Sig Akromaster – lessons learned

[Maurice Bishop]

My hat goes off to Mike Gretz.

What an incredibly resilient design. I have flown my new build twice now and at the end of each flight, I managed to embed her into grass (soft earth) at an angle of 70 degrees – at speed. On each occasion, I broke the prop, filled the carb with mud and apart from some cosmetic tears to the silkspan, no structural damage has been done. A hairline crack has appeared in the fuse that I prized open with a knife and filled with Zap.

She is powered by an OS Max 15, stock muffler and an 8x4 prop. In an effort to keep the weight down, I have finished her with 4 coats of butyrate, no paint and neglected to install the undercarriage. An additional two x quarter ounce Prather weights have been added to the tail to get the CG anywhere near the right position.

The second outing caused me some concern. Following my initial ‘lawn dart’ adventure, I cleaned the motor, started her up in the garden and was disappointed by a terrific amount of air appearing in the fuel line. Understandably, the revs were varied in the extreme. I attributed the bubbles to vibration causing the tank contents to fizz. I changed the prop to a nylon 7x6 to eliminate a possible unbalanced 8x4 wooden prop. The bubbles continued to appear in the fuel line. I thought I might have structural failure of the fuse in the nose following the first crash, or a bent crankshaft! All sorts of potential problems were going through my mind.

I eventually traced the problem to a tiny hole in the fuel line, almost certainly damage sustained by the impact of the tank shunting itself against the muffler during the first unscheduled landing. A good learning experience.

Seasons greetings

Maurice        

[Bob Furr]

The Akromaster is a great little plane.    They even fly okay very overpowered.   Years ago there was a local racing event that required 300 square inches of wing and no more than a 35 engine.   I put oversize flaps on an Akromaster and a ST 29.   It would not loop very well but it was as fast as anything else being flown in the event.    That is one strong wing!
Bob

[James Lee]

Maurice
Way to Go!!!  That is a great start in learning to fly! The minimal approach to airframe and finish is  good idea.  Less mental strain about the looks and concentrate on the flying.   
My son did an Akromaster many years ago with a bit of stretch on the span and larger flaps, plus a small stretch to the fuse, plus an old fox 25.  turned into a real FUN flyer       
I have one that I've been trying to finish for several years that will have a Russian MK 17 diesel...  A bit overweight but who cares, gonna have some fun             
Later
Jim

[Maurice Bishop]

One small point. My outside loops are not as tight as my inside loops. 

I have checked the elevator throw and there is equal moment in both directions.

Very odd.

Any thoughts/suggestions/views?

TX in advance   

Maurice

[minnesotamodeler]

Alignment problem...slight upthrust on the engine, or negative incidence on the stab, or wingtips warped down, etc.  Simplest fix is to adjust the pushrod length for more down, less up, until the respective loops are equal. 

Now all you purists can jump in here and insist the warp, etc., must be fixed instead.  And you may be right.  What I said is what I'd do first.

--Ray

[Maurice Bishop]

This is getting silly.

While light, I keep breaking props. I have broken 4 props in 2 outings.

Anyone suggest the best way to retrofit undercarriage to an Akromaster?

Cheerz

Maurice

[Paul Smith]

If prop preservation is the target, try a single wheel under the engine.

Doesn't setting the prop so it quits horizontal work for you?

[Andrew Hathaway]

That's neat!  An Akromaster was one of the first planes I flew getting started.  I used the same vintage OS 15 you're using with 8x4 props and the stock muffler (till it was severely damaged in a crash).  The plane takes a surprising bit of abuse.  I remember putting the landing gear all the way through to the top of the wings more then once.  After it started to get heavy with repairs I got a few more kits and dad put them together for me.  Finally I drove the engine into the pavement nose first and cracked the crankcase.  I've still got the first one out in the garage with an OS 15 off ebay in the nose.  The second two we had were given away a few moves back.  The last of the three had a 1.75-2" stretch in the fuselage, Fancher Twister style turtledeck, and a 15FP (way too much power) it'd do the pattern on .015x60's.  I've got a fresh kit sitting in the 'to-do' pile, can't wait to get around to it.

[George]

Doesn't setting the prop so it quits horizontal work for you?

Some newer guys may not know to set the prop so it is horizontal when it comes up on compression.

BTW, that looks like a well taken care of Supertigre.

George