• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

FTCC'18 WWII - Consolidated Vultee XP-81

Fidget

Active member
#1
For FliteFest 2018 I'm designing an RC version of the Consolidated Vultee XP-81. It was a late-war prototype combining turboprop in the front and turbojet in the back, in order to get the efficiency of the prop and speed of the jet. In prototyping it was originally flown with the Merlin engine from a P-51 but later replaced with its intended turboprop. (See both noses below)

Planning on a C pack motor for the prop and a 50mm EDF for the back, with some adjustable mixing for the throttle control.

I got a copy of this book by Steve Ginter about it. Steve Ginter book.jpg
It is awesome and has all of the images I've found online, plus several others, including some cut-away cartoons describing how it's laid out.

300px-XP-81.jpg

xp_81.gif
 

PsyBorg

Wake up! Time to fly!
#5
This is Flite Test.. There is NO throttle control its either full on or full off ;)

Will be interesting to see how you do that mix of motors. I am gonna go out on a limb and say COG is gonna be a HUGE hurdle.
 

Fidget

Active member
#6
Yeah, I'm hoping to be able to test the mixing & CG without destroying an airframe with every flight. I thought I was being semi-safe by choosing a craft that had at least flown, but in reading Ginter's book it seems they were modifying it right up until they cancelled the program. They cool flight pics all came from one photo-op flight.
 

Fidget

Active member
#7
Yeah, I'm hoping to be able to test the mixing & CG without destroying an airframe with every flight. I thought I was being semi-safe by choosing a craft that had at least flown, but in reading Ginter's book it seems they were modifying it right up until they cancelled the program. They cool flight pics all came from one photo-op flight.
 

Fidget

Active member
#9
Perhaps if you look for an EDF and Motor n prop combo that has the same Amp draw? you could just hook them up with a y -harness and do away with computer mixing.
Good thought. I'd been thinking about the EDF almost like a turbo-booster, that would be on sometimes but just along for the ride other times. Or about being able to fly with either by itself, or combine both for more oomph.

Your suggestion got be looking at more details. I'm looking at a 50mm EDF which fits in the scale fuselage. It generates about half the max thrust of the C pack Emax motor. (600 vs 1100g) Weight wise I'm looking at basically an FT Mustang plus an EDF and ESC to run it. And maybe retractable landing gear. And I tend to build heavy planes. So... I might need both together... We shall see!
 

Fidget

Active member
#10
First Draft

I thought and thought about this and finally needed to make a rough draft I could hold in my hands. Planning to build at 1:15 scale, which puts it right at the wingspan of the FT Mustang.

I took the wing from the FT Mustang and took out it's slight leading edge sweep and increased the angle on the trailing edge. Traced the fuselage and empanage from the 3 view, and printed it out at 50%.

20170810_204244 reduced size.jpg

You can see that I hate cutting out the notched spars, so I got lazy and only did it for the right wing. But it worked well, so it stays in the design.

Then cut up the Mustang canopy, cut holes for air inlets for the turbojet (EDF) in the back and used a piece of paper to mock up the air inlets. ...and apparently didn't take a photo of that before I cut it apart to trace for parts. I also took a curve out of the fuselage to be able to fold part of it together rather than assemble from individual sides, top and bottom. I think if you were laser or CNC cutting that could be OK, but when I hand cut, my 3/16" channels aren't very consistent and without being able to push parts against each other, it doesn't build neatly.

XP-81 v1.4 for printing parts 50.png

Printed this second draft last night and hope to build it today or this week.

In the 1:15 scale version, the fuselage is too long for one piece of foamboard. For last year's contest, I divided it near the front, but that ripped apart with my first nose-in "landing". (That was the only type of landing that plane ever had.) I'm thinking I will divide it where the EDF is going to sit near the tail, since that area will be getting reinforcement to hold the EDF in place anyway.

Planning on C pack motor in the front and FMS brand 50mm EDF in the back. Still trying to decide whether to by the higher KV 3S version EDF or the lower KV 4S version. I've never flown 4S, but I'd hate to get it all built and realize it's underpowered.

The aileron servos will sit at the end of the lower part of the wing, like the FT Mustang. May have to print a full-scale version to decide on where flap servo(s) and bomb drops will sit. Thinking the rudder and elevator servos will sit in the rear top deck between the intakes, sort of like the Bloody Baron.

Still debating about retractable landing gear. Trying to plan the fall in episodes of 30 day shipping from Banggood is confusing, but haven't found nearly as economical an option in the US. Maybe I'll try Hilldaflyer's method and power the servos from a separate BEC so they don't brown-out my receiver if they jam. (I've never used retractable landing gear before, but that's my understanding of why servoless landing gear is a thing.)

Trying to avoid the temptation of building an FPV guided missile from a tiny-whoop turned on it's side. The fact that it's not historically accurate seems like it could be overtaken by being contemporarily awesome. I guess I could do (try) that in the winter and spring if I get the plane working. Another option I'm thinking of is an 808 keychain camera/DVR or something similar in the bomb, so I at least have video of it for later. I'll probably tack on an Eachine E010 or similar when I order the EDF at least.

I'm enjoying watching everyone else's builds. I kind of wish the forum had a thumbs-up button. I enjoy and agree with many more things than I comment on.
 

PsyBorg

Wake up! Time to fly!
#11
I would maybe think of using two of the Emax Gremlin motors in those tiny jet intakes for the larger build. Saves weight of the huge EDF and they can be ducted using foam easily as well. Having two smaller motors would make it more scale as well and I do think they would push enough air to actually help.
 

