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Supporting Owners, Builders and Pilots of the Thorp T-18 and its variants.
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SHIPCHIEF
PostPosted: Wed Aug 05, 2015 11:43 pm 
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Mark;
The T-18 is a 1500 lb gross weight aircraft with an 86 square foot wing that has a laminar airfoil, if I recall correctly.
The Newsletter archive covers the installation of stall strips to let you know that you have slowed and the wing is within 5 MPH of stall. This is in the form of a stick shaker which you shouldn't miss. My plane did not have stall strips, so I taped on a pair. I now know the difference. It's a lot simpler than a complex angle of attack or reserve lift indicator, and you don't have to look for it. You feel it instantly.
I built an RV-8, not my T-18, so I can't comment on it directly, but you will benefit from building it in many ways you don't anticipate now. After you gain that experience, any maintenance or repair task becomes a manageable job you can do your self. I've enjoyed the time spent repairing and upgrading my T-18.
Also, don't pass up a chance to meet other T-18 builders and flyers!

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Scott Emery
EAA Chapter 326
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KWK
PostPosted: Thu Aug 06, 2015 12:57 am 
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SHIPCHIEF wrote:
My plane did not have stall strips, so I taped on a pair. I now know the difference. It's a lot simpler than a complex angle of attack or reserve lift indicator, and you don't have to look for it.

An effective stall strip would indeed be a better solution. I didn't recall the CAFE report mentioning a strong buffet, but going back over it I see they noted the plane did not have the strips. An on/off type of stall indicator is not complex, and you can use lights or a horn, but strips are much simpler.

Do the majority of T-18s have the strips?


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jrevens
PostPosted: Thu Aug 06, 2015 1:06 am 
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KWK wrote:
flyingfool wrote:
I know there is a roll bar so I'm not too worried about being crushed.

The windscreen roll bar is not foolproof. As I read (no pictures) the NTSB report, poor Ken Brock found that out the hard way. (snip)...

Karl


Someone can correct me if I'm wrong, but I believe Ken broke his neck on the flip over. It could have happened to anyone if the circumstances were right - loose belt/shoulder harnesses, a weakened cervical spine, etc. His wife, Marie, survived the crash. Nothing's foolproof, but I believe the roll bar did its job.

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T-18 N71JE (sold)
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KWK
PostPosted: Thu Aug 06, 2015 2:18 am 
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NTSB wrote:
According to the autopsy report, the pilot died as a result of "positional asphyxia with blunt head trauma." According to the coroner's report, the pilot was found suspended in the airplane's restraint system; however, his neck was bent to the left with the left ear touching his left shoulder, and his head resting on the canopy... Photographs taken after the accident showed the airplane resting on its upper nose cowling, left wing, and left seat back.


If it came to rest on both the cowl and the seat back, then the roll bar must have collapsed, at least on the pilot's side. The report had no pictures, though. It is possible the NTSB reporter's "upper nose cowling" meant the windscreen roll bar, but that seems unlikely. It's also possible that "taken after the accident," their photo may be of the plane after the rescue of the wife. Another possibility is for the fuselage to have buckled near the flap/aileron spar, raising the seat back. Sad, regardless.

This crash involved strong yawing (a wing was torn off by a post) as well as flipping over. That would strike the roll bar differently than a simple nosing over in a rough field, emergency landing, which is a more likely event.

Karl


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flyingfool
PostPosted: Fri Aug 07, 2015 11:26 am 
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Charlie Hillard met the same fate with the flip over of the Sea Fury. He died of positional asphixiation as well. But in his case there was absolutely no roll protection I believe.

The Brock crash is very sad for sure. And the twisting nature probably did apply unequal stress on the roll bar that I agree would NOT be present with a striaght ahead tail flipping over due to the gear digging into soy beans or dirt or other vegitation etc.

I know it would look odd and maybe it would end up too low to be of any practicality. But is there any possibility to have a rear roll bar installed a few inches behind the seat back that would remain below and clear of the sliding canopy. Just to allow a "bridge" between the front and rear roll bars.

I like the idea of a dual canopy latch, seat belt cutter, and canopy hammer/breaker as well as a fire extinguisher. Those are ALL very practical and doable and cost effective reasonable means to minimize risk without a ton of weight.

I think it is very interesting and important to see the lack of fire in Thorp crashes. That is VERY reassuring. While many view the fuselage tank as a huge safety no-no. Wing tanks are known to rupture and rip off in many off airport or crash situations. In which case ripping the tank open and/or the fuel lines out ensures a fuel leak. And often times I believe the fire starts by the fuel contacting the hot exhaust etc. If the fuselage tank leaks, it MAY be contained and remain in the cockpit and thus while very disconcerting, keeps the fuel or at least delays the fuel from reaching the hot engine and exhaust area. Just a theory or maybe a hypothesis.

There is little ANY airplane can do when the damage is severe enough to rip the engine away and thus the fuel line from the engine. In that case it doesn't really matter where the main fuel tank source is. The proximity of the fuel line rupture and the hot engine is there. And until we go completely electric, I'm not sure how that risk can ever be eliminated.


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fulcrumflyer
PostPosted: Fri Aug 07, 2015 12:15 pm 
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SHIPCHIEF wrote:
Mark;
The T-18 is a 1500 lb gross weight aircraft with an 86 square foot wing that has a laminar airfoil, if I recall correctly.

