Building an Eagle

I’ve been wanting an F-15 for some time now. What I want is a reasonably scale looking F-15 for a single mini fan that the design could be blown up later to a size capable of grass field operations. So I started looking and didn’t come up with much that met what I was looking for. Here’s a list of kits that are available:

EAM has a nice twin mini fan F-15 with a length of 44” and 31 inch wingspan.
Bob Parkinson Models has a twin Kyosho fan F-15 with a length of 55” and 45 ½ wingspan.
Electric Jet Factory has a F-15 for 90mm fans with a 35" wingspan.
Classic Flying Machines has a twin mini-fan F-15 with a length of 46” and 36” wingspan.
Kress Jets has a F-15 for their RK-709E fan with a length of 36.75” and 27” wingspan.
HET has an F-15 for dual mini fans with a length of 47.6" and 33.7" wingspan.

Since none of the kits fit what I was looking for I decided to get some plans and roll my own. The only plans I could find were from Kress Jets. The plans from Kress are marked as semi-scale and with a modern power system and some ducting would probably make a great sport jet. For a number of reasons I decided not to try modifying the Kress plans and decided to try drawing my own plans in CAD.

I had a talk with Kevin Cox about what he had done with some of the F-15's he had built in the past. Based on that discussion I decided to shoe horn a mini fan in the smallest sized airframe that would hold the fan. I ended up with an F-15 with a length of 39" and 27.5" wingspan. Because I wanted to keep the airframe as small as possible yet still maintain a very scale outline I ended up with the fan sitting very close to the CG. I'm throwing everything else that I have a choice about in the very back. Fortunately, the battery compartment is long which gives me plenty of opportunity to adjust things to meet the CG. I hope.

Since I wanted a very scale appearance I decided to run a bifurcated exhaust. I haven't seen a lot of EDF's using bifurcated exhaust but I think building one based on the ideas in Designing a bifurcated tail pipe is worth a shot. If the bifurcated pipe eats up too much energy I'll just up the power system. The other reason I went with a bifurcated exhaust is that if I do build a bigger one down the road I want it to have retracts. I've never used retracts but all the threads about adding retracts voice a concern about squeezing retracts into twin fan systems so using a single fan should leave plenty of room for the retracts..

So this thread will document my design and build. In the past, I've always modified someone else's design so I knew I was dealing with something that would fly. This is my first design and I think it's got a reasonable chance at flying. If not, hopefully, we at least get an amusing crash out of it.

Dan Eaton

Hi-I have one the Classic Flying Machine F-15 kits. It's a nice looking kit, good wood and pretty scale outline and has room for retracts. I'm looking forward to having the time to bulid it.
Scott

Cool Dan, good luck. I've been off and on fiddling with an F-15 design over the past several months for a single Midifan, about 52" long, with about a .1 to .2" cheat in the fuselage depth near the back of the wing. The center "valley" depression in the fuselage on the bottom is there but not along the entire length, I'll drop the bottom of the fuselage at the centerline approaching that area and then after the fan a bit the valley will reappear. A cheat yes but hardly noticeable and I think I can add the extra fuselage depth and it wont be noticeable. The little bit of extra depth lets me avoid enlarging the plane to fit the fan and it keeps the plane roughly the same size as my F-4 and highly modded F-18 midifan powered birds. With "only" 1200 watts on 5S I need to keep size down for this moderately powered system (heck that's still 200 watts per lb but lack of gear doors keeps the speed down to about 105 to 110 mph).

Here's an early drawing, I haven't done much since, added some stuff on a paper copy but I've yet to start drawing seriously. Depron is in my crosshairs lately, looking for a lighter quicker building jet but still with scale outlines and cross sections. The thicker material allows a simpler construction with less ducting work as the outer skin and inner skin in the ducting areas can be the same piece of 6mm material, at least until the ducting goes inward toward the single fan. Sorry, this is on a different tangent, I still like building with balsa and lite ply.

Hi Scott,
The Classic Flying Machine F-15 kit is the one I came closest to buying. I thought it offered the best opportunity for enlarging the plan later for a big one. Only reason I didn't buy it was I really want a larger F-15. Keeping the cost down on this first smaller F-15 makes the bigger one happen sooner. When you get time to build this kit, please do a build thread. I'd be really interested to see more about this kit.

