The International Cessna^® 170 Association

In cruise flight I observe that when my 1951 c170a serial number 19862 is trimmed for level flight that the elevator is positioned about 1" nose down (as seen between the leading edge of the elevator balance horns and the stabilizer). Is there any legal way to adjust the stabilizer with shims or other method to obtain a streamlined elevator in cruise? Has anybody else observed this problem?

Seems that if the elevator was streamlined that it would eliminate some drag resulting in more speed.

Thanks much for you input,

Jake Boyd

Jake,

Do a search on "Elevator position during flight" in the archives.

Basically, with 1 (or 2) people up front and a relatively light
fuel load, the CG is such that the elevator is not perfectly faired
with the horiz. stab. during cruise flight.

Lots of airplanes are in the same boat. If you have time, do a
search on the web for a pic of a WW-II Spitfire. The amount of
down elevator evident during (low) cruise flight is noteworthy!

PS: Don't even think of adjusting your wing trailing edge
eccentrics in an effort to go faster..... (You can think about
it, just don't post your thoughts here!!!).

Since I'm writing.... Any other Pacific NW 170-ers planning
on going to Concrete next weekend? I'll be there (weather
permitting) in my '54 C-170B. We should get together and
swap big, fat lies....

Bela P. Havasreti
'54 C-170B N170BP

My C170A SN 19853 is the same. With two on board the elevator is down an inch or so for level flight.

So's mine. But you get that thing loaded with 4, full fuel, and baggage, and it's the other way around. It's nuetral with 2, full fuel, and about 75# baggage.

Jake,
Can you clarify for me? Are your elevator horns 1" higher than your stabilizer when viewed from the pilot seat, or is it 1" lower(below)? What is your IAS at the time? When I'm trimmed up for about 122-125 mph IAS, the front tip of my elevator horn is about 3/4"" higher than the rest of the horizontal stab. My average flying weights are around 1800-1900lbs, and I try to slide my seat back to get more of an aft CG when I level off in cruise flight. Don't know what the average cruising IAS is for the c-145/0-300 guys, so I don't know if mine is where it should be.

Mike

Mike,

The elevator horns are about 1" above the horizontal stabilizer at about 110 mph cruise ( I have a climb prop, low cruise speed). This seems to indicate a tail heavy condition but most of my flying is with 1 or 2 in the front seats and little or no baggage. Seems that adding weight to the rear seats and baggage would only add to the tail heavy condition and require only more down elevator to compensate. I have been all through the inside of the aft fuselage and all is normal with no add-ons of any type to cause tail heavy condition so it is as from the factory. Further, if I was to add airspeed, it seems that the aircraft would try to climb requiring more nose down trim.

From all the comments of others of their aircraft it seems that it is normal for the 170 and I plan to leave it alone.

Thanks much for your and everybody's input.

Jake

Jake

I know this is a really late response but I just wanted to let you know I have a 1953 170 and my elevator is also positioned just like yours. I at one time or another sent in a similiar question. I thought I was the only one who looked aft when flying.

Chris Polsley

Actually, Chris,...most of us B-model owners look aft,...to see if Joe is still in view without binoculars.

I found the definitive answer to this question in Bill Thompson's fascinating book "Cessna: Wings for the World - the Single-Engine Development Story". Bill was manager of flight test and aerodynamics for 29 years, and performed the maiden flight on many prototypes, including the 180 and Skymaster. If you have any interest in how these airplanes were designed and tested, buy it! Great stories - spin testing the 195 raises the hair on the back of your neck...

Quote from the 170 development section of the book, page 22 "We looked forward to expanding the C.G. envelope sufficiently to make this an HONEST "4-place-and-baggage" airplane. Testing at forward C.G. with power off and flaps down showed inadequate elevator power for a three-point landing. To correct this the angle of negative incidence for the horizontal stabilizer had to be changed to -4 degrees. This, of course, had to be counteracted in cruising flight with down elevator, creating some drag. (italics mine). In addition, it produced a greater out-of-trim condition in a balked landing climb, requiring a more rapid application of nose down trim and/or flap retraction. However, these were small penalties to pay for such a far-forward C.G. limit. As expected, the elevator control forces were significantly greater than in the C-140 or C-195."

See, I told you Cessna knew what it was doing! I didn't know that production tooling was already fixed BEFORE the flight test program, so enlarging the elevator to overcome this problem would have been rather expensive (they actually had to do this to the 180 after testing revealed a similiar situation).

One interesting thing I learned was that all Cessna test pilots since 1948 are required to be aeronautical engineers, with at least a four year degree. Most have Masters in Structural Engineering, Thermodynamics, ect. They are the only aircraft manufacturer in the world that does this. Back in the '50s, and even to the present, it is very likely the pilot did engineering and design work on the very airplane he was testing. It certainly lowers the barriers between the eggheads and the stick jockeys! Bill Thompson himself was responsible for much of the 310's aerodynamic design.

Well worth the 25.00 I spent...I liked it so much, I bought his companion piece about the 300 series twin development. Russ Farris

That sounds like a GREAT book Russ, thanks for the heads up - going to get a copy.

For those looking for this book ("Cessna, Wings for the World") it's number is ISBN 0-89288-221-2, and it's Library of Congress Card Number is 91-61548, Published by Maverick Publications, Inc., P. O. Box 5007, Bend, Oregon 97708.
I'm curious why Mr. Thompson (after having told the story of the ragwing elevator in such detail) made no mention of the changes to the elevator in the A model, and the still further changes in the B model, the later airplanes having somewhat larger, more effective, and better balanced surfaces.

He was probably embarassed at having altered the great design of the original ragwing 170 to produce the slightly inferior but still nice A & B models!

Eric

Touche'