HilldaFlyer

Well-known member
#12
I'm enjoying watching everyone else's builds. I kind of wish the forum had a thumbs-up button. I enjoy and agree with many more things than I comment on.
Ditto! I'm a fly on the wall most of the time, quietly lurking about and getting great ideas from all the crafty imaginative folks. Here's my thumbs up for you:applause:
 

Fidget

Active member
#13
I would maybe think of using two of the Emax Gremlin motors in those tiny jet intakes for the larger build. Saves weight of the huge EDF and they can be ducted using foam easily as well. Having two smaller motors would make it more scale as well and I do think they would push enough air to actually help.
Cool idea. It's actually a one-engine jet originally, just with split intakes. But I do think I'll try an 1106 (same price as a single 1104 motor) setup. Found a "prop guard" on Banggood that is an aluminum circle with mounting holes and should help me mount the motor and shape a shroud. At 1:15 scale, the fuselage will only hold one 50mm EDF or 2" prop setup. But the 1106 setup will be about 7g, and the EDF will be 75g. (plus difference in ESC and battery weight)

Got v2 put together this afternoon. Seems like the CG wants to be just at the back of the spar at the fuselage.
xp-81 v2.png.jpg

Ordered parts and made some notes. Hopefully ready for a full-scale (well, 40" wingspan scale) in the next couple of weeks!
 

rockyboy

Skill Collector
Mentor
#15
Bump!

Any progress on this one? With EDF's taking over the FT world recently, I'd love to see a mixed power project like this in the air :)
 

FoamyDM

Building Fool-Flying Noob
#16
Fidget - How's this coming along? From the looks of it pretty well. My vote's the single motor with cowl. it will be easier for others to replicate and support FT.

There are times, I have paid $10 extra to have it ship from a US source. It sure can be a big pile of the suck though. Think of it like the Ticketmaster convenience fee... no, wait... don't do that. but you need to ask how much is the waiting costing you... There are time I don't have the monetary luxury. :black_eyed:

In the Signature be sure to look through the Team ALLIED FORCES page to talk about Color schemes insignia, and other fun things. We'd love your input!

Keep up the good work. :applause:
 

agentkbl

Illegal Squid Fighting?
#17
perhaps add set the throttle on the edf to a slider or potentiometer? or if you wanted to go really intense, and I have no Idea how you'd do that, but you could adjust how much edf to mix in to the throttle stick via a potentiometer, to adjust how much load on the prop vs the edf.
 

Fidget

Active member
#18
I've gotten "full size" plans printed and partly assembled.

I acquired both a 50mm FMS EDF and an 1105 FT Gremlin style Emax Red Bottom to try in the EDF pod. I've got a "box" to put the EDF in and slide up into the tail section, so that i can try both. There's a huge weight difference, so it'll be interesting to see how it balances.

I was all excited to start earlier than I did last year, but the late summer hasn't left much time for this. Now with work trips it looks like I'll be pushing the deadline and hoping for a warm winter again.

agentkbl, I think that would be the best way, but I have a DX6 transmitter with no variable sliders/knobs like. I'm thinking I'll set up flight modes with different mixes so that I can try them out. I'll try to maiden it with no EDF power, then start bumping up the mix.
 

Fidget

Active member
#19
Almost ready to maiden...

Thought I was going to maiden today, but my wife pointed out that maybe I should maiden it when there wasn't a tornado watch. Seemed reasonable...

I put together some photos and video of running up the motors. Several issues you'll see:
  • Assuming it would be tail heavy with the EDF in the back, I put the rudder and elevator servos far forward. So far that the pushrods basically don't reach. When I put glue on the inside of the hinge to prevent delamination, it stuck so far upward that the pushrod flexes like crazy trying to give down elevator. It is nose heavy with a 2200mAh 3s battery, so I actually need some tail weight. I'll move them way back in the next version.
  • I ended up using a regular full-size powerpod. I like it and will stick with that. I had to cut the fuselage (too long for one sheet of foam board), so I slid the powerpod between teh doublers and then slid the nose over it.
  • I made the hatch too wide, so it partially stuck to the doublers and wrinkled when I freed it. I'll make it narrower and longer. I put a rubber band over it to hold it closed, which I kind of like.

And...it's raining and my wife found water coming in the kitchen windows.
Be back later.
 

Fidget

Active member
#20
Upstairs is dry. Attic is mostly dry (drip down a vent pipe). Hopefully just need to re-caulk the window.

Back to the list:
  • Horizontal and vertical stabilizers flutter like crazy. I'm hoping this will be better with air going over them. I didn't balance the prop either, so that may be the issue. Sounds like it to me.
  • I made a "pod" for the EDF that I slid in through a hole in the bottom of the fuselage. Did that to be able to trade the 50mm FMS EDF with the 1106 red bottom motor that I bought to try out. I'm not overwhelmed with the EDF thrust, though, so I may not try the 1106. The EDF by itself won't push the plane across a table, although it's an impressive wind when you put your hand there.
  • The "scale" tail is about the right compression relative to the fan swept area of the EDF. However, I didn't really measure the intakes. Might need to cut away more of the square fuselage to increase the intake area.
  • I bought the 3S version of that EDF, but I might try 4S at some point. Also need some higher C rated 3S batteries I think. This was with a Glacier 30C 2200mAh 3S.



Probably won't get it maidened for a couple of weeks, but I'm excited to have gotten this far.

Things that might not get done:
  • Flaps
  • Landing gear (retractable or other)
  • Bomb drops