Thorp Air Command - T18.net

As mentioned earlier I have recently purchased a standard Thorp T18 with standard wings and 29 gallon tank and O320 (160HP). carbon fiber 3 blade ground adjustable prop. The battery is a PC680 located behind the baggage area bulkhead on the right (co-pilot) side. I have the original shorter gear length.
I am looking for help.

The weight and balance paperwork that came with the plane I have considerable questions over the accuracy and calculations. in fact I have little confidence at all in what the documentation shows.

It appears the original builder chose to move the datum point from the plans. The plans (and google AI searches) as best I can determine show the datum to be the engine crank propeller flange. The builder's documents show the datum to be the leading edge of the propeller. The fwd and aft CG limit is shown in the aircraft documents as 62.5 and 70.0 This pretty much agrees with the plans. Which it should NOT if the datum was changed. I estimate with a 4" prop extension plus an assumed 2 inch thick prop hub, and the leading edge be 1 inch ahead of the front of the hub would add 7 inches to the plan datum. therefore the modified CG limits should also be about 7 inches higher (add 7 inches to the plan limits) correct? This would put the range in the same location fore and aft on the chord of the wing.

Also the aft CG limit I have gotten from Dave Read and even google AI all indicated forward limit as 62.5 so that is good. However the aft limit is shown as 71.0 in one place 72.0 by another source. Which one is correct? Is one for the T18 and the other the S18? Which is correct for the std T18?

The plan datum would put the wings leading edge at 55.0 inches.

The weight and balance documents I have indicate the axles to be at 53.6 inches with the original short gear. Is this correct for the shorter gear using the plan datum?




I have come to the conclusion that I MUST put my airplane on the scales and totally redo the weight and balance in order to have any confidence in the numbers. I also want to use the plan datum so that my plane will be "normal" and not a unicorn with a different datum than the rest of the fleet. This should put the leading edge of the wings at 55.0 inches (engine prop flange with OUT the extension. (making the propeller and prop extension and spinner all negative arm distance and moments.

Can someone please post the Plan arm distances from plan datum to the fuel tank, seats, instrument panel (which could be customized and not standard) short gear axle, and baggage area front and rear. I am trying to make sense of things and get solid CG information to ensure I am safe.

I know that Dave Read has himself and on another (Scott's?) std T18 weighed two separate T18's. and weighed empty, and then with the pilot sitting in the plane (known pilot weight) and back calculated the "seat" arm. Also done on both is they filled the fuel tank half and full and also back calculated the arm of the fuel tank. reportedly in both cases the arms for the seat and fuel agree well at least between these two Thorps. This is confidence inspiring real world verification of stationing is correct for the fuel tank and seat location.

I would like to have additional confidence that the arms are very close in distance from datum for key locations. Having fixed locations will allow me to measure from these locations to the best of my ability arms of locations of various equipment in the airplane.

I am in the process to also remove and replace my tail-wheel and spring. I will put the plane on the scales AFTER this replacement is completed. So at the very least I will have a known commodity in the "as is" equipped airplane in accordance with the plan datum. I have a lead on aircraft scales from the local EAA chapter. And I know I have to raise the tail to level flight, and I have to grease poly under the wheels to ensure no side load is induced on the scales. This will get me an as is empty weight as currently equipped. Obviously any equipment of things added or removed must be calculated and/or the plane re-weighed.

Am I thinking about this correctly.

All I know is that the documents that came with the aircraft are not using the plan datum and the arms and CG limits don't make sense or match to other peoples experience. And I must correct this mess!