Inq'd Trainer

[Inq]

I know there are some very fine trainers already out there. The flight instructor at the AMA club I plan to join, trains with a Carbon Cub. Way... way too pricy for me, and if it was the only thing out there, most of the young people I know would never even get into the hobby. I also see there are plenty of trainers in the FT catalog. Those are great for people on the budget. For me... I have to be honest with myself... I like building the models better than flying them. I don't feel any deep draw to fly upside down at five feet, do outside loops while aileron rolling in the horizontal direction or any of the things that I see them do at the airfield. I can appreciate their abilities, but I don't aspire to flying like that.

If I like building more than flying, then I have this insatiable desire to design, or to improve on a design... even if it is just cosmetic. Like the F22 project I hope to be able to fly someday. Its not that I feel I can do better than the next guy here or the professional products out there, but for me, crashing doesn't bother me as much when I can make a plane for under $10. I've become quite adept at the auger-in maneuver.

In this project, I want to use some of the things I've learned from this forum. Namely using this foam-board technique. I didn't know the stuff even existed a couple of weeks ago. Now, I see many possibilities for it. I also recognized its strengths over 3D printed models. However, 3D printed parts have their strengths also, so I'm going to Inq a trainer out using the best of both techniques and see if I can come up with a better mouse trap. It may even fly.

I recognize that larger models are easier to fly (for similar designs). When I first learned to fly... oh... about forty years ago, I learned on a Sig Kadet Senior. It has a 78" wingspan and flies like a blimp. It doesn't have ailerons and uses lots of dihedral and just rudder and elevator. It was almost indestructible, but I found a way. So far my training is going well, and I might not need these dihedral training wheels, but it can' hurt to have it and I might want to train someone else someday. AND, I thought creatively... that I'd like the trainer to morph as I got better. Start out with lots of dihedral, but have it ground adjustable as I improve.

From my 3D printed J-3 Cub I also learned a few things I want to incorporate into this design. I need bigger wheels to run along the turf airstrip. The only way I can get the CG right on the Cub is to add a lot of lead weight. I can't even put the tail-wheel on it and have to drag the rudder through the turf. In this design I'll simply have a longer nose so I can push the battery further forward. The Cub is also very cramped inside. It was really designed for a smaller battery and the 2200 mAhr batteries I use are a royal pain to get in and out. I also want to add a switch into the power system. I also noted how many of the planes the club flew could slow down to a crawl with flaps. Need flaps, flaps are good! The plastic landing gear on the Cub was just not up to it. I'll use steel music wire like in the olde Kadet. The Cub did seem to have plenty of power. It was airborne in less than 20 feet, so I'm hoping I can still use these commodity 2212 motor kits. However, if I want to upgrade to a bigger motor, I'll have plenty of room for bigger batteries and plenty of location to ensure proper CG.

It is a work in progress, and without further ado, here are some details so far.

• Wings span 65" so far, but I've not designed the tips yet.
• Wing chord is 10"
• Wing can be set in slots for 0°, 3° or 6° Dihedral
• Wing has full length 12.5% Flapperons, with independent servos.
  1. Total noob mode will be with 6° of dihedral and the mixing set to do only flaps on the dial, while the right stick will drive rudder and elevator.
  2. Less noob mode will be with 3° of dihedral and mixing on the dial for flaps and the right stick for ailerons and elevator. Rudder on left stick with the throttle.
  3. No noob mode will be with 0° of dihedral and the same mixing above with the ability to reflex the flapperons for higher speed and use as spoilers.
• Wings will have 3° of Washout for gentile stall characteristics.
• Wings use a NACA 4412 airfoil that supposedly can handle Reynold's Numbers down in 20,000 range... oh about 2.5 mph. Just have to get the plane light enough and I should easily get below 10 mph for landing.
• Wing uses a hybrid design using 3D printed ribs, leading edge, trailing edge and Flapperons. Skins are foam board with paper taken off the inside, side.
• Currently the main wing structure is estimated to be 355 grams. This will go up as it doesn't include the fiberglass spar caps, 3D printed wing tips.
• The Fuselage is 48" long, not including rudder or motor spinner.
• Fuselage will also be a hybrid foam-board / 3D printed parts.
• Crude estimate for the entire plane is sitting at 904 grams. This will go up, but it favorably compares to:
  • 3D Printed Cub with 42" wings at 760 grams (ready to fly)
  • Store bought Carbon Cub with 52" wings at 1040 grams (ready to fly)

 

[Inq]

3 Stage Dihedral
The central cross-over part (Green) will be rubber banded to the body as is typical for trainers. There will be one 3D Printed version of this for each dihedral angle to allow close fitting. The half-wing sections will be slotted into the cross-over wing section and will use one bolt to hold both to the cross-over wing.

