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I live in CA and went to FFW this year so no, I have no plans to go to FFE. Thanks for the support on the build. After the B-17 I plan to do a large but simple 8' span VTOL tail sitter. I want to document a build that is actually reasonable to build at FF in a day. After 2 weekends and a week of evenings I just finished the airframe of the B-17. It is reasonable to build it at home and then bring is disassembled, but I would say it is not reasonable to build at FF unless it was a team build, and even then it would take up most of FF. I can't imagine going to FF and then spending the entire time building, so the challenge is to come up with something that gets people excited but can still be built in a day. At least this build will do a good job for showcasing a number of flat plate foam build techniques once I document it fully.
Motor pods, motors, and all wing wiring installed. I can now begin "final" assembly including the radio install. Other than the logistics due to the size of the thing it should be pretty easy. I will have to make a servo extension for the elevator servos. Hopefully I can just buy, or already have the necessary extensions for the aileron servos. Both the elevator and aileron servos will use Y harnesses just because there is no reason not to.
The main power harness is hopefully the last of the soldering but I won't make it until I can install the batteries and test for CG. I am guessing 4 batteries in parallel, but if I have to use 5 or 6 that is no problem. It will just make for a very long flight time. Weight is not good, but if I have to install weight it might as well be batteries. The current target weight is 9 pounds.
I think it is mildly humorous that the motors fit the full scale profile as the spinners!
I did a full up assembly including flying wires and found I had a problem. The 40 pound test monofilament fishing line I was using for bracing is too stretchy, and possibly not strong enough as well. When I picked up the aircraft by the wing tips the wings bowed dangerously, and that was without the weight of the batteries. As a result, I have beefed up the flying line attach points on the wing spar and ordered some 100 pound test Kevlar kite string. Kevlar is known for its low stretch properties. Hopefully it is also easier to tie a reliable knot than with monofilament which is quite slippery.
While I had it assembled I did a CG check and found that I needed 6 each 3.3 Ah batteries in the nose to make it balance properly. It might tolerate being tail heavy and fly better with just 5 batteries up front, but 6 is safer for the first flight. Aircraft with lots of aerodamping tolerate being tail heavy better than fast models. That is why 3D models can often fly with such rearward CGs.
I made the main power harness, which has provisions for 6 batteries as inputs, and 4 ESCs at the output. The BEC's in those 4 ESCs can probably be paralleled and provide plenty of current for the 4 servos, but just to be safe I also provided a JST power connector. My plan is to drive a switching BEC set to 6V which will power the RX and the servos. The extra voltage will hopefully help to make up for any voltage losses on the long servo extensions.
I weighed the plane it it's nearly complete state and it is now pushing 12 pounds, twice my original estimate of 6 pounds. With a static thrust of over 6 pounds at full throttle it should fly but it won't do much other than float around like a butterfly.
To be honest, I consider this build to be a failure of my original design mission. It's a little too big to be practical. At 10 ft. span it would be more reasonable and probably closer to 9 pounds. At 8 ft. span it would be closer to 6 pounds which would give it a static thrust to weight ratio of over 1:1. At that point it would be fully aerobatic and easier to transport as well. I feel like I should have made the 8 ft. span model instead. Since the goal is to provide a solid reference design that others can build with a reasonable expectation of success, an 8 foot span model would be more likely to meet that goal.
I will still try to fly the 12 ft span model at least once. It may or may not survive the attempt but I have put too much work into it not to make the attempt.
R-Tech Styrofoam Insulation (or equivalent) is a fine building material. It is cheap, light weight, strong for its weight, and easy to work with. It can be used to make high quality models using standard techniques, but I am mostly referring to its use in quick build models, similar to the standard Flite Test foam board designs.
The plastic coating on either side of the Styrofoam adds considerable strength and minimal weight, but it is not always well bonded. It is not uncommon for it to come lose around the edges or to have areas that are not attached in the middle. This requires that the edges be wrapped with tape to prevent the plastic coating from peeling. One benefit of this technique is that the foam is entirely sealed inside and it is therefore possible to paint the resulting models with standard rattle can spray paint.
The foam itself is relatively strong in compression, but has minimal strength in tension. This is why the plastic coating is so important structurally. Adding packing tape or strapping tape in strategic locations can increase the strength considerably.
