Thorp Air Command - T18.net

I have been considering a change from the external bullet style weights to the internal style on the horizontal tail and have been re-reading the newsletters on the topic. From my reading it appears to me that John Thorp added the tail balance weight to give the good stick feel, "the proper force per g of pitch maneuver" (NL30). During the flutter testing the aircraft was apparently tested without the weights at up to 209mph without problem but JT wanted to retain the weights. If I follow the News letter correctly it is more about the location of the weight and not strictly speaking the amount of weight that might be linked to a flutter problem. It also sounds like it was the stainless steel stiffeners on the tail tabs that had a significant impact on reducing the flutter potential.

So being both a novice pilot and novice builder wanting to reduce the weight of the aircraft i am prompted to ask the following possibly dumb questions:

How much do the 2*3lb outer tail weights contribute the stick feel and what does it actually feel like without those weight?
Do the proper stick forces contribute meanigfully to safety or is it more for consistency or ease of flying?
Could less or no weight be installed and enough stick feel be retained to keep the average pilot happy?
Is a weight reduction of up to 6 pounds worth the headache of addressing the above 2 questions and if you did, how would you determine the new max speed for flutter safety margin?

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Stug

Stug,
Those counterweights are absolutely 100% required. Do not even consider leaving them out. The counterweights (primarily the arm internal to the fuselage) do contribute to control feel but those outboard counterweights (bullet or internal to the leading edge) are part of the mods REQUIRED to the horizontal stab as a result of the extensive flutter testing conducted by John Thorp.

I have bullets on mine, they don't bother me at all.

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

Lee don't panic I am not rushing to take the weights off. I was just contemplating options for hiding them and or trimming some weight. I figure it doesn't hurt to ask the question especially if you could save some weight.

No doubt the flutter testing was comprehensive and the news letters probably provide only a brief summary of what was done making it hard would folk like me to get the full picture.
The NL recommendation is clear on what should be done but it is somewhat a one size fits all recommendation and is silent on other possibilities such as using less weight in the first place and what the compromises might be if you chose to do that.

No panic here Stuart just making sure a casual reader on this forum now or in the future understands that the tail mods are not variable or in any way optional. They must all be complied with unless you want to limit your red line to 180.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Is it advisable to keep all the counterweight in the fuse or add them on the horizontal also

This is what scared me about this thread ...

There are NO OPTIONS on the tail. The internal counterweight and the tips weights are REQUIRED as are the other modifications stipulated back in 1969!

Consult the drawings and do precisley what they say.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

From my research in the flight test data, the counterweights do not effect flutter in the flight areas we are approved to fly. The counterweights are more for stick force per g. That having been said, stick force per g is critical to safety of flight. An aircraft easily becomes uncontrollable with improper feedback to the pilot. Just read about the first F16 flight test data to confirm that.
I am trying to build a fast airplane. I did not lessen the weights in the tail at all.
Cubes

The T-18 has the best balanced stick force per "G" and stick force per roll rate I know of. If you want to see how the Thorp would feel without the weights, go fly an RV-6. I have experienced stick force reversal in a moderate "G" maneuver and I don't want to do it again. It is somewhat akin to the Mach Tuck we experienced in the F-4 when coming back through the mach after flying supersonic. The stick becomes a whole lot lighter while the elevator/stabilator becomes a whole lot more effective. The result is "G" overshoot and possibly a bent airplane. The other thing the tip weights do is move the center of resonance of the spar to a point outside the normal flight envelope. They are not so much a static balance tuner as much as a tuning fork de-tuner.

BTW, the stick forces of the standard T-18 are very similar to the artificial forces found in the F-16. Since the F-16 forces were designed/programmed in, the T-18 forces must have been near perfect to begin with.

Bottom line: Don't mess with the design!

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Bob Highley
N711SH
SN 835
KLAL

OK so the message is stick with the design.
Looking at the bullet style tip weight in the plans, drawing 623 it calls for a min weight of 1.9# and max 2.2# so I can aim to keep them light at 1.9lbs. By contrast the concealed type weight calls for 3 lbs of lead, I will be sticking with the bullet style. and saving 1.1 lbs per side.

Nothing against the "external weights." I just think the internal weights clean up the tail a bit.

RB