 

[Inq]

Wing Section
The 3D Printed parts are mostly build vertically for optimum airfoil shape and lightness. This includes the leading edge, trailing edge and Flapperon. These are created in 190 mm tall parts that constitute 2 bays of the wing. The ribs are printed flat and I'm experimenting with various thicknesses and in-fill to minimize weight and improve stiffness. The spars are also built flat and primarily use +/- 45 to handle shear web stresses. They will be printed at a different length so joints won't all be at one wing station. The caps have toughs that will be filled with uni-directional fiberglass the full length of each half wing. Fiberglass wing struts will supplement the bending loads for when I do something stupid pulling too many G's. The foam-board skins (Grey) will be full length of 30" of the Dollar Tree product and are the primary driver for wing sizing. Half wing span 30", 1 sheet makes 1 half wing.

Here is a one-bay proof of concept with 10" chord. It actually turned out better than I expected.

 

[quorneng]

That test wing section looks very nice.
When you say "Flaps" are they flaps or ailerons?
Their functions are very different. Ailerons are for roll control and work in opposition left to right. Flaps work together to maintain lift at slow speed but in doing so create extra drag.

My concern with a hollow printed leading edge is its impact resistance. Whatever the wing hits the leading edge gets it first. Obviously printing a new leading edge is not a problem but how easy would it be to extract the damaged bit?

The other issue concerns the difference in the structural properties between foam board and printed parts.
The two part rib glued to the spar will make a very rigid joint, however the foam skin above and below it will by comparison be rather more flexible. This suggests that under load the rib will break at the glue joint to the spar before the skin breaks. Put another way the skin will contribute little to the strength of the rib or to the spar for that matter. It is worth noting that the foam board is probably strong enough on its own but it would deflect much more than the printed material ever would.
Getting the maximum benefit from a component made of structurally dissimilar materials can be quite a design challenge.

 

[Inq]

That test wing section looks very nice.
When you say "Flaps" are they flaps or ailerons?
Their functions are very different. Ailerons are for roll control and work in opposition left to right. Flaps work together to maintain lift at slow speed but in doing so create extra drag.

I'm guessing you are not familiar with flapperons. This is analogous to elevons like are on most modern jets and even on the FT Mini F-22 Raptor. The difference being... flapperons use digital mixing to work the full length wing strips as ailerons AND flaps and spoilers. In the most aggressive form with 0° dihedral, I'll even incorporate some reflex/spoiler behavior (opposite flaps). The forth bullet in the O.P. itemizes how I expect to make basically three different planes in the transmitter based on dihedral configuration.

My concern with a hollow printed leading edge is its impact resistance. Whatever the wing hits the leading edge gets it first. Obviously printing a new leading edge is not a problem but how easy would it be to extract the damaged bit?

You may be right. I've never had issues with leading edge damage. Plenty of other modes... auger-in, nose first and cart wheels pretty much disintegrating the plane. But never leading edge hangar rash. Also, you might recall I use ABS. It is very forgiving. Many cars have ABS bumpers that "pop" back out. Besides, I plan to use the nice hollow tube to run the servo wiring for the flapperons instead of having to create a new path through all the ribs.

The other issue concerns the difference in the structural properties between foam board and printed parts.
The two part rib glued to the spar will make a very rigid joint, however the foam skin above and below it will by comparison be rather more flexible. This suggests that under load the rib will break at the glue joint to the spar before the skin breaks. Put another way the skin will contribute little to the strength of the rib or to the spar for that matter. It is worth noting that the foam board is probably strong enough on its own but it would deflect much more than the printed material ever would.
Getting the maximum benefit from a component made of structurally dissimilar materials can be quite a design challenge.