Since the plastic coating is not strongly bonded to the foam, anything attached to the plastic coating, such as servos with servo tape, will not be strongly bonded. When a “hard point” is needed it is necessary to use a wide plate and distribute the stresses into a large area of foam. This can be done with 3M Super 77 spray contact adhesive, or an outer wrap of tape, or both.
Sheets can be joined edge to edge by gluing the foam with any number of foam safe adhesives, but the joint will never be stronger that the foam. Any real strength comes from wrapping the joint with tape which bonds to the plastic coating on either side of the joint,
Foam can be laminated over large areas using 3M Super 77 spray contact adhesive. This is useful for making thicker stronger structures like fuselages. It is not necessary to peel of the plastic when doing this. If you do peel off the plastic then be sure to allow the 3M Super 77 to dry for a few minutes before making the bond, otherwise the solvents will attack the foam.
When laminating sheets it best not to attempt a precise fit. If the glue grabs before the pieces are properly positioned then it is virtually impossible to separate them without damage. It is better to laminate oversized pieces and then cut them to size/shape after they are laminated. Another technique involves cutting one piece to size and then laminating to an oversize piece. Then the first piece can be used as a template to cut out the 2nd piece so they match.
The best tool I have found for cutting foam is a thin sharp bread knife. The long blade allows for precise shallow cuts and straight lines when desired. Several shallow cuts results in minimal crumbling of the foam. It is also possible to saw vertically when cutting sharp curves. It is often best to cut the shape oversized by a ½” and then make the final clean cut. Depending on what you are doing in may not matter whether the edges are a little crumbly because they will generally be wrapped with tape anyway.
Tape is heavy and the weight adds up. It is easy to overlook this fact when placing small piece after piece. Avoid going crazy with the tape unless there is some good structural reason. Large control surfaces is one area where it is worth wrapping the entire surface to create torsional rigidity.
Applying tape without wrinkles or fingerprints is an art. There are many detailed techniques for doing it, but for a quick build it probably isn’t worth the effort. You can accept that there will be wrinkles and get the job done or you can fuss over it for hours. Wash your hands before placing tape to minimize fingerprints. Place the tape gently in one corner, then smooth the end, then smooth from one end to the other pressing out any air bubbles as you go. This becomes 2nd nature after you have done it for a while. When wrapping tape over corners, try to avoid large widths of tape in the over-wrap portion. This is where wrinkles will most often form. When wrapping long straight corners, use “the table as your friend” to press the piece into the table and stretch the tape around the corner. When taping free hand, use shorter pieces, like 18” or less because longer pieces are too difficult to handle. Compound curves require lots of little pieces, or you can just accept the wrinkles and move on. A proper packing tape dispenser is invaluable, but do not use it as you would for taping boxes. It is just a convenient way to hold the reel while you peel of segments with your fingers.
Packing tape makes excellent large size tape hinges. Tape the top side leaving a 1/16” gap between the control surface and the structure, then flip the control surface over and tape the bottom side in the normal manner. The resulting hinges are durable, precise, and flexible.
If you apply tape to a raw foam surface, especially a rough foam surface, it will have no strength. You must continue the tape to a plastic coated surface where it can stick. The tape adhesive is very strong in shear. Just an inch or two of lap is stronger than the tape material itself. Even a ¼” of lap is probably stronger than the plastic coating over the foam. Tape is not strong when pulled away from the surface. If the surface is the plastic coating on the foam, then the bond between the foam and the plastic coating will probably give way first, and once broken it cannot be easily fixed. Keep this in mind when designing structures.
It is sometimes necessary to tape inside corners. This is an advanced technique which requires some practice and skill. It can be done by dropping the edge of the tape to the surface and then rolling it into the corner, often with a tool like a straight edge.
For tape joints that need to be removable, it is best to use electrical tape which is available in many colors. The adhesive is less aggressive and therefore the bonds are lass structural. This technique is useful for attaching removable parts or covering over wires. If you use clear packing tape it is often best to simply cut the tape at the joint and leave it in place. Attempting to remove the tape will often do more damage that it is worth.
Packing tape is available in many colors. It is a cheap way to decorate a plane that is already largely built with tape.
Paint is heavy and foam is light, so expect to add considerable weight for a full paint job. In many cases it is better to avoid paint and fly the planes naked, or decorate limited areas with colored packing tape.
If the aircraft has the edges sealed with packing tape, then there is no restriction on using standard rattle can spray paint. If there is exposed foam then it is necessary to first seal those areas with water based polyurethane, either spray paint or with a brush.
