800mm swappable warbirds - these ones actually fly.
Like many other urban RC fliers, I have an interest in smaller planes. Having something small enough to toss in the car and fly in a small park where you can keep the space contained is very appealing. However, I suspect my experience with small foam planes is also shared with many others - they tend to suck.
I have been thinking of tackling this problem for some time, and was inspired once again by FliteTest, but this time in an unusual way. Recently they reviewed three HiTec “Weekender” warbirds, which are 800mm foamies. The FliteTest guys were struggling to say something nice about the way they flew (the consensus was they shared similar bad habits with actual WWII fighters were). More recently they also introduced their giant 200% series, which is fun, but lets face it - that is a building challenge but it’s not surprising they fly.
I think there is an idea that this is an unsolvable aerodynamics problem, but my totally uneducated guess has been that this is a weight:power ratio problem. Most small commercial foamies get around this through speed - they fly fast, or very fast, and if they don’t they fall out of the sky. Obviously this is not relaxing, small mistakes tend to lead to bad things happening, and it kind of eliminates the one advantage since you still need a big space to fly them safely. I have made several planes that flew way better than their commercial counterparts, I assume because they are so much lighter, so I thought I would give 800mm fighter planes a try. The results were portable planes with flight characteristics WAY past my wildest hopes.
I have previously posted designed of two fighters in the 1000mm-11000mm range that flew really well, much like their commercial counterparts, the Messerschmitt Bf109e (http://www.flitetest.com/articles/foamboard-messerschmitt-bf109-based-on-ft-spitfire), and the Mustang P51D (http://www.flitetest.com/articles/swappable-p51d-mustang-based-on-ft-spitfire). So I decided to make one of each with 800mm wingspans, to make 800 size powerpods, and try out some different motor/prop/battery combinations to see what happens. I basically used the exact same plans as I have posted, with modifications to account for the thickness of foam sheets (which does not reduce with scale) and to take off some weight and make the design simpler (less cutting). Below I describe the process quickly, and concentrate on how they fly.
Here are some pictures of the real things - a P51D and a desert model Bf109. I recently was fortunate to spend an afternoon at the RAF museum in London where these are (Colindale: http://www.rafmuseum.org.uk). It is excellent, and has an amazing collection of very rare planes, as well as lots of information. Comparable to the Smithsonian Udvar-Hazy Centre. I have updated all my old posts to include pictures of the actual planes and hope to keep doing this in future posts.
Here is a movie focusing on flight characteristics - launches, slow low passes, gliding, aerobatics, landing. It includes some on-board footage of aerobatics, and you will see my favorite thing is the vertical climb while spinning.
Here is another one by my flying friend Ed that has some nice comparisons with the full size planes. This focuses almost entirely on the Mustang, and at the end has some nice clips where you can hear how the full size mustang screams when it dives - a nice and un-planned feature of the air scoop I think.
Reduce - Reuse - Recycle
Another impetus for this was to use up the huge pile of leftover foam sheets I was generating. So these planes were made 100% from leftover pieces. In fact, the wings and fuselage can be cut out of the leftovers from wings and fuselage of their big brothers - the little pieces were all cut from scraps. No full foamboard sheets were harmed in the making of these planes.
Re-using excess foam sheets - this Mustang wing is cut from the leftover from a Stuka wing
Step 1 - modifying the plans
To make the plans, I simply took the existing Messerschmitt and Mustang plans, and reduced them to 80% size. When you do this a small number of changes have to be made to account for the fact that the thickness of the foam remains the same, 4mm. This mostly affected the corners of the fuselage and powerpod, the holes for the wing, and the way the tail attached, so I made these changes to the plans, and re-posted them below.
I also made another change for simplicity - I removed all “tabs” in the design except in the tail (so no more tabs in the spar, fuselage, formers, or mustang scoop). This made the cutting way easier and did not seem to affect the strength. I also simplified the spar, and added a wing servo cut-out since the thickness of the wing now no longer allows the servo to be completely inside - it now nicely sits flush with the bottom surface.
Step 2 - cut out all the pieces
This, as I noted above, is a good time to use up those weird-shaped pieces of foam you have left over from other planes. I managed to use up most of mine and used no fresh sheets at all. Hinges are exactly the same.
Step 3 - assemble the planes as described for full sized versions
This is the same as the full sized planes the following exceptions:
1. Wing Spar. the wing spar is a simple 8mm piece of foam folded back on itself, with no tabs or angles. I found it was stiffer when the two layers are sideways rather than upright (so the size you cut almost does not matter since the spar is 2X the foam thickness).
