Thorp Air Command - T18.net

My understanding is that fuel in the wings actually INCREASES the allowable G loading.

I'd like to understand how this is the case.

Not to muck up this thread with speculation as I only have a minor in physics and nothing in aero engineering.

If the G load max is based on the loading of the wing spar to fuselage connection then putting weight inside the wings wouldn't effect this and actually remove the 29gal of fuel on the fuselage.

I think the post about gross weight being higher with fuel in the wings is actually meant to say that G loading can be higher. The weight is shifted to the wings thus not putting stress on the wing spar connection to the fuselage when pulling higher loads. The fuselage is basically dead weight off a moment arm from the lifting point (wings). The closer the weight to the area of lift the more force it can take before damage. I think this is based on the fact that weight isn't just added to the wings instead moved from fuselage to wings.

The wing spar doesn't need to carry the fuel load while in flight, however, taxiing is a different matter.

Fuel burn CG transfer in flight is not enough of a problem to reconfigure the plane. A couple of trim clicks about every fifteen minutes is plenty. Having a split fuel load like the RV's and the Cherokee with the attendant tank switching is a far more labor intensive cockpit mind load. My wing tanks feed into the main, so I don't have to worry about running a tank dry. All I have to monitor is whether the wing tanks are feeding and watching the fuel remaining in the main tank. I have an automatic shutoff for the wing transfer system, so I can't overfill the main.

The Cherokee fuel weight limit is probably more of a performance issue than a gross weight issue.

_________________
Bob Highley
N711SH
SN 835
KLAL

In the 737, we have to burn center tank fuel before the wings. My understanding is that the spar load is increased with weight in the fuselage. However, any weight in your plane is part of gross weight. You and I do not know the exact structural limiting member at any given time. That's not to say you can't set your gross weight wherever you want. It's experimental after all. Bob is right in that JT did the math for Don Taylor. I have the original letter. He gave DT +3/-1 at 2200lbs. That was in a purpose built T18. Please respect the normal g load limits wrt gross weight.
Cubes

On my thorp even without baggage my cg starts to become rearward of CG range just under half tank. With any baggage it happens faster. So in my specific thorp with a 0290 the fuel burn significantly changes my CG into an unsafe range.

^^^^

Something just doesn't seem "normal" or "right" with what you are explaining compared to the vast general experience of Thorps. Being outside of aft CG with half fuel remaining I do not believe is common at all.

I wonder if the math is wrong or maybe the datum used is not the normal place.

Something seems out of kilter.

Rear CG is nothing to fool around with as you are well aware and you are being careful which is smart. Somehow I think you'll get to the bottom of this. I suspect it is datum location and/or math.

Putting your plane on the scales and starting from scratch I think is the best solution. I would have an A&P, IA or some other experienced 3rd party look at the Thorp plans and do the CG calcs. My experience as an engineer is that I can work on a complicated math problem and know the result is suspicious, and being too close to the problem no matter how many times I go over and over the calcs, I cannot see the problem. While another co-worker engineer who is not familiar with the problem can find my error in a few minutes. A fresh set of eyes can do wonders! Sometimes we can get too close and we convince ourselves we did everything right. The can't see the forest for the trees scenario.

I agree. I'm actually scheduled with the local EAA chapter to get it on the scales once I get comfortable flying it. Currently for what I am doing it's not of too much concern since I keep it topped off. I assumed it was most likely a math issue in the calcs since they look old as can be. It hasn't been checked since 1999.

I'm also wanting to move the battery from the baggage to the firewall to offset this as well. I figured I would weigh it and check the numbers then move the battery and re-check the numbers.

FINALLY!!! I was able to weigh the aircraft yesterday. TW was 51.8 pounds. I know I haven't removed that much weight. The only thing I can think of to explain the discrepancy is the airplane was weighed then the weight was added to the engine without any kind of documentation. Total weight is 940.3. I haven't worked up a new sheet yet because I don't have the rest of the info needed but the hard part is over.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I couldn't print the W&B sheet I had posted at the shop because they have an older version of Excel than what I run at home. I deleted the original post and am re-posting the sheet in the Excel 97-2003 format. As I stated in the original post, I left the numbers off of the drawing so people could write in their own. Please let me know if there are any errors.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Well I already caught one error. The weight of 8 quarts of oil is not 4 pounds. In fact that probably counts as 2 errors since I was computing most rearward CG and had no intention of using 8 quarts in that slot. There should also be a note of the aircraft gross weight somewhere (there use to be a load chart that had it but I deleted the chart because it was for another aircraft).

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I'm a bit short on time now - will look at your spreadsheet later, but you have only about 6 inches of CG range - You should have more range than that - 8 to 10 inches if I remember - Rich?.....John??......crickets.....

_________________
Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Some where on this site is several W&B spread sheets

The CG range is 15 to 31 inches, or 16 to 32 inches depending on what paperwork you believe.
Cubes

The data I have says 15% to 34% of wing chord (50") which equates to 7.5" to 17" from wing leading edge.

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA

I should have said % not inches.
Cubes