Hey Ed,
yep, I ended up using the same cheat to squeeze the mini fan into this bird. I've got the fan sitting just ahead of where you show yours. I probably could have built the bird slightly larger and ended up with the fan in the same location as yours. It makes me a little nervous having the fan on the cg but the gassers did it that way so I figure I can get it to work.

Your drawing looks good to me. That's another 1st I'm trying with this design. I'm actually creating a formal plan for this one. Right now it's mostly just a copy of the stuff I've produced parts for. Later I'll have to go back and add in all the non CNC cut parts.

Hey, no worries about Depron. I use the stuff all the time. It makes great sacrificial boards under the balsa I'm cutting on the CNC machine.

Dan Eaton

FF is also good for cut through on the cnc, gives you 1/4 inch and tends to sit flatter, which os handy cause then you can run a quik grid cut program on it, then drill the hole in the middle and keep you vac hold down if your using one. Plud its cheap sacraficial material cause its only 25 for the bundle. which will cpver you for quite a few jobs.

Barry


Barry

Hi Barry,
I was just teasing Ed about the Depron. I use FF for the sacrificial boards for exactly the reasons you state. Great idea about cutting the grid and using vacuum. I use a special fixture that matches the cutting area of my mill. Changing it over to a vacuum box would be easy.

Dan Eaton

oh man its so nice to have the hole down. Just drill your primary hole towards the corner of 0-0, my first hole was centered and that was me not thinking, cause your usually runnign or starting at 0-0 (especially on small parts) and you'll be covered on hold down there. I just run and easy quik plunge of 2 wide by about 6 inch grid. thats enough to hole down a 4X12 pice of balsa if you want just to whip something out.

Barry

Nice Dan.

What wing program do you use for ribs and spar placement?

I’m not a 3D CAD professional , nor do I play one on TV, but every time I do one of these projects I learn a little more. This time I learned that 3 views suck. Actually, I’ve known that for a long time but I finally figured out how to create a good 3D model despite the problems with 3-views.

What I did for the F-15 3d model was to break the fuselage into blocks based on the main lines in the top and side views. That insures the model at least matches the top and side plan views. I saved a copy of that as a checkpoint just in case. Then I went back to each block and used the cross sections from the 3 view to contour the blocks as close as possible. It’s not perfect but I think it gets you pretty close.

The first picture shows the F-15 as blocks, and the second is the fully rounded end result.

Dan Eaton

oh and was playing around a bit ago and found that a 620 will fit in a EAM F-15. Which should leave for scale landing gear location. I did conformal tanks on mine for a E but might be dropping twin mini's for single 620 in it.



Barry

Oh man I feel like a caveman with my 2D cad but heck even if I knew 3D I'd probably still start out with 2D to get my side and top views to match dimensions and bulkhead shapes and then go 3D.

I wonder if Geico sells 3D cad software for this caveman.

I spent some time looking at my HET F-15 for possible retract mounting and wanted to put some on the bottom and not in the wing root fairings like Ted did with his (although they look fine) but the more I looked I just started getting ticked off that they didn't do gear any favors, regular non rotating gear has so little room in the outside edge I was looking at, the ducting is so close to the edge of the fuselage I'd need a major fairing inside the ducting to make room, and the fans are way far apart, if they were closer together I'd be easier, or if the ducting came together in the middle there could be room for gear. Also thought of cutting the bottom of the fuselage off and pulling the ducting out and putting new stuff in with a different shape but then thought that's too much work, 200 dollar arf and it turns into a scratchbuild to get gear in it, I'd rather build one myself from scratch. I'm thinking along the lines of big, light airframe with maybe two fans throwing a bunch of air for tons of thrust, somthing I can rotate, suck up the gear, hold it level till the end of the runway, and pull back and climb vertical high enough to do a half roll and pull into a 3/4 loop over the top and back down the runway opposite direction for a high speed pass. My F-22 and F-18 just don't have that kind of power. Well my F-22 on 6S pulls 1850 watts and 89 amps, boy those deans ultra connectors get hot, not an everyday setup, especially this summer.

