The "8 Knot Mod"

[jrichards]

Hi All,

As part of Aerodyme's 200 Kt initiative, we are about ready to begin flight testing the so-called "8 knot mod". This is the incidence change to the horizontal stab to streamline the horizontal tail in cruise. As many of you know, Robin made this change to his 112 some dozen years ago, has flown it ever since, and reported a gain of 8 knots cruise.

In trying to engineer the best possible STC kit, it would be helpful to have additional pictures of the existing trim condition on as many of our aircraft as possible. This will help me determine if one new incidence angle will suffice or if "grades" will be needed.

Would any of you be willing to snap photos and email them to me, or post here?
Example below.

Procedure as follows: Establish your best cruising speed, trim her up nicely to center the ball and hold altitude. Autopilot, hands and feet all OFF. If she won't hold a decent heading while you try to snap the pic, go get her rigged and try again. Smooth air is best but the other conditions are flexible, just send me the following data so I can normalize the results:

Indicated altitude
Pressure altitude (dial 29.92)
Indicated airspeed
OAT (at that altitude)
Your estimation of trim wheel position (see below)

Most helpful trim report:

Before the flight, position your trim wheel to make the trim tabs streamlined with the elevators (average of left and right) and mark this position of your trim wheel. Then, report to me the in flight trimmed condition as follows:

-5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5

-5 equals max nose down trim
0 equals the neutral position you marked
+5 equals max nose up trim

Thanks,
Jim

 

[wab003]

Re: The "8 Knot Mod"

Jim,

Great news. Won't the moment-as-loaded be of interest; or are you going to assume a std pilot flying solo?

 

[jrichards]

Re: The "8 Knot Mod"

Hi Bill,

Yes, I think it would be a good idea to gather load information, although in my informal tests I have not seen much effect on the streamlining.

I can figure the moments if everyone reports:

Empty weight and CG from the latest weight & balance.
Weight in the crew seats.
Weight in the back seats.
Weight in the bag pit.
Gals of fuel on board.

Thanks,
Jim

 

[atjudge]

Re: The "8 Knot Mod"

With the sizeable weight difference hanging off the nose of the 112 vs114, would there be two different mods, one for each model? Would there be a change in the mods if one went from a TO-360 to an IO-580?

 

[Steve]

Re: The "8 Knot Mod"

I wonder if a three blade or two blade prop (weight) would make a difference?

 

[jrichards]

Re: The "8 Knot Mod"

Hi Steve and Allen and All,

Let's find out. If we can get a bunch guys to fly the evaluation, send their pics, and report their data including model, prop, etc, I should be able to spot any correlations for us. Twenty, or so, reports would be great. The more, the better.

Jim

 

[jakoehn]

Re: The "8 Knot Mod"

Jim,could you make up a template so that we are all on the same page,will help with date crunching later.

Juergen
N5893N
114A/GT

 

[Airotto370]

Re: The "8 Knot Mod"

Hi Steve and Allen and All,

Let's find out. If we can get a bunch guys to fly the evaluation, send their pics, and report their data including model, prop, etc, I should be able to spot any correlations for us. Twenty, or so, reports would be great. The more, the better.

Jim

Jim, be happy to collect numbers for you once we're back up in the air!

Everybody else, quit distracting the boy from working on the IO-390 STC!!

 

[Steve]

Re: The "8 Knot Mod"

Jim,
Attached is the requested info and pictures in a Power Point. Hope this is close to what you are looking for. I have a TAS1000 Air Data Computer attached to the CNX 80 and GX 60. Very accurate.

Looking forward to my 10% discount when the STC is approved

 

[Pete]

Re: The "8 Knot Mod"

Jim, if the pics get posted they are from 8500 ft, two blade prop, 1976 114 S/N14068, 220 ft seat, 75 bagage area, 50 gal fuel, TAS 135kts.

I give up, Jim if you want the pics send my your email and I'll forward them, I can do emial pics but not on this board. Old dog new tricks........

 

[Pete]

Re: The "8 Knot Mod"

maybe this time

 

[dgeib]

Re: The "8 Knot Mod"

Jim,

Photo taken at 5800' PA, 132KIAS, Trim about 1 nose down, OAT 10C

 

[dgeib]

Re: The "8 Knot Mod"

This one taken at 148 KIAS in a descent passing 4000 No noticeable difference in trim. Still about 1 nose down

 

[N4843W]

Re: The "8 Knot Mod"

I will take one of mine when I get out of annual ... but mine looks identical to the two other 114's posted!

 

[rcastillo]

Re: The "8 Knot Mod"

Mine looks that way too, will try to get pictures next time I fly her.

 

[jonguy]

Re: The "8 Knot Mod"

Jim,

Just flew quite a bit the past weeks, with different loadings and different altitudes, but all cases looked just like the photos shown in the thread. My typical trim is about 2-3 nose down in a 145KTAS cruise. I will do a photo and record details the next time I fly.

 

[wab003]

Re: The "8 Knot Mod"

Wonder if there is a significant trim diff between the 112 and 114... my elevator offset when I'm "on the step" looks just like the others (just starting to see the first fastener) but my nose down trim is considerably greater to get there. Trim for the tab/elevator in trail is just at the top edge of "Takeoff" so that is correct; but the at-speed (appx 135 TAS in the 3K - 5K alt) trim setting is in the 3 - 4 nose down location. Think it is just the larger motor out front in the 114 w/ corrsponding lower moment? What are you 114 guys seeing for a BEW Moment? (or CG... either will serve as a reference)

 

[jrichards]

Re: The "8 Knot Mod"

Hi All,

This is great!

Thanks very much for the reports, and keep 'em coming.

Jim

 

[Commander91gm]

Re: The "8 Knot Mod"

Jim

Here is some more data;

112TCA
Ind Alt - 4000
Pressure Alt - 3800
TAS 135KTS
OAT 5c
Empty Weight 1958
Weight in Luggage 10lbs
pilot only 170lbs
Fuel 60gal

Black line on trim photo was neutral on the ground, Picture is of position in flight.

 

[dentate]

Re: The "8 Knot Mod"

Jim, I will fly this weekend and send you the pics, though you probably remember 115CE pretty well. Mine looks identical to all those already posted. I talked about this with Vince Giampa when we went up to Paso Robles last weekend. He subscribes to the view that the airframe was originally designed around the 111 fixed gear, that the stab angle was decided upon for that aircraft, and that it was simply never revised for later models.

I don't know about that, but it seems to me that even if we all suffer from this problem to varying degrees, it is going to be highly impractical to try to produce "tailored" kits unless you, Jim, yourself, are doing every single one; the measurements and testing involved will otherwise go well beyond what most mechanics are likely to accept in a kit in terms of responsibility. It would also seem to me that if you cannot specify one specific angle by which the stab should be modified, it will create a FSDO approval nightmare. Who is going to decide what angle to use for each plane, on what basis, and how would it be checked? If the current stabs are all deviated off from ideal in the same direction, I think it would make more sense to take the average deviation and undercorrect it slightly, to be safe. Obviously this will correct it more in some planes than others, but they already vary in speed anyway. You do not want to make this too complicated, too expensive, and above all you do not want to risk overcorrecting and causing other problems.

By the way, since this thread is under the "200 kt Commander" thread, I am still seeing a max of 175 knots TAS in the most ideal circumstances, more commonly 166-170. I realize that others are doing better and that this may be due to a number of factors unique to my plane, but no one is going to get a 200 knot airplane out of this. The cost-per-knot factor needs to be considered if you plan to sell a lot of these kits, Jim.