Thorp Air Command - T18.net

Guys,

I have a question about the stabilator trim weights. My plane has only the internal weight/arm set up and to date has worked well. In the past week we have had an incident locally where a T18 got into a flutter situation where the stabilator trim tab was virtually torn off. He was able to land ok and now has a major repair to do. This has prompted me to ask about the external weights. Are they necessary?

I haven't seen the aircraft in person, just pictures and it looks to be a real mess. From what I understand the T18 tail was tested very extensively in the past.

Your thoughts?

regards,

Jim

Cubes sent me pictures of the so-called flutter event. I had something similar happen to me several years ago and the damage looked almost identical. However, the flight conditions claimed in the most recent event, 145-150 mph at 2-2.5 g, were much different than mine. I was doing over 200 KIAS @ 4-4.5 g. The horizontal reacted exactly as described in the flight test report that Cubes has.

Not only was it a gross overspeed on my part, we deduced another reason for the stable excitation of the stab was improperly built anti-servo tabs. My tabs were rounded off at the inside corner of the trailing edge and there was no rivet there to contribute to the tab’s structural integrity. The stab’s spar was un damaged; however, I rebuilt the entire stab with new parts using the original spar. We also added stiffeners to the anti-servo tabs at about a 45 degree angle from the inboard trailing edge to the front of the tab to increase torsional rigidity.

Neither my event nor the most recent event should be confused as flutter. Flutter is divergent and, therefore, catastrophic. Meaning my horizontal stab would have ripped itself apart and I wouldn’t be typing this. I may have spent my youth flying single-seat fighters for Uncle Sugar, but one of my dual majors in college was aerospace engineering. I guess I shouldn’t have tried to make my Thorp go as fast as an F-16.

Spanky

Jim,
If you're referring to the bullet style stab weights those were the original implementation designed such that they could be added to an existing completed T-18 stabilator along with the other anti-flutter mods (stainless straps at the tab, spar beefed up). The internal weights and the external 'bullets' both accomplish the same thing. I've seen pictures of the event you're referring to, and as Spanky said .. that's not flutter. I cannot fathom what type of stress is necessary to cause that sort of damage. He's a lucky guy to be alive for sure. The owner should reach out see if anyone has a stab they'd like to sell him or buy kit from Cubes.
Lee

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

As best I can GUESS, that damage was caused by either corrosion, or an improper build. It could not have been flutter, nor the harmonic oscillation that showed up in JTs flight test.
However, this is a great ( and lucky) reminder to make sure you look at the SDR list on the first page of this website. From the pictures I have seen, I do not believe this horizontal had the stainless straps that are required. I cannot comment on the other SDR changes.
Jim, there are three total weights on the horizontal. One is on a 24” beam extending forward. Two are on the horizontal leading edge. It doesn’t matter if the two on the horizontal are in the forward bullet shape, or under the leading edge skin. You need all three to be airworthy.
Cubes

The SS straps were installed, as were the weights.

This was a well built airplane and the two previous owners were excellent custodians.

I am glad he made it down okay. In addition to the corrosion possibility, something could have worn out. I had to reattach the stainless strap on the right side of mine a couple of years ago. I didn’t catch it was loose during a regular inspection. The screw holes were worn out in the aluminum wedge, the bottom skin was loose at the squeeze rivets holding the wedge in place and the pull rivets holding the strap to the leading edge were loose. I know that makes it sound like it was a sloppy mess but there wasn’t very much movement. The smoking rivets on the skin caught my attention while doing other maintenance under the stab. I think the leading edge rivets could be gone and no one would notice during a preflight. Possibly the same with the rivet heads on the bottom. I figured it was critical enough to not put off repairs until the condition inspection.

_________________
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

Couple of questions...actually a question and a request:

Lee, do I have anything to worry about with my airplane?

And, if someone would be kind enough to post a few photos of where to look for what, I would be eternally grateful.

_________________
Philip Mandel
N118BC
S/N 957
Beaverton OR

Philip,
Your plane has all of the mods.
Lee

Here's the SDR list ...

http://thorp18.com/index.php?option=com ... Itemid=125

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

The first picture shows the bend in the stainless steel strap where it turns and tucks behind the tab’s leading edge. It is sandwiched between the tab and the tab arm. It also has 2 rivets shared with the hinge along the front.

The second picture shows a row of rivets associated with having weights inside the leading edge. Mine has rivets visible on both the top and bottom.

_________________
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