Oh Dan, a thought, are you going to split the pack up into two? With the fan more foreward you'll probably have to, wont be able to slide the battery back far enough with the ducting coming together. I think you might have to place a pack on either side of the fan. Depending on the motor setup this might not be a big deal. For instance if you're going for say a Mega 16/15/2 in the MF 480, I run two 3S 2070 Extreme packs in parallel (I can use the packs in my Stryker individually), so one pack on each side of the fan could be used. The same goes for a 4S setup. I usually charge the packs in parallel using the same parallel connector in the plane (EFlite makes one for their airliner or I use one made from an FMA circuit board with deans connectors on it). I probably should charge them individually or charge most of the way parallel and then seperate them to finish off, sometimes I do that.

Of course if you want to use one large pack you can't do that...so moving the fan back to near the wing trailing edge and cheating fuse depth just a bit may be a good idea. Just trying to stir up your grey matter (brain).

Can't wait to see more progress!

What wing program do you use for ribs and spar placement?

Hi Kevin,
I used Compufoil 3D to generate the N64A and N6409 foils you suggested. I had to get Compufoils DXF module so I could export the foils into my CAD program. Once into CAD, I built a 3D model of the wing with the N64A and N6409 at the root and tip and a straight trailing edge. This gave me a smooth transistion of the foils for the length of the wing. I then sliced the 3d model where the wing break would be to get the correct shape of the foil at that point. I then deleted the original model, scaled the foil at the break to the correct size for a wing with the break, and then built a new 3D model using the root, break, and tip foils to get the wing to the correct shape.

Then I had to twist the model to get the 4 degrees of wash out. The first time I tried it I just rotated the back of the wing up by 4 degrees just like I was buiding a wing on a flat table and added a stick under the trailing edge to give me the wash out. That didn't look right so I went back and twisted the wing along the center line of the tip foil and that seems to match the wing on the full size pictures I've looked at.

Once the 3D wing model was done it was just a matter of slicing thru the model where I wanted ribs to be and then removing 1/16 of an inch from the rib outline to account for the sheeting. The spars were placed with the TLAR method. To make sure I got the wing shaped correctly when building I added tabs to the bottom of the ribs as a building aid.

Dan Eaton

Oh man I feel like a caveman with my 2D cad but heck even if I knew 3D I'd probably still start out with 2D to get my side and top views to match dimensions and bulkhead shapes and then go 3D.

Hi Ed,
matching the side and top views is the easy part. On a single engine jet like an F-16 you can even get the cross sections to mostly line up in 3D. But on the twins like the F-15 and F18 the cross sections have so much going on that it gets very difficult. Making accurate blocks from the major lines of the 3 view and then contouring those blocks using the cross sections as guides works for me. Somebody that really knows 3D CAD probably has a better way (and I'd love to hear about it) but for now I got something good enough to let me build a plane.

I can see it now, a wireframe gecko on my TV promoting Geico 3D, so easy even a cave man can do it.

I'm thinking along the lines of big, light airframe with maybe two fans throwing a bunch of air for tons of thrust, somthing I can rotate, suck up the gear, hold it level till the end of the runway, and pull back and climb vertical high enough to do a half roll and pull into a 3/4 loop over the top and back down the runway opposite direction for a high speed pass.

Me too, Ed. Me too. I like the way you think. But not for this bird. It's my first try at designing something and if it putz around the sky at 50 mph barely able to get out of it's own way, I'll be happy. Just so it flies. There's things I can do after the maiden to adjust the 'go fast' factor and if that doesn't get it then at least I'll have an idea of how to make the next one faster.

Oh Dan, a thought, are you going to split the pack up into two? With the fan more foreward you'll probably have to, wont be able to slide the battery back far enough with the ducting coming together. I think you might have to place a pack on either side of the fan.

I'm using a Mega 16/15/3 running off a pair of Tanic 2S 2480 packs configured as 4S. Looking at the amount of material going behind the CG versus the amount of material going ahead of the CG I think I'll be ok with the batteries in the battery box I designed. But that's one of those questions I won't know the answer to for a while yet.


Dan Eaton

Hi Dan,

Nice work. I like the use of blocking for ultimately controlling crossection shape, I use the same kind of techniques with AutoCAD. I guess I will sit back and follow along.

Cheers,
Eric B.