2. Wing Assembly. When the wing halves are complete, you will note the spar sticks out a lot and lacks the dihedral angle. Simply trim it to stick out about 10-20mm, then trim the bottom a bit to make some angle - you don’t have to be exact, the wing is going to be very strong. then fit the wings together so one spar is forward of the other and the leading and trailing edges match up - usually one spar is a little forward of the other anyway, and if not just force them - it won’t matter (you could probably cut them right off). When you glue the wings together, use piece of foam cut to 80mm in height to prop up the one wing to give the correct dihedral.
3. Paper upper decks. I found the tape was not holding the paper down well, so I glued it. This is likely a good idea in general. I used a piece of excess foam to press it down while it set to keep a nice flat joint (see below).
Gluing the read Mustang deck
Basic form of the Messerschmitt done
Basic form of the Mustang
Step 4. The 800-powerpod. This has proved useful when one motor failed or broke, so I am glad I kept the swappable mode. The powerpod is the same but scaled down, shorter and simpler (no tabs). I tested several motor/prop combinations and will outline that further below. With the motor I chose it is REALLY important to give it the right thrust angle (down and to the right). I used an X- configuration on the mount, with two nylon washers on the upper left bolt and one on each of the upper right and lower left. I also strongly recommend bolts and lock nuts on the top two mount holes because it’s easy to pop the motor off on a bad landing. For the bottom two I use screws since they end up over the foam and not the inside cavity of the pod.
Fitting the powerpod into place - the space between the pod and wing is EXACTLY enough for an OrangeRX reciever. I jam it in there for extra support for the powerpod.
Step 5. Painting. I skipped any fancy taping to keep the weight down, and because it is a lot of work, though in retrospect weight likely would not matter. Painting is important though, to give you good perspective when flying. I went for contrasting colours. The Messerschmitt is painted like the desert ones were, and this works great. The blue bottom and sand top stand out really well, and the top is light enough that you get good definition in the air. You can follow the plane even from quite a distance. The Mustang was painted silver with red wingtip and nose tape and also stands out well. Decals on the Messerschmitt were just printed on paper and glued on before the last varnish step.
The order of events to get a good solid finish that is sufficiently water-resistant (even for Vancouver) is:
- Oil based varnish (I use standard Minwax polyurithane like I use on wood)
- Water based acrylic paint (I use cheap craft paint, available in small bottles for $2 at Michaels craft store). I have a compressor, so I got an inexpensive spray gun (not an air brush, but a “touch-up” gun for automotive work - you can cover a plane in about 30 seconds) and do a few light coats (2 or 3 depending on the colour)
- Water based varnish (I use Minwax Polycryl, which I just had around)
Right after paining (always spray outside of course)
Step 6. Canopy. Making a clear plastic canopy actually really makes a big difference in how the plane looks, so I recommend doing this rather than the common alternatives like black paper.
The Messerschmitt canopy is the same as the larger one, scaled down. It is included in the plans. Cut and fold, then use clear packing tape pieces to tape the joints into 3D shape. Glue the whole thing into place on the fuselage, then use clear packing tape to further secure it. The frame parts are black tape.
Messerschmitt canopy ready to glue into place
The Mustang front screen is also included, but the bubble is not because it is a 3D piece of plastic. Easy though. It is taken from the shoulder of a standard 2L plastic pop bottle. Simple cut out a piece that looks like the one below, so the large arc is 110mm (i.e. the length bent up you can see below) and the length is about 90mm (i.e. the forward-aft distance of the bubble canopy). Bend it slightly so there is about 5mm overlapping with the fuselage, then mark the outline of the canopy against the fuselage with a marker. Glue it into place (put the glue on the fuse, not the canopy because the clear plastic warps when the glue is too hot), then also tape it into place with clear packing tape. Fold the front screen and tape it together with clear packing tape, then use the same to attach it to the front of the bubble canopy and glue down the front. Tape it down too. Then add the frame parts cut from grey duct tape.
Gluing bubble canopy into place first, then add front windscreen
Canopy joints are tapped with clear tape first, and then glued and taped into place on the plane. This is the Mustang front windscreen.
I tried three different motors and three props. I used a 25amp ECS all around and this seems to work fine.
First up was a HexTronic 24g 1500kv (http://www.hobbyking.com/hobbyking/store/__4859__hexTronik_24gram_Brushless_Outrunner_1500kv.html?strSearch=1500kv). I burned this out after 1 battery (and learned the Messerschmitt is a great glider).
Second up I tired the Turnigy Aerodrive SK3 2826 series. These are good sized motors but lighter than some of the equivalent powered outrunners. These functioned very well, and I used two different ones. The 1130kv (http://www.hobbyking.com/hobbyking/store/__18152__Turnigy_Aerodrive_SK3_2826_1130kv_Brushless_Outrunner_Motor.html). motor worked adequately for sedate flying, but lacked much punch. This is the motor that is usually on the Mustang, and probably accounts for why most of the footage in the movie is from the Messerschmitt.
