lml@lancaironline.net Mailing List Archive

Rob,

Just a note. The CAFE report does not indicate whether the elevator bell crank was shortened (4" to 3"). All I know is that in my airplane in a 60 degree banked turn to the left the untrimmed force to hold the stick back is quite heavy - quite (but unmeasured and also against the spring if ya got one). I did shorten the bell crank.

On the other hand, using any of the spring trim systems and trimming the plane for the desired flight (like level) results in a very sensitive elevator - especially in the same direction that the spring is pushing. Remember that one doesn't have to displace the elevator very far to get a response.

I have no idea how trim tabs affect sensitivity.

Scott Krueger

In a message dated 7/16/2010 12:58:15 P.M. Central Daylight Time, rwolf99@aol.com writes:

I looked at the CAFE reports and at FAR 23.155 (the FAR which regulates elevator forces in small airplanes). The regulation requires a minimum of 15 pounds of aft stick force to go from cruise to maximum G (4.5 G for the 320, per the POH). That would represent 4.3 pounds per G if the stick force gradients were linear. It also allows some decrease in stick force gradient (designers usually try to avoid this) as long as it is not "excessive".

Here are the aft stick forces required to obtain one additional G (i.e. to go from trimmed one-G flight to 2 Gs) for various airplanes.

1) The Cessna 152 -- 20 pounds of aft stick force

2) An RV-8 -- 11 pounds of aft stick force

3) An RV-6, a Lancair Legacy (N199L), and a Lancair 320 (Fred Baron's airplane) -- all about 4 pounds of aft stick force

So it appears as though the Lancairs are at or very close to (and maybe slightly below) the minimum stick force per G for a certified airplane, but surprisingly similar to an RV-6. The RV-8 and the Cessna 152 are well above the minimum stick force per G for certified airplanes.

But ...

Never forget that the FARs are intended to keep the lowest common denominator safe. The "Joe bag of doughnuts" weekend VFR flyer who just got his license. No offense is meant here. I was Joe for a long time. (And my favorite is a chocolate glazed -- not chocolate frosted -- doughnut.) That's why I flew Cessnas early in my flying career. There is nothing inherently wrong with a more skilled pilot flying a more maneuverable airplane. Lancairs have long been advertised as the "Ferrari of the skies" and I'd never give my sixteen year old kid the keys to my Ferrari (if I had one, that is). But it does suggest that the safety margins are smaller in the less forgiving Lancair, but also for the RV-6, which nobody thinks of as dangerous. It's obviously not the whole story.

However ...

The CAFE guys did not go all the way to 4.5 G -- they stopped at 3.5 G. So the data is not 100% there. We have to extrapolate to determine if the FAR requirement is met. The Legacy looks pretty linear (stick force vs G) and appears as though the minimum stick force at maximum G would be somewhere between 15 and 20 pounds at 4.5 G -- the FAR requirement therefore appears to be met.

The 320, on the other hand, looks pretty good at forward CG (extrapolates to 19 - 20 pounds at 4.5 G) but no so much at aft CG, where the stick force gradient decreases (although not excessively) and it extrapolates to only about 11 pounds for 4.5 G. Not so good.

That's troublesome, at least to me, and is why I have strived to make my CG forward. Long engine mount, battery in the engine compartment, hydraulic pump forward of the instrument panel are the big things I've done. I also convinced my wife that I bought the higher-power 360 engine on the basis of safety, since it adds weight to the nose of the airplane. (No, she didn't believe my rationale but agreed to look the other way, and no, she does not have a sister.)

Take this data as you will. I think we all agree that the Lancair is not for the beginner pilot, and here we have some regulatory guidance suggesting why. Experience and training act together to mitigate the reduced safety margins of a maneuverable airplane, and the Lancair is no exception.

- Rob Wolf