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The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

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The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  batjac on Fri Oct 03, 2014 6:24 pm

Since this was my first try at designing a twin, I decided I wasn’t going to say anything about it until I had made the maiden flight successfully.  That way, if it was a dismal failure I could just quietly scrap it for parts and no one would be the wiser, saving my sterling reputation.  Luckily for me, it worked out fine.

As it’s a twin, I kept debating whether to call it a BTC-4 variant, or start with the BTC-5 line.  But, I decided it’s a BTC-4 since, like the previous BTC-4s, it has:
a) ¼” balsa fuselage
b) Brodak engine mounts for the engines
c) 4” airfoil shaped ¼” balsa from SIG for the wing
d) And minimal finish other than a couple of coats of flat clear LustreKote.

I only took a couple of pics while building, as I was more wrapped up in building/thinking/building/re-thinking/building/second-guessing/building than I was in documenting.  Besides, it’s pretty obvious how it’s put together just by the finished photos.  It uses two Bee engines with 8cc stunt tanks.  One engine has a TD piston and cylinder, the other has a SPI P/C set from Bernie.  The outboard engine has washers under the left side of the engine to give it a couple more degrees of right thrust than the inboard engine has.   Finished weight is 10.15 ounces.  Plenty light for twin .049s.  








And here’s video of the test run of the engines.  I love those yellow safety props.  Really easy to see and keep your fingers out of.  I’ll have to get the fuel burn under control on the inboard engine after the proof of flight tests.




And here’s the maiden flight.  Flying on 42’ 20# Spectra lines.  It surprised me a little at the speed on take-off, but after looking at the video it seems that the speed was probably more of a perception in my mind than an incredibly fast flight.  Even after the inboard engine quits, it stays tight on the lines and very controllable.  I just did straight and level for this first flight, with a little up and down to test control authority.  I wanted to bring her home in one piece today.




I think it might need a few grams of tail weight, but maybe not.  It flies nice and easy as it is.  And is very controllable, as I said.  The glide is just not as flat as the previous two BTC-4 planes.  For this first flight, I kept the pushrod on the inner hole of the bellcrank to keep her tame.  Next flight will be with the pushrod in the outer hole of the bellcrank.  I’ll get the inboard engine to run longer and increase the throws, and then we’ll see about weight shifting.  I’m also going to pull the props and do a better job of balancing them.

The Under Wraps Mark


edit: I took a stopwatch to the video. Dual engine lap times average 3.29 seconds, and single engine lap times average 3.88 seconds. Still pretty spry on one engine.

The Spry Mark


Last edited by batjac on Fri Oct 03, 2014 10:17 pm; edited 1 time in total
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  Cribbs74 on Fri Oct 03, 2014 8:04 pm

Mark,

Your engine runs were good. It looks like it pulled well.

What's the current elevator deflection?

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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  JPvelo on Fri Oct 03, 2014 8:24 pm

Very very cool.
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  Kim on Fri Oct 03, 2014 9:13 pm

YES!!! Excellent !!!
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  RknRusty on Fri Oct 03, 2014 9:17 pm

Here's what I posted on your Tube channel:

They're on song. Blowing those bushes around pretty good back there. If you can't get that outboard engine to quit first, maybe swapping them would be a solution. Good work.
Rusty

I just watched the flight. Man, what power, it jumps off the ground like a scalded dog! Stayed tight when the first engine shut off too. Was it still the inboard one that finished first?

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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  batjac on Fri Oct 03, 2014 9:41 pm

Cribbs74 wrote:Mark,

Your engine runs were good. It looks like it pulled well.

What's the current elevator deflection?


Thanks for the compliments, Guys.  Ron, the current elevator deflection is about 20-25 degrees up/down.  It'd probably be enough, but with the speed this thing has on both engines, I don't want to find I'm just a little short on throw when I'm on the back end of a loop.  With the engines out on the wing, there's less direct prop wash over the elevator surface.

The Cautious Mark


Last edited by batjac on Fri Oct 03, 2014 10:13 pm; edited 2 times in total
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  batjac on Fri Oct 03, 2014 9:46 pm

RknRusty wrote: Was it still the inboard one that finished first?

Yup. It flew the second half of the flight on the outboard engine. I'm thinking the extra couple of degrees I put in the outboard engine was a good idea. This still had plenty of pull on one engine to keep the lines nice and tight. And on one engine it made decent lap times. I'm happy with the outcome of this experiment.

The Happy Mark
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  1/2A Nut on Fri Oct 03, 2014 10:44 pm

Impressive!!

The one outboard engine lugged that dead weight around with good authoritah Small Cox Logo
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  OVERLORD on Sat Oct 04, 2014 3:04 am

Very nice Mark. Instead of calling your series "Built To Crash", I'd name it "Bloody-good Trainer Concept"!!
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  batjac on Sat Oct 04, 2014 3:29 am

OVERLORD wrote:Very nice Mark. Instead of calling your series "Built To Crash", I'd name it "Bloody-good Trainer Concept"!!

Thanks, my Friend.  I’ll think about the name change.  In fact, after seeing how this flew on one engine, I’m half tempted to build a strict trainer variant on these dimensions, but just put an engine in the nose and do away with the wing nacelles and call it the BTC-4N (for Novice).  But that’s for later.

