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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.

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F/F: Dixielander Trim

By Joe Supercool

Well folks, I have just come next to last in a SLOP contest. For those not in the know, SLOP stands for Slow Open Power free flight, which means:

  1. No auto-rudder
  2. No VIT
  3. No Pressure feed
  4. Plain bearing engines up to 3.5 cc only

In other words, back to the 50’s style free-flight power. There are many wonderful designs from that period, magical names, such as Eliminator, Posthole Digger, Lipstick, Sweet Daddy Pearl, Skystreak, Starduster, and here in the civilised world, the grand champion of all time, the Dixielander.

It seems there is renewed interest in the Dixielander by those who remember the flood. Back in 1959, Jim Baguley wrote in “Model Aircraft” a defining series of articles called “Power Duration Models”. The date was December, 1959, and on the cover was a Ta 154, the German “Mosquito”. It might then seem strange that a proven loser like myself may choose to revisit this topic, but my regular readers know that a swollen ego like mine lets me look down on even people who are taller than me!

However, for the folk I see trying F/F and didn’t see the rainbow, it might be worth a few words of refreshment on trimming SLOP’s. I shall couch my words as being authoritative, as recently I had a dream. In the dream, God was abusing me for using weasel words like “perhaps”, “ could be”, “if”, and “etcetera”. This is what he said. “Supercool, I have given you the gift of life. Stride forth upon the Earth and enlighten the modellers of the world. Be not timid. Go forth. Go out with a bang, not a whimper. Trust your own judgement. Invest in your own talents”.

Well, with encouragement like that from on high, I sincerely recommend that you read on. Its not just me He is watching!

There are to my certain knowledge 3 George Fuller Dixielanders. Here are some pics, one of modified Dixi 2000 and one of Dixi 2000 plan.

Plan


PLane

The Yeoman kit version, the E-type and the Dixielander 2000. They run something like this:

  1. Yeoman kit. 3/8” washout at the tips, 3/8” washin on the right inner panel, heaps of tail tilt and relatively low power. OS15 Max 3, various 2,5cc diesels, probably a maximum of 0.3 HP
  2. E-type. Washout and washin reduced to ”, longer tail moment, more power in the form of Eta 29, probably about .5 HP
  3. Dixielander 2000. Much the same as the E-type, but rather surprisingly, no tail tilt shown on the plan. Hmm, looks like a mistake to me.

So we are looking at a model with built in trim. The washin gives a right spiral, and the tail tilt the glide turn. But there are devils in the detail. Here is rule number one for launching Dixielanders.

Rule 1. The launch of the model must be with the wings banked into the power turn, to the right of the wind, and nose up at an angle commensurate with the installed power.

With Rule 1 followed, the model will go into some sort of right spiral. This is not a left roll. Right rudder tab will increase the number of turns in the spiral, and left tab will reduce the number of turns. The thing to watch for is the model chasing its tail round and around, thereby reducing the climb height.

You really only want about 3 turns in 10 seconds. This will put the model into a good position for the transition into glide. It should do glide turns to the right, set by the tail tilt (starboard tip high). If the turn to the right does not appear, and the climb is too spirally, there is a C/G problem. Tail tilt only works if the tailplane is carrying a sufficient share of the model weight, which means rearward C/G.

The model doing too many turns in the spiral can be both a C/G problem and a trim tab setting problem. Here is Rule 2.

Rule 2. If the model does not turn to the right on the glide, move the C/G back. (This assumes you have plenty of tail tilt)

The plan probably shows the C/G on the trailing edge. This position is not carved in stone. My 2000 has the C/G 20mm back from the trailing edge. By now, the only remaining problem is to get the number of turns in the spiral right. Here is Rule 3.

Rule 3. Increase the number of turns with more right tab. Decrease them with left tab, even if it means the tab actually goes left. 3 turns in 10 seconds is about right.

The last problem is running the motor on suction. Small venturi’s give low power. You want the hole in the venturi as large as you can get it and still be able to tune the engine. On my OS20, the engine burps a little after launch due to starvation, but then picks up again. That is about as big a hole as I dare run.

Hope this epistle is of help to someone out there. If so, send cash, no cheques.

 

Dixie addendum:

Subsequent to the 50th centenary of the Dixielanders birth and the 63 rd MAAA Nationals hosted by the VMAA in Albury-Wodonga, some reflection on the handling of the Dixie may be appropriate.

The first flights of a new Dixie can be fraught. If you have no short DT facility, you need to get the model up fairly high in case the pattern and transition are poor, in which case you might hit the ground. Not good.

The key here is that the Dixie does need right rudder tab. With low power, you may need up to 3mm deflection of the tab to overcome the right wing washin and get your spiral. With higher power, you need less tab, maybe only 1mm. My own experience at the Nats contains a salutary lesson.

I had planned to run OS CZ11 on 40% nitro using my own 7X4 prop. For some reason I can't fathom, I found myself with no 7X4 props on hand and so retrimmed on a cut down 8X3.5 on a hot morning several days before the contest. Performance was way down, but you have to go with what you've got.

On the morning of the contest, the day was cool and conditions good for engine power. I chose not to retrim, which was a big mistake, as the little OS picked up at least 2000 RPM, which I welcomed. But I had a lot of right trim-tab from the previous session, and this proved way to much for the extra power, the model chasing its tail rather than climbing.

Another problem with the Dixie is launching in moderate winds. The usual right bank into wind results in the model being blown flat down wind. If it doesn't impact, you still lose about 3 seconds of motor run before the model assumes its correct trajectory. I have no cure for this, but wonder if a downwind launch might work. Scary.

On the subject of winds, here in the West the models need to be strong enough to be blown over on there backs, and even tumbled a few times. I use spruce spars with dihedral braces, and spruce longerons in the corners of the fuselage box. Adds weight, but the model does not need constant repairs on the field.

The final problem concerns the Dixie fin, or should I say problems. The overhanging fin trailing edge saves a gram or two, but is vulnerable. If the model DT's close to the ground, it can tail slide onto the fin and snap it off.

In addition, the fin is only 1/8th sheet, and essentially unsupported past the fuselage rear. In hot, dry weather, warpage can occur thus changing the effective amount of rudder tab. The inserted fin brace serves only to weaken the fin. I use 3/16 quarter grain for the fin, no brace and extend the fuselage to the rear of the fin. Also, I find a screw adjustable rudder tab desirable for precise adjustment on the day.

So there it is. Good luck with your Dixies, still a hot ship after 50 years!

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