I took your hybrid model you discussed in one of your threads to heart and mind and I found a LOT of merit in it. 0.4 mm or thinner plastic walls is extremely prone to oil-can buckling. At the very least, the walls don't stay flat. At the worst, they buckle long before their ultimate strength is reached. Foam board is useless for 3D curves. Using each of their strengths and avoiding their weaknesses can make extremely light, yet near scale models. It is very rewarding, CAD'ing up an original design and printing it out.

I have a little experience with disparate materials design. I routinely designed things with negative CTE carbon fiber with E11 of 120 Msi, E22 of 1 Msi and bonded it to Aluminum with huge positive CTE and 10 Msi Modulus of Elasticity. And... it has to live in +250°F and -250°F cycled multiple times a day.

Now, working with ABS and foam and paper is very enjoyable. The little test wing-foil sample feels far sturdier AND stiffer than the equilivent total plastic models and certainly better than the injection molded foam models I see at the airfield. But the truth will be in the final product. My hope is it flies easy enough for a first-time beginner, yet can be configured so even an intermediate flyer won't be bored to tears. If I get good enough, and find it too boring... I can make some symmetric, clipped wings, pop a big motor in it and make 3D flyer out of it.

Probably not, since I'll switch to fast, scale war birds long before that.

VBR,
Inq

BTW... Today I had my second day of flight lessons and the instructor had me doing, loops, barrel rolls, take-off's and landings with his plane. When this project is built, I hope to be ready to put it through its paces.

 

[quorneng]

If my experience is anything to go by you won't get quickly bored flying a plane that you have designed and built. You will always be trying to work out why it does what it does and how to make it better.
Flaperons
Full span flaps are not aerodynamically ideal. They can promote the wing tip to stall before the root. The exact opposite of the preferred result.
How significant this effect might be from using full span flaps will depend on the aerodynamics of the wing and the angle the flaperons are deployed to.

If you look at a modern airliner coming in to land they have multiple flap sections along the wing and some have drooping ailerons as well but always with the biggest flap deflection close to the fuselage with progressively less out toward the wing tip. Its done to ensure adequate roll control is maintained as the flaps allow the plane to fly slower.

It is worth a bit of reading to better understanding the primary, secondary and in some cases the tertiary effects of each control surface to try to avoid making a plane that ends up with 'odd' handling characteristics.

Glad you lesson went well. Keep at it.

 

[Inq]

UPDATE
I have finished all of the "outside" CAD work. There are plenty of details for internal structure, placement of equipment, weight and balance and wing and tail plane incidence to be finalized. I would typically prefer to put the servos up front and use control rods, but for simplicity I want to check if I can move them into the tail. Adding them further off the CG will add polar inertia and thus more benign (some may say sluggish) yaw and pitch movements. Good for my novice piloting skill level.

I've also completed some analysis using this designed wing and projected weight and it should stall below 10 mph with 15° flaps.

Dimensions so far:
Span: 1770 mm (69.7")
Length with rudder/spinner: 1265 mm (49.8")

 

[quorneng]

Inq
I am a firm believer in placing the servos close to where they are needed primarily to eliminate the weight, slop and friction of control rods or snakes. The resulting improved mechanical efficiency of a short direct link allows smaller lighter servos to be used as well.
Is the polar inertia, particularly in pitch, likely to be a problem? For most planes pitch is far more sensitive than roll for both for inertia and aerodynamic reasons.
A minor point but does the nose need to be that long? It puts the prop a long way ahead of the undercarriage resulting in a prop strike at only a modest nose down pitch. May be not ideal for a trainer.
Just saying.

 

[Inq]

Good points...

• Short distance - sounds better to me. The Piper Cub 3D Model had a curved printed tube going to the back and had lots of friction. I thought it was way too much, but I assumed they had decided the friction was a good thing. It would have been a simple matter to make the tube strait as an arrow if they had wanted to.
• Polar inertia - I was thinking for a trainer this would be a good thing to increase the polar inertia to make it less sensitive to pitch.
• Pinocchio

You're probably right. 3DLabPrint recommends balance on the spar for experts, 5 mm forward for us Noob's. In the Piper Cub 3D Model, I had a 2200 mAhr battery full forward (they recommend 1500 mAhr) and I removed the tail wheel and I still couldn't get a forward CG. The instructor added a half-ounce weight up front and it still looked nose high to me.