Since I’m happy with the way this flies, I figured I’d go ahead and post my working drawings for anyone to look at.  They're not pretty.  And they're certainly not to scale.  But good enough to understand what I did.  You'll notice I didn't have the fin/rudder on the drawings.  That's because I just winged it when I got to that point. Also, you'll see that the nose is double wide. Originally, I had planned to put 1/4" balsa cheeks on it to flesh out the nose. But in the end I decided against them, as they were only cosmetic and went against the BTC concept.

This plane took a little longer to build than it could have, as a high wing version would have built quicker and been a little easier to assemble. But, to me, a twin just has to be a low wing plane.





Looking at the plane, if I wanted to pretty it up, I could add a canopy and taper the nose. It'd look like a nice profile twin. In fact, I can see it as a German twin fighter-bomber if I changed the fin and rudder to a twin tail look.

The Sharing Mark

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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  Kim on Sat Oct 04, 2014 9:22 am

Stick a bunch of wires or toothpicks in it's nose and call it a Night Fighter!  'Course, you'll need to make an extra effort at keeping intruders out of the circle so you don't become "Mark---The Impaler"!

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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  roddie on Sat Oct 04, 2014 10:56 am

Yet another great accomplishment in your BTC series!! For such a small/lightweight model.. the extra drag of a second prop is going to affect your off-power glide I would think. The balance is probably right where it should be. That thing took-off like a sling-shot!!!!!!!! Shocked

P.S. if you're running a TeeDee cylinder on one of those engines.. It might be worth switching to identical-ported cylinders.. and swapping the outboard tank to a 5cc.. and make it the 2nd engine in the starting sequence. This "kind-of" assures that the inboard engine quits last. You may need more outboard tip-weight though. Two Cents
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  navion34 on Sat Oct 04, 2014 11:05 am

Congratulations ! Bravo,


Twin engines planes have an awesome song, not a noise, but a real song.

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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  RknRusty on Sat Oct 04, 2014 11:15 am

I think it's great that you guys are so innovative and contributing so much truly useful and fun stuff. When I first hit the forums a few years ago I was energetic and ambitious, just learning how to build, doing some simple engine experiments and making fun movies. Lately I've been only coasting and spending my hobby time away from Cox powered planes. You guys have picked up the torch and are now inspiring everyone including myself. My 1/2A activities will be picking up over the winter. No big builds this off-season. Thanks for all you do, this Bud's for you!
Beer Cheers
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  rsv1cox on Sat Oct 04, 2014 12:35 pm

Very nice, love those twins. Enjoyed both of your videos.

I hadn't thought of building a little east/west deflection into the outboard engine of my P-38, but that is a great idea if the inboard engine quits first. That has never happened to me though as I have two bone stock and well matched Golden Bees on mine. I lean them both out prior to flight then top off the inboard engine last. So far so good.

I would love to scale build an old Ford Tri-motor with three Bee's pulling.

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twin teedee

Post  faif2d on Tue Oct 07, 2014 8:55 pm

I built a twin flight streak at 150% of the 1/2 A version. I was going to use 2 teedee's running off 1 latex tank. I patterned this after a twin flight streak that I had seen at the 1962 Glenview nats. I built one of those in the motel room and mounted 2 K&B .35's on it when I got home. It flew very nicely but it was also very nose heavy. I never flew the teedee version but I did verify that the one bladder would feed both motors with no change between the needle valve setting on the motor started first. These had the custom kraftsmanship fine thread needle valves. My hobby life was winding down at the time and I ended up selling the model at Mike's in Dallas.
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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  RknRusty on Tue Oct 07, 2014 9:13 pm

faif2d wrote:I built a twin flight streak at 150% of the 1/2 A version.  I was going to use 2 teedee's running off 1 latex tank.  I patterned this after a twin flight streak that I had seen at the 1962 Glenview nats.  I built one of those in the motel room and mounted 2 K&B .35's on it when I got home.  It flew very nicely but it was also very nose heavy.  I never flew the teedee version but I did verify that the one bladder would feed both motors with no change between the needle valve setting on the motor started first.  These had the custom kraftsmanship fine thread needle valves.  My hobby life was winding down at the time and I ended up selling the model at Mike's in Dallas.
I like that idea. After cranking #2 did you have to re adjust the needles, or did the pressure of the bladder maintain the same flow? I'm guessing the answer is yes it did Maintain. If not, it seems like a thicker bladder would solve it. I use pretty thin tube for the 1/2As.

If my oldest Baby Streak gets much uglier, I might try it.
Rusty

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Re: The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Post  faif2d on Tue Oct 07, 2014 10:02 pm

There was no change in the tune of the first motor when the second was started and that line was opened.  I simply used a T fitting to split off the bladder into two streams.  I built a tank compartment in between the two fuselages at the LE of the wing.  If I had to do it over again I think I would arrange the bladder in line with the outer fuselage on top of the wing.  Biggest problem I had was one of the motors was a Gene Hempel tuned one and the other was a stocker.  I ended up with a stock venture on the GH one and a low compression head.  The other had a large venture and a Nelson plug head and it was still 2 thousand slower.  One of the reasons I never flew it.  I was going to use 52ft .012 lines.  The pull was very strong which makes sense when you figure the (2) 5" props had the same disk area as a 10" prop and they were turning18000 and 20000 rpm. I think the tubing was 1/8" x 1/32" wall as that was what I used on the 1/2A combats. Any higher pressure made the needles to sensitive.


Last edited by faif2d on Tue Oct 07, 2014 10:05 pm; edited 1 time in total (Reason for editing : answer question)
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