Cub Training (J-3)

Inq

Elite member
570 gms of thrust should be good thrust to weight should be 1.5:1 for beginner planes and 2:1 for aerobatic

Where did you get 570 gms of thrust? Is there some chart of typical motor/propeller combinations that provides that? Now that I've got the basics of the electrical side of motors under my belt... KV, watts, watts/lbs ratio, flight time, that would be my next hurdle to understanding how to size the motor and more specifically the propeller diameter / pitch. - Thanks!

Also, didn't you mean... 0.8 : 1? ... as in 570 g of thrust / 700 g of model weight = 0.8?

I've always heard the ratio expressed in thrust TO weight ratio. Thus anything over 1.0 : 1 allows the model to accelerate while going vertical. This one isn't even close. :LOL:
 
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quorneng

Master member
Inq
Being printed items it was not too hard to made the internal diameter just a bit 'tight' at each end but only for the last turn of the spring.
Get it right and the spring strut is free to move but it takes a bit of force to actually pull it apart.
The 70mm wheels don't weigh much either. The hubs are hollow (printed) and the tyres are sanded from Depron.
70mmWheel2.JPG

It was always intended to be a "light weight". ;)
My AN2 is just a bit smaller than the 62" Maxford ARF one but it is less than half its weight.
 
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JetCrafts

Active member
Where did you get 570 gms of thrust? Is there some chart of typical motor/propeller combinations that provides that? Now that I've got the basics of the electrical side of motors under my belt... KV, watts, watts/lbs ratio, flight time, that would be my next hurdle to understanding how to size the motor and more specifically the propeller diameter / pitch. - Thanks!

Also, didn't you mean... 0.8 : 1? ... as in 570 g of thrust / 700 g of model weight = 0.8?

I've always heard the ratio expressed in thrust TO weight ratio. Thus anything over 1.0 : 1 allows the model to accelerate while going vertical. This one isn't even close. :LOL:
oops I took the model weight as the landing gear etc weight if the weight is 700g then you'll need 1050g min for good flight
 

Inq

Elite member
Inq
Being printed items it was not too hard to made the internal diameter just a bit 'tight' at each end but only for the last turn of the spring.
Get it right and the spring strut is free to move but it takes a bit of force to actually pull it apart.
The 70mm wheels don't weigh much either. The hubs are hollow (printed) and the tyres are sanded from Depron.
View attachment 231179
It was always intended to be a "light weight". ;)
My AN2 is just a bit smaller than the 62" Maxford ARF one but it is less than half its weight.

Wasn't familiar with the AN2, Googling... That's your avatar???

I'll have to look into the Depron. I'm having a lot of trouble with trying to print the PLU+ tires. The overhangs are way over 45 degrees on the bottom curve and adding support doesn't work well either. I guess this is the perfect case for having the dual extruders... use a water soluble for supports and then print the PLU+ on top of it.

BTW - I checked Amazon for "RC Springs" and there's a whole slew of RC car parts with shock absorbers and springs to choose from. Who knew? :ROFLMAO: I've been away from RC for a long time. The hobby has changed tremendously... electric engines, electronic autopilots and now tiny oil-filled shock absorbers. How cool! :cool:
 

Inq

Elite member
oops I took the model weight as the landing gear etc weight if the weight is 700g then you'll need 1050g min for good flight

Is that some RC Jet rule of thumb? Because real planes didn't have greater than 1.0 till at least the 60's or 70's. I'm sure no Cessna or Piper ever had greater than 1.0. No RC prop plane of mine ever had greater than 1.0... yet.
 

JetCrafts

Active member
Is that some RC Jet rule of thumb? Because real planes didn't have greater than 1.0 till at least the 60's or 70's. I'm sure no Cessna or Piper ever had greater than 1.0. No RC prop plane of mine ever had greater than 1.0... yet.
ya its an ft rule o thumb
 

quorneng

Master member
Inq
The 'required' thrust to weight is largely dependent on the planes aerodynamic efficiency. For model planes as the size is reduced the aerodynamic efficiency declines so the power to weight has to rise.
Many models are designed to be 'built' rather than trying to achieve the best possible aerodynamics for their size so generous thrust to weight ratios give a better margin of safety and climbing ability.
 
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Inq

Elite member
I finally got out to a club's airstrip that was recommended to me, but no one was there able to help train me. The one individual there was very helpful, but didn't feel comfortable flying my plane. He did check out my rigging, servo settings and CG trimming. He did suggest I taxi it around. Good thing I did, I found the scale wheels (about 2" diameter) kept getting bogged down in their finely golf-green mowed strip. I decided to make some tundra tires for the Cub. Like they say, "If you're going to go, go BIG!"

Although I'd prefer to just scale up the original styled wheels, they would have been over 160 grams for the pair. The TPU+ is far heavier than the ABS. I tried to go for minimal thicknesses and came up with this design. These are 58 grams for the pair.


DEC Wheel.png
 
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Inq

Elite member
Last Friday, I had my first training session. Fortunately, the club has a designated trainer and he has a Carbon Cub all decked out and has wireless trainer/student transmitters. For some reason (either not installed or not working) the SAFE feature was not available. My three flights during the day were without SAFE. He did take-off and land of course. I did the oval circuit around the field, with emphasis of maintaining altitude in the turns and then proceeded to do figure 8's. He said I did great, but these days, everyone gets a blue ribbon and of course these clubs seem to be dwindling away with old timers (like me). Getting fresh-meat is a priority.

Toward the end of the day after my last training flight, I asked him to take up my plane. He checked it out thoroughly for settings and balance. Of all the setting (limits, mixing, centering), he had me change only the exponential rate. The instructions with the plane suggested 80% on the first test flights, but he had me reduce it to 30%. He was also concerned that the CG was not forward enough even though the 2200 mAh battery was full forward and I had replaced the rudder with a backup that didn't have the tail wheel. He added about 1/2 ounce.

The new tires worked great on their grass landing strip, but the plastic landing gear is too flexible and the tires tended to toe-in or out causing ground loops. I blame my use of ABS for this as PLA is stiffer. Also, the stock landing gear location does not seem to be far enough forward. It is closer to the spar location instead of the leading edge. This also exacerbated the ground loop and nose plants. After the third attempt (no damage on the previous two) he got it in the air, but I could tell he was fighting the aft CG problem and it was VERY touchy trying to climb/dive. I wanted no part of piloting it! He did several circuits of the field and did improve quickly and did not need to adjust any trim settings. I felt he did a great job of getting it on the ground, but it ground looped almost immediately at full speed and nose planted hard.

The wing came off (rubber bands) but otherwise was unharmed. The landing gear broke right at one of the wheels... understandable as this is where it torques in/out when it ground loops. The fuselage cart-wheeled and broke the rudder off. Otherwise the plane is easily repairable.

As my primary interest is designing and building the airplanes and flying them is really only to validate the desired design or improvement to a design, I'm debating about doing a total blank sheet design. I'd like to explore using the FT foam-board technique / 3D printing design like I'm doing on the F22 Raptor project. Since I'm also tutoring a student who wants to learn aeronautical design concepts AND learn to fly also, it seems like the perfect opportunity.

I may come back to this plane, but I've learned what I need from it and there are lots of things to explore.