For this model, I wanted to be able to move the battery as far forward as I needed. I also wanted to have a good tail wheel and the afore mentioned servos in the tail, so I needed more distance up front. I could have done some moment calculations, but I don't have good handle on estimating of the printed body, especially using the foam-board panel concepts. I have done a simple stick with main weights and found I can shorten the nose some. The wheels are shown with a 10" prop clearing over an inch when horizontal, plus I'll be using an 8" prop to start with.

But I think your point is significant. I'll plan on finishing and printing the wings, body from wheel back and be able to completely assemble the wheels back. Then I can accurately measure exactly how much nose I need. I can then simply scale the longitudinal length of the nose, CAD internal structure, print... FLY... CRASH. ... print again.

I was kind of thinking the functionality of...

... with some cosmetics of...

 

[Inq]

Wings Coming Along...

Wanted to give an update before I buttoned up the wings. Here completed...

• Ribs printed
• Leading edges printed
• Trailing edges printed
• Flapperons printed
• Wing-tips printed
• Servo / Strut attachment box printed
• All internal structure fastened to the bottom skin foam boards.
• Fiberglass re-enforced spar caps
• Finalized wing dimensions:
  • Span = 69.7" (1770 mm)
  • Chord = 10" (254 mm)

In an endeavor to have parts perform more than one purpose, I've created an ABS box that both holds the flapperon servo in place and is the mounting point for the diagonal wing strut. The box is ABS welded to the main spar and the leading-edge tubes. The servo wiring is routed through the forward, hollow leading-edge and is fully replaceable at any time. The door is fastened with the same mounting bolt used for the strut and is angled such that the strut will not require a bend. Here one is shown from the bottom of the wing while below shows the box before installation in the other wing.

Next

1. Install the port servo/strut box
2. Print the jigs to incorporate the wing washout twist
3. Attach the top foam board top skins
4. Attach the wing tips
5. Some clean-up, filling, sanding, polyurethane, painting
6. CAD up the (3) different dihedral Wing Boxes
7. Head on to finishing the CAD on the fuselage, empennage and print.

 

[model14]

I am enjoying following your project.
Are you using 9-gram servos? If so, I would question their power being sufficient for your project. Curious of your thinking here.

 

[Inq]

I am enjoying following your project.
Are you using 9-gram servos? If so, I would question their power being sufficient for your project. Curious of your thinking here.

Wishful thinking? Penny pinching? Massive rationalizations?

I have to admit, that it is probably my biggest questionable decision. (Savvy catching that model14) I certainly don't have enough experience to know what the practical limitations of these servos are. But, since you asked, here is my thinking...

1. It's a big plane, yes...
2. ... but it's light. Should be around 2 to 2.5 lbs. (But I'm typically optimistic AND I build things like a Brick Shit House)
3. It's got a big wing... analysis says stall speed is 10 mph with flaps, 12 mph without. Cruises at 15 to 20 mph.
4. Top end... with just a 2212, 1000KV, 10 inch prop and 3S probably won't be much higher.
5. In my first flights, I'll be using it with the 6° dihedral and aileron function won't be programmed. Exploring flap functionality won't be that risky unless only one deploys.
6. Rudder and elevator are critical items. Comparing to the FT Mini F22...
   1. This is 4x the weight
   2. This elevator area is about 2/3 of F22
   3. F22 flies 60+ mph - air forces are (20/60)^2 = 1/9 of F22
   4. It only uses the micro servos that have 1/3 the torque of the 9 gm model.
   5. I sweat (fret) the rod positions to eek out the most torque. Using the full 120° throw, I should be able to get 3x torque into the control surfaces. According to the Math, that should be about 4 pounds pushing on the control's horns. I can't imagine the horn would take that.
7. At the end of the day... if I auger-in, I'll have lost about $10 in materials unless I damage equipment. It costs me $10 just to drive to the airfield.
8. I can change the CAD drawing, press Print and install expensive/bigger servos and try again.

You have any recommendations for bigger (cheap) servos? I guess I could always use two 9 grams servos per flapperon.

 

[model14]

I like your thinking on using 9gm servos. Keep the info on your build coming. I am learning here also.

