Thorp Air Command - T18.net

Well, let me think about this weight problem. As I stated previously, I had two model a ford blocks, two crankshafts, 25 pound tool bag, case of beer, clothes for two weeks, 62 gallons of fuel and departed out 2200 AGL airport(density altitude ??) and flew non stop from KCKB to KLAL. My estimated gross weight 2000 lbs, which I believe is under the utility weight. I did land with empty tanks.
What effect does an engine have on the wing? same as fuel?

How are weight limits determined? The gross weight set by the builder of my airplane is 1600 pounds. Others have used different numbers but I do not know how to determine the limits. I only have load tables for normal category and “g” limits for a couple of different weights. It almost appears as though I could build a T-18 and claim a 4000 pound gross weight if I can can find a long enough runway to get it off the ground.

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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

Put enough horsepower behind a turd and it will fly !! This subject may have been covered a few years back , so pretty sure the info is hidden in the Forum .

RB

The designer, John Thorp, stated that he engineered the T-18 airplane to withstand +6G (9G ultimate) and -3G at 1250# gross weight. If we limit the plane to 3.8Gs, simple math yields 1974# as the max gross weight. Using 4.4Gs, we get 1704#. These are flight loads on the basic structure, however. No account has been made for what the landing gear, baggage compartment floor (if you have one), or other accessories may withstand. To further complicate the issue, Lu Sunderland told me when he designed the S-18, he strengthened the main spar to withstand +6G at 1500# gross weight. The rest of the plane, particularly the stabilator, is essentially unchanged. One could calculate the fact that the stab is 5" further back on the "S", so has a longer moment arm, and therefore has a slightly greater advantage in producing the downforce needed load the wing. Beyond my feeble brain, but I suspect the difference is negligible. So, on council of several aeronautical engineers, I would stick with the design limit of 6Gs at 1250# and extrapolate from there. Certified airplanes use more than just the strength of the structure to determine gross weight. Performance factors play a big part. Ever see an STC for higher gross weight based a horsepower increase? My airplane has an IO-360 with a constant speed prop up front. It can still muster 1000 feet per minute rate of climb at my arbitrary gross of 1850#. I would not try that in an O-290 powered bird.

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Bob Highley
N711SH
SN 835
KLAL

It is rumored that one of the Piper PA-20 150HP upgrade STC's includes a gross weight increase. The only way to confirm would be to contact the STC holders individually or to contact someone who has the paperwork for their aircraft since STC descriptions aren't very descriptive. I do know the TC for the PA-22 lists different gross weights for the different HP versions.

I believe mine would easily handle a higher gross weight than the current 1600 pound limit in the paperwork. I have flown it with nearly 1600 pounds (I didn't put the passenger on a scale but he was bigger than me @195 lbs so I could have had it over 1600) and landed with about 1500+ pounds on grass strips. I had to carry a little extra power into the flare but the takeoff and and flight didn't feel very different than if I had been alone. If I was to add some fuel capacity I would need to increase my published gross weight but I am concerned about how the landing gear would handle it on the grass if I needed to land right after takeoff for some reason. I guess I could rig a way to dump fuel.

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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

My experience is in seaplanes where they put higher HP engines on and increase the gross weight. To put your mind at ease, the gear bends before it breaks, should you need a rough landing.

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Bob Highley
N711SH
SN 835
KLAL

Well I proved it was simple because I sat here until I figured it out without knowing anything except what you posted. It's a percentage extrapolation that was rounded up to get the answers. ie. 1250 is 83% of 1500 so the g limit for 1500 pounds is 83% of 6 which is 4.99 (or 5) g's.

I would prefer not to bend the gear on my bird. That is a whole can of worms I don't have time to get into. If I screw it up bad enough to need a replacement, no one is making the short gear anymore so it would mean I need to either build a new A-frame myself or modify my airplane to fit the longer gear (brake lines, fairings...). Even just straightening it may cause the aircraft to sit for years before I got it back together.

I agree that seaplanes often get a higher gross. I mentioned the PA-22 specifically because Piper offered it with engines from 108 HP to 160 HP with corresponding gross weights of 1650-2000 pounds without floats. There are several aircraft that have similar power to gross weight changes from the factory but I am hard pressed to think of any with such a range.

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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

And replacement gear are available.
Just so we are all on the same page. The T18 designer set the gross weight to 1500lbs. The S18 gross weight redesign is set at 1600lbs. The rules for experimental aircraft allow the builder to set gross weight wherever they want. All you have to do is load it to that weigh and fly it....demonstrating the capability. I have a letter from JT to Don Taylor giving him 2200lbs with +3, -1 G limits.
Having said all that, do you really need to push those limits? I'm not a designer/structural engineer. I do not know where the increased gross weight will show up. My bet is that it will show up somewhere. Dons airplane (hanging in the EAA museum) was built for one reason..to fly around the world. Most of us build and fly these beautiful birds for enjoyment. We tend to want to keep them, and us around for a long while.
Cubes

I think only the builder (manufacturer) can set the gross weight and all the other operating limitations. Can someone else years later, put the aircraft back into flight testing mode and essentially re-certify the aircrafat with different operating limitation such as gross weight.

