Could this be a problem or it is normal!!!

[juasiel123]

I have been flying my 1950 C-170a for about 3 years now and 500hrs I have build with it.
This is my first plane and since the first time I flew it, notice that during level flight the elevator remains in the down position and that in order to avoid claimbing even more, the trim is set to nose down position (that is something I don't find too odd, but it is the way the elevator looks or at least the tips). I flew some other aircraft in the past, like the C-150 and C-172 and during cruise flight the elevator and horizontal stabilizer remain parallel. The only way I can make the elevator and the stab to remain parallel or in one single line is during slow flight.

So I took some pictures during straight and level flight and during slow flight.
Level Flight
110 mph Indicated
1000ft altitude
68* f
2300 RPM s
Trim set to nose down position
1/2 fuel on both tanks
No cargo
Just Crew No passengers


Slow Flight
70 mph Indicated
1000ft altitude
68* f
1800 RPM s
Trim set to nose up position
1/2 fuel on both tanks
No cargo
Just Crew No passengers




Talking to some friends that flew with me and looked at the elevator during flight, they think that something must be wrong and mention that if the plane is flying with no weight or overweight, the elevator should remain straight as the H.Stab.
Would like to know if this could be normal on this plane or if something else going on.

Any thoughts? I would appreciate.
Blessings for 2010....
Juasiel

[Bruce Fenstermacher]

This is normal. My elevator is rarely matched with the horizontal stab. I quit looking back at it and feel much better know.

What is your CG? Do you know if the eccentric bushings in your wing are adjusted both neutral? Both would effect where the elevator flies in relation to the horizontal stab.

[jrenwick]

I don't see where a little down-elevator in cruise is a problem. It means you've got less tail-down force than if it were in the neutral position. Less tail-down force means the wings have to lift slightly less, you can fly at a slightly lower AoA, and a better cruise speed should be the result.

[3958v]

Thats the way Cessna built them all. The horizontal stabilizer is trimmed up a little so that you can get the tail down far enough for a three point landing at the forward CGs. The original prototype did not have enough negative incidence and it was increased in production aircraft. From William Thompsons book Cessna Wings for the World. "We looked forward to expanding the C.G. envelope sufficiently to make this an honest "4place 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 of the horizontal stabilizer had to be changed to -4 deg. This of course had to be counteracted in cruising flight with down elevator, creating some drag. In addition, it produced a greater out-of-trim condition in a balked landing climb requiring a more rapid application of nose-down 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 and C-195." Bill K

[GAHorn]

Yep. Normal. Completely normal.

[juasiel123]

Thats the way Cessna built them all. The horizontal stabilizer is trimmed up a little so that you can get the tail down far enough for a three point landing at the forward CGs. The original prototype did not have enough negative incidence and it was increased in production aircraft. From William Thompsons book Cessna Wings for the World. "We looked forward to expanding the C.G. envelope sufficiently to make this an honest "4place 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 of the horizontal stabilizer had to be changed to -4 deg. This of course had to be counteracted in cruising flight with down elevator, creating some drag. In addition, it produced a greater out-of-trim condition in a balked landing climb requiring a more rapid application of nose-down 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 and C-195." Bill K

Wow.. . I didn't have any idea that it was that much into it.... thanks for the explanation 3958v. I knew dip inside that it could not be wrong, since my plane flyes great.. even with the four speakers mounted on the wings..hahahaha !!!
However I thought I had a drag problem in my hands. Good to know that everything is ok.

Thanks to you all for your comments, and if you ever come to BAJA, please give me a call. Whales are a beautiful sight from the air .

Juasiel

[Bill Hart]

How the hell did you see that?

[GAHorn]

Did you know the 170 and 170A both have a negative incidence of minus-Four degrees of incidence of the horizontal stabilizers....and the 170-B incidence is only a minus-2 degrees?(approximately...it's actually -2 and a few minutes but I am not where my data is so I cannot post it right now. I'm in La La Land (greater Los Angeles, Van Nuys Airport hotel) for the Rose Bowl.)


Anyway, the B-model has less negative-incidence than previous models to deal with the increased angle-of-attack created by downwash of the larger flaps. This usually results in only a slightly less nose-down elevator in cruise than that displayed by 170/170A aircraft.

[juasiel123]

How the hell did you see that?

See what??

[hilltop170]

....and the 170-B incidence is only a minus-2 degrees?(approximately...it's actually -2 and a few minutes


Anyway, the B-model has less negative-incidence than previous models to deal with the increased angle-of-attack created by downwash of the larger flaps. This usually results in only a slightly less nose-down elevator in cruise than that displayed by 170/170A aircraft.

Funny you should mention that George, (why don't you carry all your reference material with you on those trips so we can get instant answers? ), I was just discussing that with Del at Mountain Airframe yesterday (the 170-B horiz stab incidence is -2°48' by the way). That makes the 170B stab 5/8" higher at the leading edge of the stab.

We were trying to decide what speed increase if any would be realized by a 170A by reducing the angle from -4° back to -2° 48'. For a guy who always carries 50lb or more of survival gear and stuff in the back anyway and does wheel landings only, the extra negative angle is just extra drag being dragged around for no good purpose.

That 5/8" would also seem to make the B slightly faster but maybe not because of the airspeed loss due to the dihedral on the 170B vs. the 170A straight wing, so it's probably not an apples to apples comparison.