The T-18/S-18 does not have a laminar flow airfoil. A laminar flow airfoil's thickest part is at about the 50% chord line. The T-18/S-18's wing is thickest at about the 25% chord line. Since this thread's about stall speed and keeping it to a minimum; if you want the stall speed to go up, put a laminar flow airfoil on the airplane. You'll go faster for sure, but you'll run out of liftees sooner too.

Spanky


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KWK
PostPosted: Fri Aug 07, 2015 3:18 pm 
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I don't recall what Thorp wrote in the EAA articles, but the CAFE report lists the airfoil as the NACA 63(1)-412 slightly modified near the trailing edge. This is a modest laminar airfoil, with maximum thickness at 35%; the NACA turbulent airfoils are thickest at 30%.

If I recall, the S-18 airfoil uses a different nose rib, slightly dropped to try to improve the stall --or was it a larger leading edge radius? I'm not sure if it was tested in a wind tunnel for laminar effects, but I recall reports saying it has a gentler stall.

I claimed "the NACA laminars are not noted for good stalls." I should correct this a bit. The thin ones can be sharp in the wind tunnel, but some of the thick ones are very gentle, for example the 63(3)-418, yet it retains a fairly low C.d (if you can make and keep it smooth enough). It seems difficult to believe, but this 18% thick airfoil has about the same total drag (including trim) as the 12% used on the Thorp.

Karl


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flyingfool
PostPosted: Fri Aug 07, 2015 4:29 pm 
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If the goal is to simply lower the stall speed (increase the Critical AOA). Then Vortex Generators would seem to be a possible solution.

However I don't think they will pay off in the landing phase. As I could foresee that with a higher angle of attack, the tailwheel would hit first and potentially slam the mains on, setting up a bounce and a possible PIO situation. So they really might not result in a lower landing speed. But they MIGHT increase the safety margin of a stall/spin base to final turn situation. But it doesn't really SOLVE the problem. It just changes the speed at which the identical situation occurs to something slower on the airspeed indicator.

And besides, I think VG's look terrible on a sleek airplane.


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KWK
PostPosted: Fri Aug 07, 2015 10:41 pm 
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flyingfool wrote:
... the tailwheel would hit first and potentially slam the mains on, setting up a bounce and a possible PIO situation.


On the T-18, does the plane stall before the tailwheel can touch first?

My tailwheel time is limited to a Stinson four seater. My very best landings were to touch the tailwheel first, with the mains a little above the ground. There is no PIO. When the tailwheel touches, the plane noses down, reducing the angle of attack, killing the lift. You really plant a landing nicely this way. I was amazed how little runway you needed landing so, especially compared to touching first on the mains.

Karl


Last edited by KWK on Fri Aug 07, 2015 11:22 pm, edited 1 time in total.

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KWK
PostPosted: Fri Aug 07, 2015 11:20 pm 
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flyingfool wrote:
... possibility to have a rear roll bar installed a few inches behind the seat back that would remain below and clear of the sliding canopy...

I think it is very interesting and important to see the lack of fire in Thorp crashes.


The roll bar thought occurred to me, too. I don't have a set of T-18 plans, so I don't know what structure is available behind the seats to carry such. Unless you're short, a front roll bar assumes the fin doesn't crush.

I wish to correct an impression I may have left. T-18s do burn in crashes, but it appears to be less likely to do so than other designs with a fuselage tank. With that enormous header tank, I guessed it would be common, but it is not.

Karl


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James Grahn
PostPosted: Sat Aug 08, 2015 11:09 am 
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The Thorp has been built with two different lengths main gear and many types of tail springs. Some may touch tail first. Others cannot. The gear is extremely stiff. I found it less than optimum to control the roll out after tail first landing, so I changed to the Trusty spring I now sell. It made a ton of difference.
You could put an aft roll bar in. My new design has only an aft roll bar as I eliminated the forward one. That is a one off design that I do not intend to bring to market.
Karl, I have flown a 6'4", 245lb man as a pax in my Thorp. It's a T18, not an S. It was snug, but he fit. You can modify the seating to fit large folks, especially the S which has a 3/4 inch taller roll bar.
Cubes


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KWK
PostPosted: Sat Aug 08, 2015 12:25 pm 
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James Grahn wrote:
I have flown a 6'4", 245lb man as a pax in my Thorp. It's a T18, not an S. It was snug, but he fit. You can modify the seating to fit large folks, especially the S which has a 3/4 inch taller roll bar.
Cubes, thank you for all the replies.

As mentioned in the other thread, we're 6'5", but under 200 lb. I don't mind the snug fit of a C-150 (especially with the gals), but I never cared for flying with my head up inside its wing roots --tunnel vision, you know. Even the narrow cabin of the T-18 would be adequate I think.

Usually leg room is not a problem for me; headroom always is. I'm tall, and I'll insist on at least 2" of headroom clearance. The T-18 Ambassador here in Illinois has graciously offered to stop by some day this fall, when off on a joyride. My son and I will look over his plane carefully.

While it's premature for me to ask for a picture of that rear roll structure you mention, others might be curious. I do think that's the better place for a roll bar, with all due apologies to Mr. Thorp.

Karl


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flyingfool
PostPosted: Sat Aug 08, 2015 1:18 pm 
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KWK, where do you live in IL? I'm just outside of Madison, WI.


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James Grahn
PostPosted: Sun Aug 09, 2015 12:56 am 
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Again, I do not intend to bring anything to market except exactly what John Thorp and Lou Sunderland designed. Sorry.
Cubes


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