Battery Cut-Outs and Spar Installation

I again used a dremel cutter to remove foam for the two battery locations. The 1050 mAh battery location only extended 1/2 way through the center wing section. The 2150 / 2200 mAh battery location, on the other hand, requires a full depth removal of foam. I'll have to create a "floor" for the battery compartment with lite-ply or 1/4" foam. The dremel works, but it is messy and time consuming. I think I'll be building or obtaining a small hot wire cutter to make future foam cuts.

 

After cutting the battery compartment holes it was very obvious the wing needed some sort of wing spar. I was hoping the prototype wouldn't need them, but after the cuts the wing had very low flexural rigidity. I chose to use some 1/8" x 1/2" scrap to fashion spars rather than carbon. I'll save the expensive carbon for the real high speed machine. 

I used my soldering iron to cut the spar channels rather than the dremel. This worked, but the slots were a little too wide. I would have preferred tight-fitting channels. To fill the gap, rather than heavy epoxy or hot-glue I used white Gorilla Glue. I soaked the spars in water, wiped the excess off with a towel, then set them in the channels. I tried three techniques to apply the glue, one for each spar section:

1. Apply gorilla glue to each side of the spar, then insert in the channel
2. Place spar in channel, squirt glue on each side of spar
3. Squirt glue into channel first, then place spar.

After the glue is cured it should be obvious which technique is best and will be used for future aircraft.

Once the spars were in place I put a stripe of "extreme" 3M packing tape over the top, to keep the Gorilla glue in place and to add additional strength.

Batteries & CG

A word about buying batteries from www.hobbyking.com: high quality batteries, low price- but it's a lotto pick on which connector you get on your batteries. For some reason on my last order I received two batteries with EC3 connectors, and two batteries with a "mystery" connector. Hobbyking, if you are out there- just ship your batteries with bare wires. Soldering new EC3 connectors on is pretty easy.Horizon Hobby has an excellent video on their Youtube channel.

 

Mystery Battery Connector

  

Control Surfaces- Active!

Tonight I covered the elevons with 3M "Extreme" packing tape. The elevons went from being super floppy- basically no torsional rigidity at all- to being reasonably stiff. At least I think they'll be stiff enough to work. I attached the elevons with 3M Blenderm tape. When I connected the servos to the surfaces using some old control horns from an old gas model, I cut the horns down to use the most aggressive hole. After I did this I realized the disadvantage- this hole not only gives the most surface travel vs. servo travel, it also gives the most slop. I'll have to watch this when I build the 'real' speedwing.

I tested the surfaces, and verified they were moving in the right directions. The elevator / aileron mixing seems to work well. This model should have plenty of control authority!

 

After using a dremel to core out space for the ESC, receiver, and various hook-up wires it was time to verify the CG is still ahead of the neutral point.  I tried balancing the wing using a 2200 3s, a 1050 3s, and a 2150 4s. All appear to work assuming that the battery has to fit within the boundaries of the wing. Note the batteries are all pictured in approximately the location required to have the CG just ahead of the neutral point (NP).

 

 

Finally, I attached a 9x6 APC E prop to try out the motor. Even with the 3s battery it seems to have a much greater than 1:1 thrust to weight ratio. Provided the wing is controllable it should be a blast to fly! With the smaller batteries it should also be a good floater.

I'm rethinking my initial plan of not adding any spars or reinforcement except for external packing tape. After I core out enough material for the various battery options there isn't going to be much left at the wing root. I haven't yet decided if I'll add simple carbon spars, or else cut some plywood spars to join the two wing cores across the battery cavity.

Flying Wing Neutral Point & Center of Gravity (CG) location

Next, I had to find the CG location on the wing in order to start placing components. I found this flying wing design website via google: http://www.mh-aerotools.de/airfoils/flywing1.htm

It outlines a pretty easy process for finding your neutral point and center of gravity for both straight (plank) flying wings and swept / tapered wings.

I used the formula and method for the swept / tapered wing. From the listed formulas to find the mean chord length and location of mean chord length, you simply draw a perpendicular line back from the chord/4 length from the leading edge at the mean chord loction back to the wing root. This locates your neutral point. (NP) The CG should be placed just ahead of the NP location.