Most types of paint do not stick very well either to the plastic covering or to packing tape. Spray paints will remain slightly gummy for a month or two and appear to stick but eventually they will harden and peel off in sheets. For throw away projects this is not a problem, but for longer term projects a solution is needed.
A very light misting of 3M Super 77 spray adhesive can be used to create a tooth to help the paint stick. It will create a slightly bumpy surface but no more so than the native surface.
A very thin dry brush coat of white latex house paint can be used on the silver side to dull the shine. It is not necessary to create an opaque white surface, only to create a dull film of white that you can still see some silver through. The side with the blue lettering is more difficult. You can selectively brush thin coats of white latex over the letters and markings to keep them from showing through, but it is a bit tedious. Keep this in mind when choosing your paint scheme.
Once the surface is prepared standard rattle can spray paint works fine. Latex house paint also works well and can be found in many colors, and tinted to almost any color. Masking tape is not a great idea as it will likely pull up the paint. Flat card stock or other non stick-masking is preferred. When spray painting, always use multiple mist-coats. When spraying a large horizontal surface you can spray over the surface and let the paint fall on the surface due to gravity.
For large projects, hand painted edges with latex and a good brush can work fine. You can sharpen these edges with a straight edge and a large permanent marker. The dark boarder not only cleans up the line, but also makes any color difference pop. Obviously you can use various colored markers as well. A sharpie can make as many panel lines and rivets as you like. If you want to protect the finish a coat of clear spray paint or brushed water based polyurethane works as well.
Painting does not have to be about bold monochromatic panels and straight lines. These aircraft will probably be imperfect in many ways, so use that. Design paint schemes that are imperfect, weathered, dirty, or burnt. Let your inner artist out to play.
This build is using a number of new techniques, or at least I am using them for the first time in this context…
The wing and horizontal stabilizer are removable. This is accomplished by extending the spar 6” beyond the stabilizer root and 12” beyond the wing root. The spars fit through holes in large plywood plates on either side of the profile fuselage. Since the spars overlap, one ahead of the other, there has to be a slot in the foam, either ahead of or behind the spar for the wing. In the case of the stabilizer the rearward overlapping spar encroaches on the elevator so there is no elevator hinge in that area.
Just to close this gap visually a thin strip of plastic is placed over the gap and covered with tape. The plastic is used because the inside of the tape would be sticky and would make it difficult to insert the spar from the other wing-tail. Closing the gap top and bottom turns the gap into a rectangular tube similar to a wing tube, but it has no significant strength. After the wings and tail are installed a pair of cable ties are used to cinch the spars together near the tips. This is structural, and while the spars can still shift past one another slightly, from within the cable tie, it greatly reduces the vertical flex of the spars. These cable ties are sacrificial, meaning they are used once, then cut free for disassembly.
For the horizontal stabilizer, these overlapping spars are all the strength that they need. The wings, however, are much longer and need to be externally braced.
The vertical stabilizer is just stuck to the top of the tail with several strips of Velcro. The Velcro has almost no strength in terms of holding the vertical stabilizer vertical, but it is quite strong in shear. This implies that the vertical stabilizer must be braced to the horizontal stabilizer, which is done with 40 pound test monofilament fishing line. It is difficult to tie the line with the proper tension so what is needed is something like a turnbuckle, and cable ties are used for this purpose. A cable tie is inserted through the hole in the plate at the top of the vertical stabilizer, and then the head of a 2nd cable tie is installed a short distance to hold it in place. The monofilament fishing line can then be tied through the hole in the head of the first cable tie. After the bracing lines are loosely tied on both sides of the vertical stabilizer the cable ties are gently tightened taking care to keep the vertical stabilizer vertical. These bracing lines do not need to be super tight as they work at a good angle. The bracing lines and cable ties are sacrificial in this case. As a matter of convenience, and to speed assembly, a piano wire clip is added between the bracing line and the head of the cable tie. This allows the cable tie to be cut and thrown away, but the bracing line is reusable.