Seems reasonable to have different gross weights based upon max G-loadings at the various given weights.

A person wanting to do some light acro most reasonably would not fill up auxilary fuel tanks etc.

The Cessna 172 has the ability to go from normal to utility and again it is based upon max gross weights and G loading. Seems like the THorp would (or could) be no different.

I was thinking you told me you didn’t sell the short gear.

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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 think it could be done. The PA-20 I use to own had a gross weight increase done on a field approval along with a HP increase and if the rumor about the STC I referenced above is correct, then that would be another case where a limit was changed on “certified” aircraft. Changing a similar limit on a homebuilt shouldn’t present a problem.

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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

Whether the fuel is in the fuselage or in the wing, the wing spar and skin are going to have to carry the load. To say that the skins support the fuel weight for wing tanks assumes a wet wing. In Cubes' racer he has internal metal tanks near the wing tips that are supported by the ribs. What happens in that case? Free weight? Sarcasm aside, on the ground, the wing spar and skin (The wing is a box structure. The upper skin is in tension and the lower skin is in compression. Skin loads are opposite in flight, assuming positive g) support the weight of its structure and anything you add to the wing - fuel in this case. In flight, the wing spar and skin support the weight of everything minus lift produced by the fuselage, which is probably very little, and the horizontal stabilizer. In a positively-stable aircraft such as a Thorp, the horizontal stabilizer's lift vector is down in order to keep the nose up. The spar/skin have to carry that load also because the wing has to work harder to produce the lift required to maintain equilibrium. As the CG moves aft, less down lift vector is required, but it is always down. If the CG moves aft of the center-of-lift, then we need a quadruple-redundant computer flight-control system like the F-16. In flight, even if there is no fuel in the airplane, the spar/skin are still supporting the weight of the aircraft.

Some argue that adding fuel to the wing relieves some of the bending moment on the spar and allows for a higher gross weight. The bending moment on the wing is relieved mostly if that fuel is out at the tips and the aircraft is at more than 1 g. Fuel in the tips, as mentioned in the quote above, can have an adverse effect on rolling moments-of-inertia. Holders of STCs for add-on wing tip tanks to production aircraft also advertise that adding wing tip fuel allows for a higher gross weight and it's not for bending relief on the spar. For example, a Bonanza owner buys the FAA-approved D'Shannon wing tip tanks. As part of the STC, the owner gets a gross weight increase. That's to make up for the useful load lost due to the increase in the weight of the additional fuel. Beechcraft is not going to do the engineering to certify the new gross weight. What's happening here is that the Bonanza is going from utility class to the lower g limit of normal class.

Reducing the g limits with increases in gross weight has been mentioned earlier in this thread and is certainly a strategery. However, there are multiple reasons for setting a gross weight limit and Cubes is always happy to sell you a new set of landing gear. It's a little extra jingle in his pocket.

In the end, WTH do I know? My degree in aerospace engineering came from a land-grant cow college, which means low standards. Besides, I figured it would be more fun flying single-seat fighters for Uncle Sugar, so I didn't use that college degree much (except for two years pre-USAF working for Boeing doing, among other things, load analysis on airframes).

Over and out,

Spanky

Jeff,
The gear I produce is the long gear. I have recently run across an existing short gear, which I acquired for someone exactly like you!
Cubes

About that turtle deck tank and angle of attack;
The Bellanca Viking has long (fore & aft) main tanks in the wings. These tanks have 2 fuel pick ups, one at the front of the tank and one at the back to get all the fuel out whether climbing or descending. They join in the fuselage bottom (low point).
I just measured 'Feisty', the bulkhead is about 2" measuring down from the top deck. An 8" tall tank would interfere with the opening to the baggage compartment by 6". What are the dimensions of the successful turtle deck tanks? I did see 32" wide mentioned, and that measured well in 'Feisty'.
I have a sheet of aluminum 54x33, so I could make a 33W x 8H x 12L tank 14.9 gallons with 2 internal baffles. That seems too big, so I have enough to make a reasonable sized tank.

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Scott Emery
EAA Chapter 326
T-18 N18TE

An opportunity for auxiliary fuel presented itself when I addressed the fuel tank valve and tank strainer.
I selected an RJS Economy Fuel Cells 3006501, 8 gallons:
https://www.summitracing.com/parts/rjs- ... /overview/
This system is removable from the selector valve to the tank vent. It's light weight and the big ring on the cap makes it easy to lift the whole tank out of the baggage compartment.
The tank can slide around the floor a bit so I'm considering ways to secure it. A larger tank, an inch wider and five or six inches longer would fit.
I'm building an under wing tank using aluminum, but this was much easier so it is getting done first.

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Scott Emery
EAA Chapter 326
T-18 N18TE