Thorp Air Command - T18.net

Tony,

The design engineers, John Thorp, and then Lu Sunderland have both been gone for some time now, as you know of course. There has never been any statement from either one of them about -4 g's to my knowledge. It's always been +6 -3. Lu said the same to me as he did to you about treating the 2 wing designs the same... I could probably find the correspondence if I look. But he also said that the critical limit was due to the joints between inner & outer panels, the "weakest" link if you will. He stated that the new wing was good for +6 -3 @ 1500 lb. vs 1250 lb. for the "standard" wing. No "guessing" about this. I believe that John Thorp checked Lu's calculations. John also wrote to me about some "hidden" strength in the spar, but that is something usually found in most designs and should be treated as a sacred asset not to be spent. That belongs to the designer.

I mentioned an airplane that survived over 10 g's. Those were 2 very lucky people, one a good friend of mine. I don't want any of the new guys on this forum to think that an average T-18 can be abused like that & survive. Many of the "older" guys know the story. The airplane did not come out of it undamaged. I also saw an S18 doing aerobatics at a Thorp fly-in years ago that landed with wrinkled wing skins... very stupid.

The legality of doing aerobatics in airplanes with load limits less than +6 -3 g is another conversation. It's also illegal to do aerobatics without parachutes when carrying a passenger, when below 1500 AGL, etc., etc. Flying safely, and doing it legally are two different things not necessarily interdependent. I'm not advocating that anyone fly illegally.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Thanks John and Rich.
I did mean +6 -3, my mistake. I hope newbees will heed your sage words. The T/St 18 is a slippery beast down hill and with powerful controls it is easy to pull g if you are not used to that kind of performance.
I don't aerobatic my T18 often, just odd aileron and barrel roll to break the boredom.
I love my T18!
Tony S
ZK-VMS

One needs to remember that the horizontal tail is also loaded during increased g applications. An increased load on the wing will increase the load on the tail plane. The horizontal tail is the same for the T and the S. OK, for the purists, the S is mounted 5 inches aft, but I doubt that decreases the load on the tube spar significantly. Stick with +6 /-3 for both. Most garden variety acro can be accomplished with no more than 3 gs.

See archived posts for an important discussion on rolling gs.

_________________
Bob Highley
N711SH
SN 835
KLAL

This 20% increase in the acrobatic rating didn't come with a 20% increase in the gross weight. Apparently stress in the wings wasn't the limit on gross. Perhaps there wasn't enough tail volume to provide the desired stability in cruise?

Karl

Those are load ratings at specific weights... nobody's saying anything about gross weight. The design gross weight is 1500 lbs.
If you increase the gross weight the load ratings decrease accordingly.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

I had noticed a chart on T18.net giving the gross weight for each as 1650 lb. I had assumed those were limits set by the designers, thus my question.

Karl

That's an arbitrary figure chosen by someone other than John Thorp or Lu Sunderland.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Years ago I saw an aerobatic demonstration in a T-18 at either KY Dam of McAlester OK. The pilot was from S America as I recall and had flown in the Luftwaffe for Adolf. He was very good and did everything except Outside stuff. Does any of the old timers remember that occasion? I used to roll mine but got that out of my system quite a long time ago. I never have liked to get the nose much below the horizon because the slippery little jewel picks up speed so quickly. I limit myself to gentle wing overs now...you know, there are old pilots and bold pilots etc.
Steve Hawley

Don't kid us Steve, you are an OLD BOLD pilot!!!
Cubes

Setting MAX Gross Weight . NL#135 , pages 7-9 . OH MY ! We have all the knowledge locked up , we just need to take the time to find the key . Amazing we have $600 smart phones and multi Gigabyte computers , but in the end it is the good old printed page that wins out . BOOM !

RB

The printed page tells us a T-18 is, for Normal category use, good to 1970 lb and the S-18 to 2370 lb --if you can keep the CG within the limits. So loaded, the S-18 is approaching the wing loading of a WW-II fighter but without all the horsepower. I think I'd just stop at a useful load of 600 lb.

Regardless, that's one stout wing on the S-18.

Si'

(Deleted: I see my previous post could still be Edited for corrections.)

Don Taylor flew his T around the world at 2200lbs. JT gave him +3/-0 at that weight.
Cubes out

Disregard last. Just found JTs letter to Don. At 2200lbs, JT gave him +3.4/-1.7. I stand corrected. That was a T.
Cubes really out