weight of the battery)? I think this should be the starting point when selecting motor & prop. Then I would consider if the model will use gears or just belly land. If you use gears, then prop clearance is one variable to consider.
I'm thinking of putting 3S 2200mAh battery. The plane will have a landing gear.

I have also done some work on the plane today and I'll try to post some photos later
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Old Mar 03, 2015, 01:26 PM
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Could you take some measures on how high the plane will stand on it's gears? Try to measure from where the motor is mounted / prop middle is located. Then we'll have estimate on how big prop you can mount there safely.

(Figures below are just my guesses / opinions and I'm learning this hobby all the time)

Very rough estimate on plane weight:
- Battery 180g
- Motor 50-100g
- ESC 30-50g
- Servos (4) 40g
- Fuse & wing 200g
- All the paint, horns, push rods, gears, prop etc. 100-130g
600g - 700g

At that weight the plane won't be a slow flyer anymore. So I would aim for quite powerful setup. 35-30 size motor maybe? But please measure / guess the prop clearance first, then will have very good idea on what prop size would be ok.

And to compare, my Tony will most probably be in the same weight category and I'm planning to use NTM 3530 1400kv motor on it with 8" or 9" APC prop.
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Old Mar 03, 2015, 01:53 PM
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Quote:
Originally Posted by Late99 View Post
Could you take some measures on how high the plane will stand on it's gears? Try to measure from where the motor is mounted / prop middle is located. Then we'll have estimate on how big prop you can mount there safely.
Well I tried it and it can take approximetely 11 inch prop, 12 inch could work out but it would be pretty tight fit and I think the prop could snap easily in landing so I guess I shouldnt be going for more bigger than 11 inch. I was thinking of putting DT750 for the motor with 11 inch prop but do you think that the plane will be too slow with it?
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Old Mar 03, 2015, 01:56 PM
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And heres a few photos from the progress.
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Old Mar 03, 2015, 02:02 PM
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This motor worked well for me http://rctimer.com/product-117.html
I got 750g thrust with a 8x4 prop.
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Old Mar 03, 2015, 02:07 PM
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As it looks like that you've got the DT750 already available I would go with it. Just leave the option to change it later (build it so that you can change it relatively easily). That motor should have enough thrust, but the max speed won't be phenomenal. I would start testing with something like 11x5.5 APC prop.

One idea... If possible try to get the battery quite close to CG. This way you could easily start test flights with smaller & lighter batteries (1000-1300mAh).

That fuse starts to look really Vinkaish. You're making good progress.
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Old Mar 03, 2015, 02:14 PM
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Quote:
Originally Posted by Late99 View Post
That fuse starts to look really Vinkaish. You're making good progress.
Heh thanks! Let's hope I just wont ruin it in any stupid way
I also might try to fit the motor inside the fuselage so it wont be visible, I first thought of just putting it on top of the fuselage as you might see from my wooden plate, but I'm having second thoughts about that.
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Old Mar 04, 2015, 08:59 AM
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R3gou: looks like a cool subject for an all around fun flyer, should be nicely balanced and acrobatic! On the motor selection, I too wouldn't use the hextronik DT750 as a first choice. It looks heavy, and has that really low KV which will translate to lower rpm/volt and lower speeds. You will have to use a really high pitch prop (hard to find) to get any speed out of her. The big question is how much do you think she will weigh? You can weigh the foam airframe for a start, and then total up all the gear weight if you have a gram scale.

I have several planes in the 40"/1000mm span that run around 450-600gm. The lighter ones I fly with an Emax CF2812 40 gram, 1543KV motor and 7x6 prop for a good balance of acceleration and top speed with good low speed performance as well. The heavier ones, I use a 60 gram 1000 to 1200 KV motor (forgot the brand, was either KEDA or Turnigy/HK). Said all that to explain that your motor is on the heavy, slow side of the equation.

Additionally, I know that the 2200mah packs are popular with rtf's and arf's, and if you have some of them or a quad copter that needs that size pack, then ok, but my largest pack is an 1800mah 3s and I can fly my biggest plane for around 10 minutes on it. (the others fly for 8-10 with1300 - 1500mah packs). There is a point of diminishing returns with batteries. You have to lug it around the air regardless of whether it is fully charged or depleted, so if you pick one too large it will hurt performance and not give longer flight times. Lighter flys better, and that includes motors and batteries.

Just food for thought... (as if you didn't already have enough of that with your first scratch build!)

Definitely put the motor inside the nose!
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Old Mar 04, 2015, 09:23 AM
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I will try to see how much the final weight will be. I guess I need to order some another motor for this project after all?
I also have few 1300 mah 3s packs laying around so I can use those too if you guys think that those will suit the plane better
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Old Mar 04, 2015, 09:27 AM
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Actually, I do have one motor laying around thats around 1000kv and spins 9x5 prop resulting to give me approximetely 1 kilo of thrust. Do you think that it would work?
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Old Mar 04, 2015, 09:43 AM
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I'll bet it would. If you can keep the AUW under 750gm, then the higher Kv, smaller motor and battery will be a good fit. Be careful about covering, it adds weight fast, and adds it in a way that shifts CG aft, sometimes requiring addition of weight in the nose to compensate (horrors!) I like to use coffee filters (the round pleated ones) and water based urethane varnish (minwax polycrylic popular here in the US) for a poor man's covering. it makes a paintable smooth and more durable surface than bare foam. Of course the lightest is still just foam.

I built a Rollason Beta racer that is a great flyer, but it is my heaviest plane, and needs to be flown all the time (it flys fast and lands fast - I have rebuilt it several times.....), where as several others will float without lots of focus and input because they are lighter, but still perform when I add throttle (and I don't have to rebuild them...). That's one of the fun aspects of scratchbuilding, selecting how the plane is to fly based on power and weight options.
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