After watching two fine gentlemen putting themselves in a state to cut off their fingers the other day, and scaring the crap out of me, I thought I might go back through what we mean by "back-bumping" engines to start them.

   First and foremost - you DO NOT FLIP THEM THROUGH COMPRESSION BACKWARDS!!!!!  The entire point of doing a back-bump is to keep your fingers away from the prop when it comes up on compression, if you flip it through backwards, your finger is pushing on it very hard when it hits. That is sometimes useful but you have to be extremely careful when you do that on AAC/ABC engines, because the situation you need to do it is that they are already flooded, and and then can easily kick back.

  Done correctly, you are never in contact with the prop through compression, reducing the chances of getting caught up in it. We are not playing games here, we have engines capable of putting out up to 5 HP in ideal conditions, with razor-sharp and very strong prop blades and they run up to 12,000 even in normal conditions, and we are working fractions of an inch away from it.  What we are doing would get us all arrested by OSHA if it was done in an industrial setting. sharp spinning blades at extremely high RPM and no guards or safety provisions, close the place down.

   The good news is that these engines start very easily inverted or on their sides, and can be remarkably consistent in all respects including starting them, compared to the good old days.

SO, the general procedure:

Start with the battery off and the airplane fueled. Your helper must be holding on throughout this process, it can start any time there is fuel around and the prop can move, do not stick your hand/arm in the way of the prop arc when its has fuel in it because it *can easily pop or start with no battery attached* in almost any conditions.

Choke then engine by pulling it forward through compression "N" times with your finger over your venturi, completely covering it. As you get to the "Nth" pull, *simultaneously pull your finger away and flip it forwards through compression 3-4 times*. The idea is to flip it foward before all the fuel runs out on the ground.

      What this does is first get fuel up to the spraybar and into the venturi, then the flipping forward sucks the fuel in the venturi stack into the engine - but not very much, and any excess falls out. This gets a small amount of fuel and some air into the cylinder. At this point, the chances of it popping without the battery goes up astronomically, so continue to be careful. Correctly done, it will make a very slight "sloshing" sound as you flip it through compression forward, it takes some experience to determine when it is too much or not enough. Not enough and it will sound dry.

   Then *with the helper holding the airplane firmly*, *grab the spinner or prop and hold it in place and attach the battery*. DO NOT, repeat DO NOT, reach through the unrestrained prop disc to attach the battery, because the attaching the battery and throwing all that heat into it is absolutely the most likely point at which it might start unexpectedly.  Expect that it can start any time from now on, even if the prop windmills onto compression.

   Gently rotate the spinner in a *forward* direction (CCW from the front) onto compression. It's possible it will pop or start as you are doing this, but that has only rarely happened to me. Then, from the spinner and root of the prop, briskly fling the engine *backwards* towards compression in the other direction, moving your hand away from the prop before it gets to compression. You turn it forward up against (but not through) compression in order to give yourself enough travel to get it spun backwards and still get your fingers off the prop before it comes up on compression backwards. It is NOT NECESSARY TO WHACK IT LIKE YOU ARE CHOPPING WOOD, just a reasonably brisk twist. Some engines can easily start with just a twist of the spinner, but I usually give it a pretty sprightly twist with the LE of the prop at the root. Some people only know this method from combat in the era of the Fox 36xx, that may have needed a big whack with a teeny prop and poor fits, but not a modern stunt engine with a big old stunt prop.

    The engine should start, or at least give some indication of starting. If it speed up, and then quits lean, you didn't choke it enough up front, so go back the the beginning and start over. Advanced users can do other things, but I hate to discuss them in a tutorial. Generally you are better off having too much fuel than too little.

  "N" from above indicates some integer number of pulls-through to choke. This depends on the engine and conditions. When the engine is very cold and on the first start of the day, it will typically take more than normal, and when the engine is hot, like on back-to-back flights, N might be 1 or even 0. For PAs, I would suggest maybe 3-4 under normal conditions, 5-6 for the first run of the day, and 1 or 0 for a back-to-back hot flights. 1 is usually safe, 3-4 is too many, the cold fuel will get on the piston crown and shrink it and the compression will drop until it warms back up and as the cylinder cools. For the RO-Jett, I usually use 5-6 cold, 2-3 hot, and I have never had a case of "shrinky piston syndrome".

   Note that this is all for conventional inverted mounts. Variants on this work for side-mounted engines and will not generally work with upright engines. With upright engines it is very difficult to get enough fuel into the cylinder without getting way too much in the crankcase, which will result in repeated pops but no running because all the excess fuel in the crankcase just gets sucked up into the cylinder and stops it, while sucking more in from the tank. The solution on the bench is to detach the fuel line, keep back-bumping until it starts cleanly forward and then quits lean, evacuating the crankcase of excess fuel. Then re-attach the fuel line, one pull through, and bump it, is should go. It works when you are burping the engine in the morning because even though you are putting a lot of fuel in the crankcase, no new fuel comes in from the (empty) tank

    But the kickers are:

      NEVER EVER FLIP IT THROUGH COMPRESSION WITH THE BATTERY ON IN EITHER DIRECTION - you fling it backward up against compression, but you fingers are not on the prop when it gets to compression.

    HAVE SOMEONE HOLD THE AIRPLANE THROUGHOUT THE PROCESS - whenever there is fuel in it, it can start, battery or not

    DO NOT ATTACH THE BATTERY WITH THE PROP UNRESTRAINED.

    Brett

     

Brett;
I have been starting all my motors (except the 1/2A's. use a starter there) like this for more than 30+ years.
Started doing it when I was flying F3A with the Hanno Specials (61's) and then the big (then) YS 120's.

I do this starting method at the flying field and just about everyone thinks it's magic.
Not magic, just a tried and true very easy and safe starting method.
I do it on ABC, ABN, AAC, ringed, lapped motors. No problem.

One of the flyers is getting better at it at teh field now days tho.   
Just have to get one of the other flyers to quit using that big chunk of garden hose he uses when starting his motors and trying to start it going thru
compression back flipping it.

Carl

I have NO idea who Brett is referring to. Since it involved "fine gentlemen" it certainly wasn't me, but coincidentally I was present to observe Brett's starting tutorial. By the end of the morning and 8 flights in, I was getting reliable first-spinner-back-snap starts. Went home with same number of fingers I started with. The frustrating part, solvable only with lots of experience apparently, is knowing how much choke the engine needs, and what to do then. The explanation above of WHY the motor needs choke, and why each successive step in the process is necessary, really helps. Jim Aron added that, once the motor is primed and ready to start, with igniter attached, flip it immediately or the "starting" condition of vaporized fuel will dissipate and it won't start.

I have learned a lot about this hobby in the last 3 weeks, more than in probably the previous 20 years.  Thank goodness we have experienced people who know what they are doing and are ready to share their wisdom.

By the way, the reason you were both having so much problems starting was because flipping it through compression backwards also tends to blow the fuel from the choke back out the venturi. That's why it perpetually acted "dry" - it's actually a reasonable advanced technique for clearing floods.

     Brett

Thanks for the tutorial.  I had several of those pesky Fox 36x Combat Specials.  Flipping them through compression with those tiny combat props was down right dangerous.  I wish I had had your course on prepping them to start.  Now days I use a completely different method.  Push the red button and count the beeps.....  However, I do still have one IC that I will use your method on in the future.

Ken

I agree with Brett on this. When I was flying the OS VF 40/46 pipe ships I leaned the "spinner flip". This follows Bretts pre-start routine right up to the flip part. At the point of starting the engine one simply rotates the engine back up against compression then grabbing the spinner, close to the prop, between the thumb and index and pointer fingers and simply snap the fingers as you twist your wrist which spins the prop clockwise against compressions and it just goes. If it doesn't go, remove the battery, pull a choke turn then flip forward several times then backward several flips, connect the battery and spinner flip.

I now do this with all my engines even Fox 35 and OS 25's I use in OTS. For these engines the important part is to have a fully charge starting battery and a good hot plug. One rule with any engine - don't over prime.

Best,   DennisT

I've been back-flipping racing motors for over 60 years! Virtually all except the F2C diesels.

Bill

Starting is even more fun when you are left handed and a back flip is a forward flip(!)