Flite Tip Ep. "Scratch build guidelines" (i.e. Body/Wing ratios, proper motor, etc.)


Mad Scratch Builder
Flite Tip Ep. "Scratch build guidelines" (i.e. Body/Wing ratios, proper motor, etc.)

Don't misunderstand... This isn't in the wrong thread. Keep reading, my show suggestion is coming...

My son Benjamin and I recently built a cub centered around the power-pod and the concepts learned from the swappable series. We used many ideas from some of the other forum articles (thank you all for your ideas and plans), looked at a lot of photos, and also made up a bunch of stuff on our own. It really was an experiment, and we had no idea if the thing would even fly. Success really wasn't even the point... we just wanted to try it!

The maiden flight was this last Sunday. Amazingly, the thing FLEW!!! However, we noticed many problems right away. First it required a LONG take-off run, and that the throttle was pinned most of the flight. It flew like a brick even though when I weighed it (flight ready) it was 1.3 lbs. Any turn more than about 20 deg. caused it to stall and lose altitude like crazy. The flight surfaces all had HUGE authority. Eventually dialed them all down to about 40% with 30% expo and they were much more manageable. Fish-tailed like CRAZY on take off. It's amazing we got it off the ground. Crazy, with a 1.3mAh battery in the nose, it had PERFECT CG (measured from the tallest part of the wing) and required very little trim. Flew very straight (long as it was at full throttle) but even when I dropped throttle, it was still pretty flat. Just dropped altitude like crazy. The rudder, which I thought was small and had tiny throws, had HUGE authority and sent the plane spinning all over. Basically couldn't fly with it.

Long story short, after doing some research I came to the following conclusions:

1. The wing probably needed to be longer by about 4" and 1" additional chord (if you consider the dimensions of the real Cub. We're aware that dimensions don't necessarily scale). How big should a wing be in relation to the fuselage/in relation to total weight/in relation to motor power? We know this is a HUGE area of debate.

2. We have no idea when building a plane/wing/etc. how big control surfaces need to be. Should the ailerons be 1/2 the wing span? 1/3? Right now they consume 2/3 the wing span on either side and are about 20% deep (not sure how else to explain that).

3. We probably made the body too heavy. Not sure on this one, it might even out if we fix the wing size. Could still probably ditch some weight.

4. We understand that the tail-draggers have an issue with ground loops. And, that a little toe-in on the front gear will help. And, that you need to hold a little right rudder during throttle up. And, the wider the gear, the less it tends to turn.

Anyway, here it comes:

Show Suggestion

"Scratch Building for NOOBS"


Considerations when scratch building (wing size, fuselage size, gear size, motor size).

What is "Wing Loading" and "P-Factor" and why should I care?

Creating a plane design from scratch (scale plans, drawing plans & software, printing plans, adhering to foam surface, episodes regarding other mediums (balsa, plastic, etc.))

Tips for reducing weight while not sacrificing strength.

How to diagnose plane problems (watching it take-off, fly, and land) and how to fix them once you know what they are.

*And here's a long shot... Invite guests to bring their scratch builds onto the show and help them diagnose, repair, and then successfully fly them in the episode. :D Ben and I will volunteer!

I'll post in another thread details of our build, photos of the plane, and a video of our maiden, but won't waste space with those details here.

FT Guys, you are AWESOME! My son and I are having SO much fun getting into this hobby, and exploring the science behind flight. Four of our first 5 planes (ever) have been FT swappables (FT flyer, Wing, Cruiser, 3d, in that order). We crashed the crap out of the flyer learning how to fly. We filled the wing with expanding construction foam, and now call it the "TANK" because we've run it into walls, ground, and multiple light poles at full throttle, and literally NOTHING happens to it. NOTHING. The cruiser flies like a dream. I'm too scared to fly the 3d yet. Anyway, we hang on baited breath for each and every episode of your show and are (to quote a phrase) "so grateful" to you guys for your amazing contribution to the hobby and to my relationship with my son. THANK YOU, THANK YOU! :)
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Mad Scratch Builder
Check out my finished version! Flies amazing!!!!

http://forum.flitetest.com/showthread.php?14482-Swappable-Super-Cub(ish)-Trainer-Super-easy-flying!&p=155135#post155135 20141220_032317.jpg


Flugzeug Liebhaber
"Beginners are advised to choose cubic wing loading values no greater than 8, as
it's likely to give relatively low take-off and landing speeds.
At higher cubic loadings one should expect increased landing and take-off speeds
assuming no special lift devices are used, such as flaps."

In mech eng school I kept chasing performance numbers on a homebuilt. Running the numbers manually (wish I had that site back then) made me realize why those who can afford it have multiple aircraft (cross country cruisers, cub back country access,) At the end of the day I had the numbers for two aircraft - Pilatus PC-12 and PC-6 they encompass, at full scale, the performance we enjoy at scale in this hobby.