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Airman 98,Any update pics of your F15C?

mikes68charger

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I worked on mine this week, put the single pipe in angled motor 8 deg to outward. K hope it's enough. I did add 7oz of led in wing to balance left and right weight. But had to move batteries out if nose due to lighter tail setup.

So im sitting at 39 pounds with UAT full, with a K180 i think I should be good.

Just waiting on the grass to dry and harden up

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Thanks for the update. Mine is getting close. Just finished the pitot airspeed sensor, and air brake. That was a chore. Waiting for a dry day to paint the fuse. Main wings and rudders are all painted. Tasks left are: battery mounts, stiffer main gear springs, and pressure test the fuel tanks again (just to make sure they're still sealed).

Airman98

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Couple of weeks ago it was warm enough for me to get all the parts primered. Then it turned cold again, but its starting to warm up so I can start sanding and primering all the parts. I too am intrigued by the single turbine setup so Im going to make it so one of the turbine mounts can be removed and another installed that has the offset to work with only one turbine. This will give me the option of twin or single whichever one I prefer. So really looking forward to seeing how the single setup works on Mikes F15.

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there was another guy who flew on one offset turbine and it worked fine.

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Guys, report your elevator travel. I can only get 1” up and 1” down at the rearmost point.

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Here is the pitot tube. The pylon is 3D printed. The prandtl tube is Spectrum and the electronics are Carsten from Germany. Works great on Futaba!

frankle951

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Thanks guys, for your updates, very impressive work.

Slotus7

Hello
Great work, I also have a F15 Kit in box, I am more and more motivated to start it, I think I will start the assembly, what are the references of the electric cylinders used for the retracting gear and the back brake?
Thanks in advance

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Airman98 appears to have used Actuonix 16L actuators. Probably 50mm travel but I'm not really sure. My airbrake used 16L 100mm. Same motor but twice the travel.

mikes68charger

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Without pulling mine back down, thats about all i can get also. It must of been enough for my maiden flight. But I would also feel better if someone who Is flying there could commit on elevators travel

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thanks for the info! Anyone else? If one uses the original Byron elevator horn, the ball link hits the fuse preventing much more. Maybe it’s a blessing as I’m not sure the airframe could handle a hard positive g from any more travel.

Airman98

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Well after 8 years of working on and off I finally got a maiden on my Byron F15 today. All I can say is Wow, didnt need any trim even when I put the flaps down. I have twin KT85s in it and to me its just the right amount of power, flew great at half stick. Have 80 oz tanks in it for each engine and should get 10 min flights. My cg is half inch ahead of what byron recommends and on landing its about one inch ahead. Elevator had 2 inches of up and down 1 inch on ailerons. I have documented most of my mods on my utube channel. The linear gear actuator I used is a firgelli L16-100-150-6-R. Will be finishing the detail work out this winter, just rushed to get it in the air this year. Any questions will be happy to answer. Tim
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Congratulations! Great first flight! As you know, I'm building one of these too. Mine is ready for maiden, but the summer has kind of slipped away from me. I read yours weighs 45 lbs dry. Mine is 48lbs dry. HOWEVER, I don't have the 2 inches up & 2 inches down of elevator travel. Do you think you really needed that much? I have Byron's recommended 1" travel in each direction. Appreciate your take on this. Was your elevator sensitive during flight? And did you have to move the stick very far back to get it to rotate on takeoff? Thanks!

Airman98

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Ok so my main wheel axle is 4 inch behind byron's cg. Had I thought more I would of moved the mains at least a inch farther forward to take some of the weight off the nose. I have 8 lbs on my nose which takes a bit of elevator to get to rotate. One more inch forward would of cut it down to 6 lbs on the nose. The original F15 only weighted 28 lbs so they probably only had 3 or 4 lbs on the nose that the stabs needed to rotate. It took a bit of elevator to rotate mine, hard to say how much I was giving it. Inflight I doubt if I used more than a inch. Elevator was not sensitive but I run 50 percent expo. My main springs are a bit stiff so it barely sits level on take off and its nose down a bit on landing, if yours slopes upward that would help with takeoff. I also didnt use any flaps on takeoff, next time I will use half and maybe this will help it rotate easier. Hope this helps.