My results for the 40" wing:
Mean Chord: 9.44" C/4=2.4"
Mean Chord Location: 8.38" from wing root

Then, graphically solving on the wing, the NP is 6.75" back from the leading edge.

Sanding

I did a rough finish sanding using 100 grit sandpaper glued to a scrap block of foam. This worked great- yielded a nice smooth finish without ripping the foam. The foam didn't load up the sandpaper either. I used Elmer's glue to attach the sandpaper.

I only took the high spots off of the wing- I'm not going to try and remove all of the spanwise artifacts from the wire cutting process. I'm afraid I'll remove too much of the airfoil. I also made sure the leading edge was nice and rounded.

Control Surfaces, Dihedral & Winglets

The prototype wing is coming together well. Last night's activities:

• 
  
  Trimmed trailing edge of wing to square it up and bring it to a uniform thickness
  
• 
  
  Cut out elevons and prop hole
  
• 
  
  Mitre cut wing root to add 0.5" of dihedral
  
• 
  
  Glued wing halves together (using 5 minute epoxy)
  
• 
  
  Attached winglets (using 5 minute epoxy)
  

I've deviated from the original design. This prototype is going to be built up as a lightweight floater, just to test the wing geometry, construction techniques, etc before I put together the final high speed wing. The elevon size, motor location and winglets were all eyeballed- no calculations used. This may prove a mistake, but it will be interesting to see how it flies. Since it is so lightweight it should be durable enough to try several different thrust angles and motor offsets. I also extended the elevons behind the prop to give some measure of vectored thrust to try and improve extreme slow speed maneuverability.

The resulting glued up fusalage is darn lightweight- only 95.6 grams.

Major Component Weights

• Wing, Wingtips & elevons, no cutouts 95.6g
• Turnigy plush 18A ESC 25.4g
• Esky 1000 mAh 3cell LIPO battery 87.5g
• Emax CF2822 1200kV motor, prop adapter, & APC 9x6 67g
• 2x Parkzone DSV130M Servos 31g
  

 total: 306.5 grams

 Using the WebOcalc calculator:

• estimated stall speed: 9.8 mph
• maximum speed: 50.9 mph (pitch speed)
• 3:1 Thrust to weight ratio
• 910mAh 3 cell 20c minimum battery
  

So far, looks good. The Esky 1000mAh batteries I have are total junk, so I'll try to fly this with the 500mAh 3 cells or 800mAh 2 cell batteries until the Rhino 1000mAh's arrive. I may also cut out the wing enough to allow trying the 2200mAh 3cells.

The next steps are to find some of the fancy 3M double reinforced packing tape for "spar" reinforcement and wrapping the elevons, plus some colored packing tape to cover the wing. I may also insert some balsa into the elevons behind the prop to stiffen them up.

 Top view, components arranged before cutting elevons or gluing wing cores together

Dry fit components, including pilot

 

Components epoxied together & drying (notice sweet painter's tape clamps)

Estimated Wing Core Weight vs. Actual

Based on an estimated foam density of 2.2 pounds / cubic foot, Solidworks estimated the foam cores would weigh 3.81 pounds. (this is the raw cores before cutting the lightening holes, prop cutout or area for electronics.)

After cutting the cores, I weighted them with a precision scale: 97.1g = .21 pounds

This is way off from the estimated weight! Assuming the real parts have the same volume as the Solidworks model, using this value to back-calculate the density of the foam, the actual foam density is 1.48 pounds / cubic foot. So, for anyone using the Blucore foam (sourced at my local Lowe's)- you may want to use value. I'd also recommend checking for yourself what your foam density is to account for mfg. variability.

I adjusted the inputs to Solidworks, and the new total all-up flying wing weight is 1.3 pounds. Besides making the aircraft lighter, it also has the welcome effect of moving the CG forward- it is now in the desired CG zone. (based on the Ritewing Demon CG location)

I also ordered Model Aircraft Aerodynamics. I'm going to do some more thourough calculations to locate the center of lift, thrust angle, etc.

In the mean time, I'm planning on doing a "quick and dirty" build up with the initial wing cores to do a rough feasibility flight test to make sure I'm on the right track.