The same technique is used on the wings, but there are some differences. The bracing lines need to be much stronger and are made of 100 pound test Kevlar fishing line. Kevlar is used because it is both strong and much less stretchy than monofilament. It is also easier to tie a knot that will hold under heavy tension. The wings have bracing wires on both the top and the bottom, as well as 2 bracing lines per side, 8 total. One of the bracing lines goes to the spar, and the other goes to the leading edge. This combination gives the wing a degree of torsional rigidity while still allowing the trailing edge of the wing to flex up and relieve stress in response to lift. The bracing lines on the wing are also much tighter in order to minimize dihedral flex.
There is one more pair of bracing lines between the wings and the horizontal tail. These prevent the tail from wagging side to side, which is quite possible since the fuselage has very little stiffness in that direction. As with the other bracing lines, these are tensioned using cable ties.
The wings on the model are essentially flat and mounted in the middle of the fuselage. The full scale aircraft had low mounted wings and considerable dihedral. This change in configuration is a structural compromise related to the use of external bracing in place of a tall spar inside a thick wing. Such a wing with a scale airfoil could be made from foam but it would make for a more complex and heavier build which conflicts with the original design goal.
The wings also have aluminum brackets attached to the leading and trailing edge at the wing root. These are used with a sacrificial cable tie and separate cable tie head to pull the wing halves together through a small hole in the fuselage. This also helps to align the wings angle of attack, but the primary “structure” is a patch of Velcro at the wings leading and trailing edge. Once again, the Velcro is quite strong in shear and should be adequate for a gentle flying model. These wing root pinch brackets are primarily needed for takeoff and landing. During normal flight the flying lines will pinch the wings together strongly, but on landing the wings will be thrown forward as the lower flying wires catch in the grass. This could pull the horizontal stabilizer forward and break the spars. Hopefully the landing speed will be quite low with minimal remaining energy. At 12 pounds and a 12 foot wing span, it should still float like a butterfly.
The B-17 is actually a simple 4 channel airplane. I did some transmitter programming and used a 10 channel RX because that is what I had but you could do this with the simplest 4 channel computer radio. You could even use a non-computer radio if you added a rudder servo, but at this point I am not even sure if a rudder is necessary.
The power harness has 6 each XT-60 connectors to accept input from up to 6 batteries in parallel. This is mostly to provide nose weight. Just one 3S, 3.3Ah battery would be adequate for flight. I have a lot of these batteries that I use for many projects so it is just a matter of convenience. The power harness has 4 each XT-60 connector outputs that provide power to the 4 ESCs. The ESCs are from the Power Pack D (Standard Quadcopter), currently $100 in the FT store. This includes 4 ESCs, BL Heli 20A XT-60 and 4 motors Emax 2213-935kv (CW Threaded Shaft) as well as 8 propellers, 10x4.5 CW and CCW 4 each plus some tools. I have measured about 6 pounds of static thrust with this setup so it should provide sedate performance with a 12 pound airplane.
The motors do not come with the standard “X” type motor mounts but you could bolt them directly to the plywood firewalls if you wanted. I used the X type motor mounts so I could more easily remove them for use in other projects.
Note that the battery connection to the ESCs is short, and the wires to the motor are long. It is important that the ESCs not be placed next to the motors with long wires to the battery. This would quickly destroy the ESC’s due to the “water hammer” pounding of pulse currents on their input capacitors.
Also note that all motors, ESCs, etc. are left in the stock configuration. No wires are cut or extended, and no connectors are changed. We will want to use this gear in many future projects.
The power harness also has a JST connector output which feeds a switching BEC (Battery Eliminator Circuit) which is set to 6V output. The higher voltage is intended to help make up for any losses due to the long servo extensions. Any reasonable BEC will do, even a linear BEC. It is probably possible to use the linear BEC inside one of the ESCs. It is probably also possible to just plug all 4 ESCs into the RX so the BECs are all in parallel. In my experience it does no harm to parallel linear BECs of this type, but always read and follow the manufacturer’s recommendations.
The wires from the ESCs to the motors are 16 AWG (American Wire Gage) stranded hookup wire from the hardware store. There is no need to use expensive wire from the hobby store. This sort of wire is flexible enough for the application. The motor wires are 20 AWG and the 16 AWG hookup wire has a lower resistance per foot by about 2.56 times, which helps to make up for the extra length. The motor “extension cables” have a 3mm male bullet connector on one end, and a 3mm female bullet connector on the other, and are just taped along the bottom of the wing.