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My mains are also roughly 4" behind the CG. Must be something about putting modern retracts in an old Byron. When you say 8lbs on your nose, are you talking 8lbs to balance the plane? Or is that the weight required lift the nose up by pulling up on the nose, pivoting on the mains? And is that wet, or dry? Mine, without actually weighing (I will), figures like this. 48lbs dry. The CG is around 43-1/2" from the tail tip. Overall is 103". So the nose weight zero if the wheels were on the CG. But they are 4/39ths back from that point which means 2.4lbs get added to the nose side. That's dry.....and the fuel is all a bit forward of the CG, AND THE WHEELS. So I'm guessing your 8lbs to lift the nose is with fuel. Does that sound close?

Airman98

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I use xicoy cg scales, so when I say 8 lbs on nose thats what the scale under the nose is reading. So my stabs are going to have to generate 8lbs of down force to lift the nose of the jet. This is with full fuel, my tanks are 4 inches behind the cg so when I land the cg moves about .5 inch forward of this. I might get some slimer tanks that can be moved forward and leave room for the wheel but it flys great as is and I didnt notice any trim change during the flight. I currently have 2.5 lbs of lead in nose but my cockpit and pilot weight 2 lbs so I will end up with almost no nose weight and thats with my engines mounted in the rear.

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I looked back at the actual calculations that I did a while ago after the plane was basically complete. I needed around 9.5lbs force on the tail surfaces C/L to get the nose to rotate, fuel full. This calculation was done assuming my limited 8 degrees of elevator deflection. The simulation was done at 40mph (17.88m/s). I can generate a theoretical 4.95lbs per elevator half or 9.9lbs total force downward, exerted thru the elevator pivots. Even through the center of lift of the elevator planes is a bit behind the pivots, the down force is still felt thru the pivots because that's what attaches them to the rest of the airframe. My all up weight is about 50.6lbs wet. I think the main reason I need more elevator downforce than yours is because my fuel is almost all forward of the CG. I guess we'll find out. I assume any bump the nose gear hits will bounce the nose upward, and catch some air under the wings. So no worries.

Airman98

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If you can get it to rotate I would think you will be fine for the rest of the flight. Take off was the only time I got into the elevator a decent amount. Next time I fly it I will use half flaps and see if that lessons the elevator input for rotation.

Airman98

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Pull up Now, I came across this thread yesterday. It a build thread of a feibao F15c, he started out at the recommended cg that feibao had for the d and e model. He said it took 500 ft to rotate, so he and his dad did there own cg calculations and instead of the 9.5 inches behind leading edge they came up with 11.75. Said it flew amazing and tried 12.5, still flew great and now flys with a cg at 13 inches behind leading edge of wing. He can really hold the nose up on landing now. So I think next year when I start flying it again I'm going to start moving my cg rearward. My cg is around the 11 inch mark so moving it to 12 would take a lot of weight off the nose. Here is the thread.
​​​​​​feibao f15c​

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Thanks for the info. Great discussion. I read through most of the Feibao thread. I couldn't find his wet weight, but could have missed it. I measured my CG-to-wing root leading edge distance and it's 10-1/4". I also compared the Feibao main gear location compared with Byron's and they are exactly the same. I scaled the calculation because the Faibao is an inch or two bigger. They are identical, so the rotation pivot point is not a source of difference.

I suggest before moving your CG all the way to 13, try a landing using the dorsal air brake. That will cause some pitch up moment, and also add drag which also will cause the plane to pitch up to maintain lift. Of course, a bit more throttle will be needed to keep from stalling.

