In a recently revived thread regarding relative build times of the 601HD, Sonex, and RV-7, the subject of the T-18 as an LSA was brought up. It might be worth starting a new thread on that topic.
I do not think it possible to build the T-18 as an LSA. The highest wing loading for a (claimed) LSA I’ve seen is the Sonex at 11.2 psf, and frankly, I doubt their claim. (Perhaps someone has come across GPS runs at gross which prove the clean stall of the Sonex is under 52 mph, but I haven’t located such evidence.) Most don’t try to load the wing beyond, say, the RV-12’s 10.4 psf. The T-18 has a wing area of 86 sq ft, so gross at 10.4 psf will be 893 lb. The lightest, modern T-18 built to plans I’ve read about was 780 lb. This plane had no electrical system, a J-3 style instrument panel, and a fixed pitch prop. So, we have a (un)useful load of about 120 lb.
It is true Thorp’s original article for the T-18 claimed 600 lb empty. I don’t know if any were built to that weight. The modern plans call for a big bubble canopy, flaps, full engine cowl, and fully faired mains. I suspect the sheet metal was thickened in places by Thorp over the years, and I’ve read many builders have used thicker sheet to prevent unsightly oil-canning in regions of the fuselage.
I recently bought T-18 plans from Eklund, but I have not inspected them with great care. So far, I’ve seen no places where it calls out alternate metal thickness for different gross weights, so I must conclude the 780 lb is about as light as it gets. The engine and gear mount is stoutly constructed, not light in weight.
An aside: The owner of that light T-18 encouraged me to build an RV instead, with its bigger wing area. He didn’t elaborate (and my phone call was not invited, so I didn’t press the issue), but I suspect his T-18 has scared him at times. With it’s sharp nosed airfoil, perhaps the stall has proved wicked as built. In the NTSB data base, the T-18 has a poor safety record; stall/spin accidents are many relative to the size of the fleet.
Now, don’t get me wrong. The T-18 done right is a brilliant airplane, and I did buy plans. I’ve not flown a great variety of airplanes, but the T-18 I flew in was definitely the best handling airplane I’ve flown. The control harmony was superb. This example had the S-18 airfoil, and the owner noted he’d taken great care to ensure the wings were level with each other (no wing drop to one side). The clean stall was more gentle than any factory plane I’ve flown. I have to believe the NTSB data reflects planes with poorly formed leading edges on wings not square flown by pilots not accustomed to the performance.
In that other thread, there were questions about the T-18’s airfoil. It is the 63-412 with the lower surface cusp removed by drawing a straight line, starting at the trailing edge and ending tangent to the NACA profile at about 46% chord. This has the effect of pulling down the camber line in the rear, and this greatly lowered the pitching moment. It also thickened the airfoil at the ailerons, which would help lighten aileron loads and give a thicker, stronger section at the aileron cut out. Further, it retained the more rounded upper surface, which in NACA tunnel data proved to have a decent stall. Using the 63-212 instead would not have given the same result.
The Sunderland airfoil changes only the nose rib. It is much more rounded at the leading edge, and the top surface arches sooner. The coordinates are in the UIUC data base, as LDS-2.
Back to the LSA question. You could modify the design to make LSA. If you eliminated the flaps, the tail volume could handle a longer span wing. As a rough estimate, lengthen the span while keeping the spar cap sections proportional (along the span) to the existing design. Increase the span so that the design 6 g wing becomes no less than a 4 g wing at the same 1250 lb loading. This would give about a 29’ span at 10.4 psf. If it could be built at 800 lb, you’d have only a 450 lb useful load. Design concerns are increased torsion of the wing box as well as forces in the coupler plates at the dihedral break, and you still wouldn’t have much of an LSA.
I don’t think the T-18 is a good starting point for an LSA.
Karl
I do not think it possible to build the T-18 as an LSA. The highest wing loading for a (claimed) LSA I’ve seen is the Sonex at 11.2 psf, and frankly, I doubt their claim. (Perhaps someone has come across GPS runs at gross which prove the clean stall of the Sonex is under 52 mph, but I haven’t located such evidence.) Most don’t try to load the wing beyond, say, the RV-12’s 10.4 psf. The T-18 has a wing area of 86 sq ft, so gross at 10.4 psf will be 893 lb. The lightest, modern T-18 built to plans I’ve read about was 780 lb. This plane had no electrical system, a J-3 style instrument panel, and a fixed pitch prop. So, we have a (un)useful load of about 120 lb.
It is true Thorp’s original article for the T-18 claimed 600 lb empty. I don’t know if any were built to that weight. The modern plans call for a big bubble canopy, flaps, full engine cowl, and fully faired mains. I suspect the sheet metal was thickened in places by Thorp over the years, and I’ve read many builders have used thicker sheet to prevent unsightly oil-canning in regions of the fuselage.
I recently bought T-18 plans from Eklund, but I have not inspected them with great care. So far, I’ve seen no places where it calls out alternate metal thickness for different gross weights, so I must conclude the 780 lb is about as light as it gets. The engine and gear mount is stoutly constructed, not light in weight.
An aside: The owner of that light T-18 encouraged me to build an RV instead, with its bigger wing area. He didn’t elaborate (and my phone call was not invited, so I didn’t press the issue), but I suspect his T-18 has scared him at times. With it’s sharp nosed airfoil, perhaps the stall has proved wicked as built. In the NTSB data base, the T-18 has a poor safety record; stall/spin accidents are many relative to the size of the fleet.
Now, don’t get me wrong. The T-18 done right is a brilliant airplane, and I did buy plans. I’ve not flown a great variety of airplanes, but the T-18 I flew in was definitely the best handling airplane I’ve flown. The control harmony was superb. This example had the S-18 airfoil, and the owner noted he’d taken great care to ensure the wings were level with each other (no wing drop to one side). The clean stall was more gentle than any factory plane I’ve flown. I have to believe the NTSB data reflects planes with poorly formed leading edges on wings not square flown by pilots not accustomed to the performance.
In that other thread, there were questions about the T-18’s airfoil. It is the 63-412 with the lower surface cusp removed by drawing a straight line, starting at the trailing edge and ending tangent to the NACA profile at about 46% chord. This has the effect of pulling down the camber line in the rear, and this greatly lowered the pitching moment. It also thickened the airfoil at the ailerons, which would help lighten aileron loads and give a thicker, stronger section at the aileron cut out. Further, it retained the more rounded upper surface, which in NACA tunnel data proved to have a decent stall. Using the 63-212 instead would not have given the same result.
The Sunderland airfoil changes only the nose rib. It is much more rounded at the leading edge, and the top surface arches sooner. The coordinates are in the UIUC data base, as LDS-2.
Back to the LSA question. You could modify the design to make LSA. If you eliminated the flaps, the tail volume could handle a longer span wing. As a rough estimate, lengthen the span while keeping the spar cap sections proportional (along the span) to the existing design. Increase the span so that the design 6 g wing becomes no less than a 4 g wing at the same 1250 lb loading. This would give about a 29’ span at 10.4 psf. If it could be built at 800 lb, you’d have only a 450 lb useful load. Design concerns are increased torsion of the wing box as well as forces in the coupler plates at the dihedral break, and you still wouldn’t have much of an LSA.
I don’t think the T-18 is a good starting point for an LSA.
Karl
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