 

[quorneng]

Inq
Even a basic 9g servo has a stalling torque of 1.8 kg or 4 lbs.
This means such a servo will be able to directly lift nearly twice the total weight of your plane. Could a pilot lift twice the weight of say a Spitfire with just his arms? Yet he can do aerobatics quite happily and do them flying several times faster than your plane is ever going to go.
Your first crash is most likely to be the result of many factors but most unlikely from a lack of servo power.
Just saying.

 

[Inq]

Wing Update

The last wing is Gorilla Glue Curing now. I had some trouble keeping all the wing jigs in place and weights to compress the skins on the first wing, but I persevered and after removing all the paraphernalia, the wing retained the 5° washout twist. It's a little messy with the oozed GG, but I figure, I can sand and fill and by the time I poly and paint, I won't be able to tell.

I am a firm believer in placing the servos close to where they are needed primarily to eliminate the weight, slop and friction of control rods or snakes. The resulting improved mechanical efficiency of a short direct link allows smaller lighter servos to be used as well.

Took your comment to heart, plus I want to try to hide the horns internally. Not too important on this plane, but future war birds look better without the horns sticking out in the wind. I may look into printing some gears and get rid of the wire rods completely. Both are buried inside the fin/body as shown with a 3:1 ratio... +/- 60° servo travel = +/- 20° control surface travel. No external horns, no bulges. Now, I've got to figure out how to service them.

 

[quorneng]

Very neat.
However short servo arms or gears does not bode well for reducing backlash. Any free movement in a control surface can lead to flutter which because it is most likely to occur at speed tends to be serious. As rule the longer the arms the less the backlash and that includes the effect of the backlash in the servo gears themselves.
It all comes down to what penalty is acceptable for something that is really only a cosmetic issue at model plane speeds.

An 'external' linkage can always be aerodynamically faired in. They were on Concorde.
http://www.concordesst.com/pictures/fwtsb4.jpg

 

[Inq]

Had a setback... Totally irresponsible of me!

Even though I have been bookkeeping all the individual weights of pieces I'm designing (plastic, foamboard, parts) in a spreadsheet, I was mainly doing it for total weight calculation, I should have simply done the Moments calculation also before printing. AFTER I had the Empennage totally printed and assembled, I hefted the brick. 150 grams at 30" aft CG... says I have to have the motor/ESC/battery at least 16" forward CG... IN A 15" nose that some think looks like Pinocchio telling a lie already.

Now that I've done it, I realize those two servos in the tail... in addition to the non-optimized Empennage can't be balanced even with my long nose plane and 2200 mAh battery/ESC/Motor. As I think my biggest issue with the J-3 Cub trainer was my marginal CG location ... yeah, let's go with that... I've done an about face and now I'm re-positioning, re-designing, re-CAD'ing, re-printing as I write this. I'll at least get the elevator horn inside the plane. The rudder will have to waggle in the wind. C'est la vie!

 

[quorneng]

I fear what you have discovered is that a printing is heavy when compared to using 'conventional' aeromodelling materials. In the tail any weight penalty is very significant.
Bear in mind a long linkage from a mid mounted servo has some weight too.
Have you considered using pull/pull wires? Done well it can be light and mechanically efficient.

 

[LitterBug]

I fear what you have discovered is that a printing is heavy when compared to using 'conventional' aeromodelling materials. In the tail any weight penalty is very significant.
Bear in mind a long linkage from a mid mounted servo has some weight too.
Have you considered using pull/pull wires? Done well it can be light and mechanically efficient.

BINGO!

Most of the successful 3D printed planes use a completely different design for internal reinforcement and single wall printing. LWPLA is helping reduce the weight for 3D printing too. Need to complete a few of my 3D builds this winter to compare PLA to LWPLA.

 

[Inq]

Have you considered using pull/pull wires? Done well it can be light and mechanically efficient.

Actually, I considered using the braided fishing line... 0.1 mm is good for 10 pounds test, weighs essentially nothing since it's Dyneema. Has specific stiffness higher than steel. I'll keep that bookmarked for something like WW1 and WW2 planes with the short noses and big radials... DR1, Sopwith Camel, FW190A.

For this size model 70" span, 50" length, I think the pushrods will be fine... now that I've done the calculations I should have done up front, I've got plenty of leeway with up-front weight placement.