The 2 aileron servos are physically arranged so they can be driven by a single channel and a Y harness, but as a matter of convenience I have them plugged into 2 separate RX channels that output the exact same signal. The two Elevator servos are are driven by a single RX channel with a Y harness in the tail. The motors on the left side could be connected together with a Y harness, but I used 2 identical RX outputs, the same for the 2 right motors. I have 30% rudder programmed into the motors for differential throttle yaw control. There is no physical rudder or rudder servo as I don’t think it is needed, but one could easily be added.
I am using the main RX and two separate satellite receivers. This is probably overkill but I am concerned about all the metalized (silver) plastic acting as an RF shield. One receiver is on the left side below the wing, one on the right side below the wing, and one on top of the fuselage. Hopefully this gives a clear path to one of the antennas from every angle.
I am using one heavy duty Y harness in the tail to drive the 2 elevator servos. This connects to a long custom servo extension back to the RX. All of the more or less permanent connections between RC connectors are tied off with dental floss. These large aircraft can flex in flight and pull connectors apart, even if the wires are taped down. It is a good idea to leave a tiny bit of slack where they connect to prevent this. There are 2 long custom made servo extensions from the aileron servos to the RX. All of these custom servo extensions were made from 1 foot heavy duty extension cables that were cut in two and spliced with a length of heavy duty 3 wire servo cable. Each individual wire is cut about ½” apart so the solder joints do not overlap, and a single piece of shrink tubing can be used to cover all 3 joints. This technique avoids having to buy a crimping tool for the contacts inside the RC connectors.
The 4 servos are standard Flite Test Tough Tilt Servos – Emax ES3154. They have a lot of mechanical slop in the output shaft but should be adequate for this application. The output spline fits JR style servo arms and I am using very long heavy duty servo arms. This is necessary because the tape hinges on the side opposite of the control horns. These long geometry linkages are a good thing because they reduce slop and stress on the linkage.
When the plane is assembled for flight, 6 bullet connectors and 1 servo connector must be connected for each wing half. These connections are made at the wing root near the leading edge. There is 1 servo connection to be made for each horizontal stabilizer half as well. It doesn’t matter if any of the servo connections are reversed. The motors might run backwards, but it is a simple matter to swap any two wires for any motor to fix them.
RX – Receiver
TX – Transmitter
ESC – Electronic Speed Controller (Motor Control Module) Different types are used for brushed or brushless motors.
BEC – Battery Eliminator Circuit – Converts Power battery voltage to approximately 5V to 6V to drive the RX and other radio gear. Also known as a DC to DC converter
SBEC – Switching BEC – A more efficient BEC that uses a slightly more sophisticated and expensive technology that wastes less power as heat.
Ah as in 3.3 Ah – Amps times Hours – A measure of the total energy stored in the battery.
S as in 3S – S stands for “Series”. 3S means 3 cells in series. Series means “in a row”, like cars in a train.
P as in 2P – P stands for “Parallel”. 2P means 2 cells in parallel. Parallel means side by side, like lanes on the freeway.
Y Harness – A special cable that allows 1 RX output to drive 2 (or occasionally more than 2) servos or ESCs
AWG – American Wire Gauge – Larger numbers are for smaller diameter wire.
JST – Japan Solderless Terminal as in J.S.T. Mfg. Co, a company that makes many types of connectors, some of which are used in the RC (Radio Controlled) industry. Within the RC industry the small red 2 pin male and female connectors that are used for currents up to about 12A are called JST connectors.
XT-60 – A popular polarized battery connector, or the mating connector, most commonly yellow in color, that is rated for up to 60A.
The B-17 has flown and landed safely. It is underpowered but otherwise OK. Video soon.
It took about an hour to set up. Some of that may have been due to answering questions and taking video.
The rigging techniques all worked as planned. One cable tie head broke during cable tensioning so I doubled up on the cable tie heads for the main Kevlar flying wires. It is fairly amazing that a plane of this size can be held together with tape, Velcro, and cable ties, but it works. There is very little glue involved, mostly 3M Super 77 spray contact adhesive to laminate the fuselage, and just a little foam safe CA.
The CG turned out to be on the edge of tail heavy. It’s not a problem. Big light weight models such as this have a lot of aero-damping and they tend to go where you point them. They also tolerate being tail heavy much better than heavy models.