One big question besides the weight, did the Feibao use takeoff flaps during that 500ft takeoff roll? That is a key piece of information to compare takeoff distance. Did yours need that much runway? Also, the Feibao maidened with 50lb thrust AMT Olympus, yours is 38lbs from the twin 85N Kingtechs. Mine has a single Jet Central Mammoth 250 with 52 lbs of thrust. The Feibao took 300ft of runway according to post #26, and it was on grass. Not too bad, as grass has tons of drag. So I'm not sure where the 500ft number came from. That's a lot of variables. I can't increase my elevator travel without a ton of modifications, so I probably will rely on my 52lbs of thrust to get me to my 45mph calculated number to generate the 10lbs of elevator downforce I need to rotate before I run off the runway. And then throttle back almost right away after airborne.

I'll be waiting with a lot of curiosity if your landings flare more with the big brake deployed. Your thoughts?

Airman98

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No he never mentions his wet weight but from other threads I have read those come out at around 60 dry. Here is the video that Lighter side of RC did on his F15 with him doing the cg, it looks like he did the 9.5 from leading edge and if you look at the maiden video it takes forever to rotate. He added 3 lbs of lead to nose. . Yeah I will start moving the cg back about 1/2 inch at a time. It looks like I might get a chance to fly it again this coming wed after that the temp drop 20 degrees and that will probably be it for this yr. Yes I will be using the air brake on landing for sure, one thing that caught me off guard was how much it floats and I was wishing I had used the brake. On landing I could of flared it a lot more but as I was running out of room I had to land it flat and get on the brakes. Mine did not use that much runway to takeoff but I did have to get into the elevator I think another lb less on the nose would really help. I think he talked about the 500 ft take off on one of his videos, if you go to his utube channel he has several videos on the 15. So I will report back on Wed if I get a chance to get more flights on my 15.
Tim

Airman98

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So was able to get 3 flights on my 15 today. Got to say it's now my favorite jet to fly in my fleet, the previous champ was my Cougar but the 15 is just so enjoyable to fly. I could of got a couple more flights on it today but didn't want to push my luck with the season over and want to give it a good going over again to make sure everything is in good shape. So I took a 1lb out of the tip of the nose which took 2 lbs off the nose wheel. So now the nose wheel has 6 lbs on it compared to 8 on the maiden. The cg is now 12 inches from the leading edge, it's still super stable so I'm sure you could take more out but when I did some inverted on later flights it required no elevator input to hold inverted which to me is a perfect cg. I tried half flaps on take off and I could not really tell any difference in takeoff distance to rotate, guessing 200 to 250 ft to takeoff. Used full flaps on landing, not sure if it adds much lift but it sure adds to the drag, made it a lot easier to nail the landing spot. Also used the speed brake, on the first flight I deployed it at too high a speed and it only partially opened but the 2nd and 3rd flight on opened it on base leg and like the flaps really creates a lot of drag, was hold at least half throttle until probably 5 ft off the ground. With the new cg it was touching down so much lighter. I took a half inch out of the main gear spring but think I need to cut another half inch off to get the correct amount of scissor action. Thats a wrap on my jet flying for this year. Everyone keep cranking out those 15s.
Hope this info will be of help.
Tim

Airman98

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So I flew my Byron F15 a month ago as seen in the video above and I loved it so much I got to thinking I wanted to start on another one incase anything were to happen to it I could get another one going fairly quick. But the thought of all the work that it takes to update a Byron kit into a modern jet was something I wasnt really excited about doing again. But I remembered a guy used to advertise a byron F15 fuse made out of epoxy, he also sold canopies for byron kits and I had bought some of them in the past. So I reached out to him and honestly its been so long since he had posted those ads that I really never expected a reply. But to my surprise he answered that day telling me all the work he put into making the molds and never sold a single one, but that he still had one for sale if I wanted it. Well I couldnt say yes quick enough. So looks like Ill be building another F15 soon. My current 15 I didnt put a lot of work into making it scale I just wanted something that would be big and fun to fly but I think this one I will put more scale details and put more effort into it. It should go a lot quicker because I have solved all the engineering problems in converting one to turbines so its just a matter of duplicating the parts on my old one plus change a few things that will make it better. One change will be to use electron retracts, the retracts in my current one work great but it was a lot of work and I think going with electron retract will simplify the build. So I made a unboxing video for anyone that wants to see the fuse, it really is amazing compared to the poly one that Byron made.
Tim