The Motor I ultimately recommend is the Aerodrive SK3 2826 1240kv (http://www.hobbyking.com/hobbyking/store/__18115__Turnigy_Aerodrive_SK3_2826_1240kv_Brushless_Outrunner_Motor.html). This motor performed very well with a number of props (see below), and I recommend it or something like it.
9X4.5SF - This is a floppy SF prop, and worked adequately well, but led to a lot of vibration, so it was noisy and a bit sloppy.
8X4 APC - this worked ok for sedate flying.
9X6 APC - I thought this was going to be overkill, but it worked VERY well when used with the third motor. Makes for a nice quite flight, with huge amounts of power. Can go fast if you want, but also makes for nice slow flights too.
This is a 3S system, but to keep the weight down I used 1300mAh batteries. These are fine and give 10-12 minute flight times (with the crazy vertical spins featured in the movie you get around 10 minutes, but I have never actually killed the battery). I also used a 1500mAh and it worked about the same with expected longer flight times.
A nice tip is to put an elastic on the skewer to hold to in place, and at the same time you can use it to hold the battery in place.
The whole point of this is to make an 800mm plane with excellent flight characteristics, so lets break this down.
C of G. The Centre of Gravity should be right under the wing spar. This works well with the battery (1300 or 1500mAh) placed about even with the firewall, so nothing unusual. I put Mobius and HobbyKing Wing Cams on periodically and they were fine. I made the Messerchmitt tail heavy with the heavier HK camera once and it flew about as badly as you would expect (the vide of flying with Eagles is like this), but gave no real problems except landing was very poor. The Mustang seemed more sensitive, but this was maybe because it had the smaller motor.
Take off. Easy hand launch at 50% throttle and a gentle throw at 45 degrees. I discus throw from the wing, but underhand also works. I tried a vertical take off from stationary and it worked too, but I don’t know why you would bother. My planes needed a little trim because I make the tail really badly crooked.
Calm flights. These planes are easy to fly at low speeds or scale speeds. They are typical bank-yank controls, and I mixed a little rudder in with the ailerons to smooth it out even more. They do nice wide banking turns, or can come around very tight in a small area. If you dial down the ailerons a bit (say 80%) then you have power to climb out of any trouble without loosing it. They look nice on low passes.
Aerobatic flights. I was shocked at how nimble these planes were. They have great roll rates and very scale loops. Rolls are very nice, without a huge loss of altitude (just a little pulse of down in the middle is all you need). With the power of the 9X6 and a decent motor one of the funnest things i found to do was high speed vertical spins! Check out the movie. They just love doing this and climb out of the spin nicely. Pull out of dives well and do a nice job with snap rolls, hammerheads, and other basic moves I can do. I have not tried actual 3D stuff, but that is not the point.
Gliding. This is an acid test for a small plane. Kill the throttle and what happens? The answer is, nothing much happens. Check out the movie for a couple of glides. It is smooth and steady - no inclination to stall or tip stall from the Messerschmitt in particular (did not test the Mustang so much). You can glide the Messerschmitt across a soccer pitch without much altitude or effort, and when you hit the throttle it recovers from the glide very nicely and jumps right back to normal flight.
Wind. Another acid test for small planes. Often the wind bashes them around like crazy and tips catch the wind badly on turns. These planes maidened on a windy day as it turned out, and they flew fine. They bump around as you expect, but importantly they stay level and self correct very well. They have flown in very windy conditions, including regular gusts of around 15k or more and it was nothing that the power could not compensate for.
Landing. Kill the throttle and glide, since with the 9” prop you get a lot of leverage against the firewall if you hit it spinning. give it a little flair right as it hits the ground, and you are good (see the movie).
There are examples of most of these things in the video linked above, so if you are interested have a look. Its short and I tried to make it amusing. in summary, I have flown a lot of 800mm or so planes because I want one badly, but they are all pretty crap. I built these plans hoping that they would be tolerable at best. So I have been very very pleased. In fact, these are my goto planes now for any reason. They fly better then the full size ones, so why not?
Below are plans for the 800 series Mustang and Messerschmitt. The wing and the fuselage are shared (the front of the Mustang fuselage is actually slightly different, but it's trivial so I did not bother with two different plans. If you want just freehand the difference).
Messerschmitt Wing & V Stab (also Mustang Wing)
Messerschmitt Fuselage & H Stab (also Mustang Fuse)
For full build instructions, please see the original posts (and also linked below)
Mustang Build Instructions:
Messerschmitt Build Instructions: