Upright Mounted Engine
vs.
Inverted Mounted Engine
By: Dale Williams



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A Challenger with the engine mounted upright instead of inverted is not often seen, but many owners and builders express an interest in the possibility of mounting the engine upright. Here is a brief summary of my observations.


The engine, a Rotax 503, is mounted on a Challenger II Clipped Wing. No belt drive is used. Instead, a B-box drives a 66" TPI prop. It was mounted that way when I bought the airplane. I know the builders and it wasn't that way to start with. (I'm the second owner.) I don't know why the builders elected to mount the engine upright.

I have heard many comments about upright engines on Challengers and I know the subject has been discussed, or debated, at length. I'm not disputing anyone's opinion by any means, but here is what I have learned.

As a BFI in a fairly active flying club, I have flown ChII LW (Long Wings) and Clips as well as ChI LW and Clips, all models, and I do not find any noticeable differences in flight characteristics. The man from whom I bought the plane changed it over and told me that he liked the upright mount better.

In favor of the upright engine, I would say,

  1. There is no concern with belt drive problems such as loosing it in flight, sideloads on the crank (which I don't really believe is a problem), or routine belt adjustments.
  2. The engine gets good, undisturbed air and runs cooler.
  3. I can use plugs with the screw caps (BR8ES) from the auto store at 1.49 each.
  4. Plug wires don't fall off. (I still use wire ties to secure them).
  5. Mounting and removing the engine for service is a breeze.
  6. Preflight is much easier to do.
  7. Routine maintenance is very easy as far as changing plugs, carb sockets, CHT or EGT sensors, fuel and primer lines, and so forth. I can stand next to the engine to do this.
  8. Access to the rear area of the airplane is easier. This is where the wiring, solenoid, electric fuel pump, and electric primer are located.
  9. The conversion to an HKS or similar 4 stroke engine should be easier if and when I decide to do it.
  10. I can turn a longer prop with the upright engine.

Some of the disadvantages are,

  1. Some loss of speed is caused by the extra drag of the upright engine. While this doesn't appear to be a lot, it is no doubt there. I fly with other members of our club and usually don't have a problem keeping up with them, but doing so does require about 200 RPM more than they run.

    Some of this extra RPM requirement may be due to the prop, which I think could be pitched more for speed rather than climb as it is now. (I also have a leading edge wing wrap.)

  2. It just ain't as pretty as one that is tucked out of the airstream.
  3. A heater is not easily mounted to an upright engine.
  4. Changing or adjusting the cooling fan belt requires a ladder and more effort.

As to any other concerns, let me add that at most fly-ins, I'm the guy answering the "Why did you do that?" questions. I have considered changing it back to the original configuration, but honestly, I just haven't seen that it would be worth the time and cost to do so. I do like it for the reasons mentioned above but I'm not dogmatic about it.

Hopes this helps answer some of the questions that others may have. Incidentally, I would be interested in hearing from some of the other "upright" pilots. ( I am a member of the FlyChallenger list.)


UPDATE
Just Inverted the 503 Engine

Well, I finally did it. I changed my upright engine to inverted. I haven't finished all my testing but so far I'm very impressed. I'm now using the Hager 2.2-1 redrive with the 54/37 TPI prop. In addition, I've also done a few other things of which the most noted was to begin to lighten the airplane by removing useless weight and moving the CG back. I've also lost about 30 lbs myself. Here's what I've observed so far.

Stall now is at 27 solo (down from 35 solo) and 35 dual.

Cruise is 70 mph at 5600 RPM, and 80 - 85 at 5800-5900 (up about 10 overall).

Slow flight at 40 with no loss of altitude was achieved at 3950 RPM with very good control response still.

Glide is much better also. Just seems more "spirited" and not as "laden" as it used to be.

I also believe the 66/34 prop wasn't enough to properly load the engine. Before the change back, I had tremendous climb but poor cruise. Also I had CHT's of 250 and EGT's of 1100 before, which is an indication that the engine was not loaded enough. Now the readings are 325/1050, which is where they should be.

The biggest difference is that now I don't get the pushover that the larger prop was giving (and which is noticeable even with the 60" high drive).

A disadvantage of the inverted position would be that annoying 2.2-1 redrive whaa, whaa, exhaust-in-the-prop sound. However, I understand that a 90 degree turnout on the muffler helps that a great deal (or a 60/44 high drive).

The airplane did handle differently with the top-mounted engine but not anything that I would consider dangerous or undesirable as far as handling alone is concerned.

Performance in almost all phases of flight seems to be improved, but I'm not yet finished with the follow-up testing. So far so good. I still maintain that the top mount is doable but I now believe that the loss in performance is greater than I first anticipated.


Dale Williams