The B-17 is not difficult to fly for an experienced pilot. We passed the transmitter to several pilots and flew for about 20 minutes. After the flight the batteries were at about 33%. The controls are sluggish but effective. The biggest issue is that it barely has enough power to climb slowly at full throttle. It is necessary to fly efficiently in order to do so. The angle of attack needs to be managed within a narrow range. If the angle of attack is too high or too low it won’t climb. Sloppy turns that lose altitude are not acceptable. It’s a bit like flying a large sailplane. Rudder via differential thrust is quite effective. Ailerons are more slowly effective. I saw no evidence of any bad characteristics like tip stalls. For such a large airplane it makes fairly sharp turns with low bank angles, mostly because it flies so slow. It has lots of drag, so the glide angle is poor. Basically you either fly it at full throttle to climb, or ¾ throttle to maintain altitude. It descends nicely at ½ throttle and is easy to land. Below ½ throttle is pretty useless.
It looks great in the air, very stately. The way it flies so slowly makes it look like it is really big and far away. In other words it flies at something close to “visual scale speed”. The fact that it is a flat plate profile model mostly disappears at a distance.
There were no structural issues, but we did not stress it either. The winds were light, about 3 MPH, and there were some mild thermals and sink, but it handled the resulting turbulence without difficulty. I would not want to fly it in heavy winds.
I think it would be a better model at 10 ft. span instead of 12 ft. More power would make a big difference. I am looking into upgrading the motors but at 12 ft. span and 13 pounds it will always be a very gentle flyer. At 8 ft. span it would probably be mildly aerobatic. It remains to be seen if the structure would be strong enough but I suspect that it would be if built with the same techniques at a smaller size.
Full disclosure, I did managed to hit a tree…. I made my final approach well above the tree, or so I thought. Being so big, it was farther away than I thought, and I managed to nick a branch with a wing tip. The tree bounced around, but the plane didn’t wiggle. The landing was otherwise uneventful.
The B-17 flew again, this time in a bit more wind and turbulence and with a bit more power to handle it. I upgraded from the Emax MT2213-920KV to the Emax GT2215/09 1100kv motor and used the stock Emax 10x4.5 props. The motors draw about 20A and get fairly warm when run on the ground but I think they unload adequately in the air. The motors did not seem overly warm after landing. The APC 10x5.5 multi rotor props draw about the same current on the ground, but I have not tested them in the air yet.
I am making good progress on the build instructions. They should be done in a week or so.
The 2nd build in the "Go Big At Flite Fest" series has begun. This time it is the "+ Size VTOL", which is a simple cruciform tail sitter with an 8 ft. span, 22” chord, and 4 ft. “span” on the verticals, so it’s a + Quad. I plan to give it 4 control surfaces for yaw control in hover, and more importantly solid control in forward flight. It will be externally braced with no spars and break down easily for transport. An entire sheet of 4’ x 8’ x 1” insulation foam is just slightly over 3 lbs. and it will use less than ¾ of a sheet. I figure I will need 2 each 3.3A 3S batteries in parallel since each motor can draw up to 20A and produce about 2.5 lbs. of thrust with a 10” x 4.5 prop. Hopefully it will be between 5 and 6 lbs. complete. I am also hoping that it will be able to do an auto-rotation tumble among other tricks.
This is definitely a build that could be done at Flite Fest in a day, especially as a group build. There is a catch though. You have to build it at home first, then strip it down to a small pile of parts, less the Styrofoam. The theory is that you can buy the Styrofoam insulation at a local home improvement store after you get to Flite Fest. The one sheet of 4' x 8' x 1" Styrofoam is only $11. Of course, if you have more room to transport parts, I will also design it to break down into 2 pieces roughly 2' x 4' and 2 pieces that are 2' x 2'. My personal version will have 2 each 2' x 2' pieces and one piece that is 2' x 8', since I have the ability to transport it like that.
Since this is a VTOL it will have a flight controller. I will be using the KK2 with OpenAero VTOL firmware of course. It supports tail sitters like no other flight controller that I know of. Given that it has a flight controller it will be tunable for anything from a totally tame beginner plane to a wild 3D machine. It will not be fast, and it will not handle much wind though. You could probably fly it in higher winds, but it wouldn't be much fun. At some point you spend all your time just plowing into the wind to stay in one place. Tail sitter landings that are dead easy when it is calm become challenging in the wind. You can still hand launch it and fly it like a regular plane though. If it tips over on landing, so what? It is not likely to hurt it much.