As a side note which is more apples to apples, the same 5/8" difference was also done on early vs. late C195s but in the opposite direction. The early 195s had LESS negative incidence than the late by the same 5/8" even though like the 170, the later 195s have larger flaps and the truncated elevator like the late 170B does. The story why, as told to me by Mort Brown Cessna production test pilot, was the jigs were moved to a different location for the late s/n planes and when they were re-set, they didn't re-shoot the alignment on the jigs and they were off 5/8" more negative. So as the story goes, it was done by mistake, not design. Don't know how accurate that story is as Mort was not in engineering at Cessna and he was 100yr old when he told the story to me. Again, it would be really interesting to know the details of Cessna's decisions on thing like this.

Anyway, the early 195s with the less negative horiz stab incidence are a few mph faster than the later 195s. Guys have flown side-by-side more than once to check it out (I know, not scientific proof but close enough for a BS session). Everything else is the same on the early vs. late 195s while in cruise so it stands to reason the 170A MIGHT get a similar boost in cruise speed. Then we could end the "who is faster" debate once and for all!

[GAHorn]

I'm not a 190/195-knowlegeable person and so these comments are purely speculative.... but if the 195 is a development from the 190 which was powered by a W-670 220 hp Continental with a dry weight of 465 lbs, then the CG of the 190 would likely be aft of a 195, which first started out with a 300 hp Jacobs R-755, and subsequent versions (195A/195B) had even less hp, 245 and 275 respectively I believe) which had a dry weight of 505 lbs. If the Jacobs were heavier than the Continental then that may explain why greater negative incidence would be desireable to provide similar elevator effectiveness. The increased hp would easily overcome any minor drag of the changed incidence.

(And, by the way, we are talking about really minute amounts of drag. In fact it's such a small amount that other aircraft condition items are probably more important....such as paint and color, so I recommend RED.)

[hilltop170]

George-
Your assumptions are plausable but the 190, 195, 195A, and 195B were identical airplanes made on the same line with the only differences being the engine assemblies (190/Cont 240, 195/300 Jacobs, 195A/245 Jacobs, 195B/275 Jacobs). The 190 engine is positioned to make up for the difference in weight and a 190 cowl will not fit on a 195. The 195 was actually made first and the other models followed. There was no distinction made between models in the airframe. The horiz stab incidence difference, flap, and elevator changes were made after a certain serial number without regard to engine or model. I'm not sure but I don't believe any 190s were made after the change.

So what say you George, does the horiz stab incidence difference make a measurable difference in airspeed?

I agree with red.

[Bruce Fenstermacher]

I'm not a 190/195-knowlegeable person and so these comments are purely speculative.... but if the 195 is a development from the 190 which was powered by a W-670 220 hp Continental with a dry weight of 465 lbs, then the CG of the 190 would likely be aft of a 195, which first started out with a 300 hp Jacobs R-755, and subsequent versions (195A/195B) had even less hp, 245 and 275 respectively I believe) which had a dry weight of 505 lbs. If the Jacobs were heavier than the Continental then that may explain why greater negative incidence would be desireable to provide similar elevator effectiveness. The increased hp would easily overcome any minor drag of the changed incidence.

(And, by the way, we are talking about really minute amounts of drag. In fact it's such a small amount that other aircraft condition items are probably more important....such as paint and color, so I recommend RED.)

Yep it's obvious your 190/195 knowledge is lacking George. Otherwise this short paragraph would stretched out to at least 10.

[GAHorn]

Richard, of course the engine mounts and cowls are not identical, they cannot be due to the dissimilarity of the two basic engines, but the later change of incidence might the the result of Cessna having a learning-curve on the airframe due to the engine weight diffs...???
WD Thompson, who unlike production test-pilot Mort, was an engineering test-pilot, wrote about the removal of the C-195 assembly jigs and the subsequent problem they discovered during stall/spin testing of the newer airframes, blamed on improper jig set-up. Perhaps that was what Mort was considering when addressing this question?
Thompson wrote that the improper jig-set up was related to the leading edges of the stabilizer being improperly applied, not the angle of incidence. He observed that corrected leading edges solved the problem.
I wonder if the later serial differences weren't somewhat further down the line following flight-characteristics...., as opposed to a "lets make this change" following an improper jig-setup. (The more likely result of such a discovery would be a re-shoot of the waterline and re-set of the jig to the proper incidence.) I would be more inclined to believe Cessna wanted to refine the handling or possibly even reduce an assembly-operations-cost, rather than believe they would spend money on redesign when all they needed to do was correct an error in set-up. I really don't know, tho'.

Bruce,....Thhhrrrrrrbbbbbbwwwwwpppppppttttttt!

[hilltop170]

The point about the 190 cowls not fitting was not a very good point because obviously the engines are designed differently, my intention was to point out the Continental engine was positioned differently to compensate for the different weights of the Cont. and Jacobs to keep the cg in the same place. The airframes are all the same no matter what engine. All airframes can use any of those four engines by the type certificate. Most 190s now have Jacobs engines for obvious reasons, there is no substitute for horsepower.

I don't know George, but whatever caused the horiz stab incidence change, it stayed the same on all the airplanes made after the change. It was never changed back to the original incidence. I've never heard of the horiz stab leading edge problem, that must have been taken care of before it reached production airplanes.