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!

Have you looked through the newsletters? I think the W&B stuff, to include arms, are listed somewhere in the newsletters. I also recall seeing both 71 and 72 in the newsletters, but don't remember anything definitive of using one over the other. Good luck.

I found this thread that may be helpful: viewtopic.php?t=7766

here are the attachments from that thread

Thank you all for the responses!

still leaves open the hard aft CG limit. AI google search for what that is worth indicates the aft hard aft limit CG is 71.0 which is 32% of the MAC. the linked discussion stated that one individual flew it with 71 inches and while it was controllable it was uncomfortable. The AI search results state the same thing (possibly from this very post from many years ago) that many builders will set the aft limit at 30% of MAC which is 70.0" which is more conservative and results in diversity of load carrying.

Has anybody else more recently flown with CG at 71.0 and attest how it flew?

Without any other answers, I think I will set the aft CG limit for normal use at 70.0 and reserve the 71.0 as a ultimate emergency use situation.

Once I get my new tailwheel set up in place, and I re-weigh the plane, I am curious how this will all work out for the existing bird and what baggage and fuel limitation scenarios would be allowed given myself and my wife in the plane.

I bet your baggage limit weight will come it at 75-100 pounds max with full fuel and a passenger. Thats about what most people find I think. Let us know what you come up with.

It is not the max fuel situation I am worried about. It is the low fuel condition.

According to my plane documents. doing nothing, zero fuel at landing with my wife and myself will only allow for 10 lbs of baggage under the seats and I am at the aft CG limit at 70.0" and I could add 30 lbs in the baggage compartment at 71.0 aft hard limit. Not particularly a useful Cross country airplane unless I leave half tank of fuel at landing! I DO question the accuracy of this. However I have a very light 21 lb propeller and a 15 lb battery behind the baggage compartment bulkhead (former). Also I may be saving about 4 pounds of with the replacement of the tailwheel. SO that will help. The existing records show the tail weight at 59 lbs. Which seems heavy to me. But with light weight prop, heavy tailwheel and rear Battery maybe it is realistic.

I have done some math and using the arm lengths that Bfinney above uses which also correspond with Dave Read's numbers for seat and fuel locations. And assuming some alternative battery weights I came to some estimates. (does not take into account the lighter tailwheel)

I used three scenario's. All with myself and my wife on board. Zero fuel in tank and also with 4 gallons (about half hour). I then calculated weights of baggage at both the 70.0 and 71.0 inch aft limits.

Scenario 1: Replace the PC680 battery 15 lbs to an EarthX 5 pound battery in the current location immediately behind the baggage area bulkhead.
Result: Zero fuel. we can have 20 lbs in under seat baggage and 8 lbs in baggage area to reach 70.0 CG limit. If used 71.0 I could add up to 42 lbs in baggage compartment.
4 gallons remaining: both of us, 20 under seat and 32 lbs for 70.0 and 68 lbs for 71.0

Scenario 2: Scenario 1 AND relocate the EarthX battery to immediately behind the passenger seat at far forward baggage area. (about 22" forward from current location)
Result: Zero Fuel: Both of us 20 lbs under seat and 27 lbs baggage compartment for 70" and 62 lbs at 71.0 inches
4 gallons remaining: Both of us 20 lbs under seat and 39 lbs baggage for 70" and 75 lbs for 71.0 inches

Scenario 3: Scenario 2, PLUS adding 15 lb saber propeller spinner crush plate weight
Result: Zero fuel: both of us 20 lbs under seat and 53 lbs baggage for 70" and 90 lbs baggage compartment at 71"
4 gallons fuel: both + 20 under seat and 66 lbs baggage compartment @ 70" and 103 lbs at 71"

Mind you this is all mathematics only. Putting my plane on the scales and getting more accurate arm locations for the battery will potentially change things.

The extra cost of the EarthX battery, and the saber 15 lb crush plate weight due to the simplicity over moving the battery to the firewall may be worth the trade off. 70.5 inches would allow 20 lbs under seat and 84 lbs in the baggage area. Seems like acceptable utility to me. And who knows maybe there is an error in the weight and balance documents and the rear CG isn't as bad as the math indicates.

What does all these things do to my overall empty weight? Very small additional weight .

Minus 15 lbs for the removal of the PC680
Add 15 lbs for the Saber crush plate weight
Add 5 lbs for EarthX battery
Minus 4 or 5 lbs for the tailwheel replacement.

Yet I gain a LOT of loading utility.

sounds like you would benefit from relocating your battery to the firewall. I think most of our planes have a tailwheel weight somewhere in the low 40 pounds.

I agree 100%. Ideally moving the battery to the firewall would be the best solution.

I will have to determine if there is room for a battery on the firewall and evaluate the amount of work effort that may be required to move items to make that happen.

Added benefits would be the MUCH shorter cable from battery to starter.

My first step is to get the new trusty tail spring and API tail wheel installed information I searched indicated this would save about 4 pounds. The second step will be to put the plane on scales and know exactly where the wight and CG is located.

I have several winter projects to do. Moving the battery is the first. Then constructing a baggage compartment will be the second. after those are completed I intend to put the plane back on scales so I am not only relying upon math to determine empty weight CG. I will get it exactly.