The airframe is complete at 2.77 lbs. Now it is just a matter of adding the radio gear, batteries, wiring etc. The batteries weigh 1.164 lbs. so it might make it in at under 5 lbs. Each motor has roughly 2.5 lbs. max thrust so performance should be quite good. Forward flight duration was never a goal, but it would not surprise me if it makes 15 minutes when flown gently.
The + Size VTOL has flown, hover mode only, indoors (Yes, I fly 8 ft. span VTOL's in my spare bedroom).
All control functions check out, so it is ready for an outdoor maiden, which will hopefully be early tomorrow morning. The power is good, it hovers at a little over half throttle. It weighs 5.85 lbs and the CG is 5.5" behind the leading edge which places it at 25%. 25% is a little tail heavy for a plank wing, but I am guessing it will be fine thanks to the combination of lots of aerodamping and a flight controller.
Tail sitters are nothing new for me so that is not where the risk lies. I am using a new technique for external bracing and it might shift under load. The front and rear bracing lines are each one long line that goes all the way around the aircraft. It makes a loop around the plywood plates attached to each wing tip. Hopefully the "cinching" effect of the loops will keep it from shifting. At the end of the line there is a small hook made from a paperclip. It hooks into the tip plate to complete the circuit. So long as the lines stay reasonably tight they should hold.
I am using 40 lb. test monofilament fishing line so it has a bit of stretch. I just attach the top and bottom center plates with Velcro and wrap the bracing lines around the aircraft. The setup should take about 5 to 10 minutes as compared to about an hour for the B-17.
As with every new aircraft, there are tuning risks. I know it hovers reasonably, but I may have too much or too little feedback gain in forward flight so it might try to oscillate. Eventually as I fly it and gain confidence I will shift the tuning parameters to allow for more extreme maneuvers. Eventually it should be fully aerobatic.
This is the biggest VTOL I have ever built by span. It will be interesting to see how it handles. I do not expect it to handle much wind but I am hoping it will surprise me.
She flew this morning with little drama. I have some tuning to do, but it was nice in beginner mode, with AL (Auto Level) in both hover mode and FFF (Fast Forward Flight).
For the 2nd flight I turned off the AL in FFF and did some minor aerobatics, loops, rolls, etc. The structure held fine with no slipping of my "quick turn" bracing wire system. She didn't feel tail heavy at all, nose heavy at 25% if anything. I need more aileron throw, it is sloooow in rolls. So much so that it is hard to do a roll and keep it flying in a straight line. Rudder and elevator are quite effective so you have to fly it through the roll and my 3D piloting skills aren't that great. I have already increased the throws and made some other changes that I hope to test tomorrow morning.
Oh, I did manage to crash it once. I was having too much fun and ran the battery too low. It tipped out of a hover from about 20 feet but she fell so slowly that there was no damage, not even a broken prop.
I usually provide 3 tuning options for VTOLs:
Very Easy Mode - AL in both hover and FFF. This is the VTOL version of the most gentle beginner plane you ever flew.
Easy Mode - AL in hover but more or less regular airplane mode in FFF. This is actually my favorite mode because you can still safely hover even when the aircraft is too far away to be seen well and you lose visual orientation.
Expert Mode - Rate mode, meaning no AL in hover, and the same regular airplane mode in FFF. In this case hover mode isn't really just for hovering anymore. Since it can fly in any attitude it can hover or fly in FFF just as easily. The use of integral feedback in all 3 axes makes it quite precise. It probably can't do any real flips because it is just too big and slow, but it can do all the normal airplane stuff and more.
This mornings flights were in wind up to about 5 MPH and it handled it just fine. The only real issue is takeoff and landing. It has a wide enough ground base so it doesn't tip over easily. The other problem is that it could quickly get blown downwind in the instant after takeoff. This is best handled by jumping it off the ground quickly, and then immediately pushing the nose forward. Even flipping into FFF mode immediately after takeoff would be fine. It is also possible to flip into a half-way mode between hover and FFF. In this mode It naturally wants to fly at 45 degrees nose up, but the actual nose angle depends on the throttle. At lower throttle settings it will drop the nose and fly almost level, or at full idle it will glide, though poorly. All of these characteristics are easy to handle and fun to explore. I still would not recommend it in over a 10 MPH wind though. You might get away with it but it would not be much fun.
I will get some video after it is fully tuned. I will also write up the build. It's pretty easy and should make a good Flite Fest build.