Why is my trim backwards?

[Jr.CubBuilder]

I hadn't really given it any thought till the other day when a friend took me for a ride in his 185. When he put the flaps on he trimmed the plane up to take the backpressure off the yoke.

The 152s I first started learning in wanted some nose up trim. The 172 that I used to rent always wanted some up trim as the flaps were deployed, and I don't remember doing anything drastically different when I was flying the warrior years ago.

When I add flaps on my 170B I have to give it a swipe of down trim with each notch. Is this because of a difference in thrustline?

[GAHorn]

It depends upon the speed and which model aircraft is being flown.
In his book "Cessna, Wings for the World", Cessna Test Pilot Wm. Thompson addressed pitching moments in the B-model.
When the B-model with it's large "para-lift" flaps was introduced, it was first feared that the strong nose-down pitching moments of the A-model would be greatly exaggerated. However a pleasant surprise awaited when it was found that these moments were ..."counteracted to a large extent by a nose-up tendency resulting from the (larger flap's) stronger wind down-wash striking the horizontal tail. Thus, very little retrimming was required from an 80 mph flaps up glide to a 70 mph flaps down glide. I recall project engineer, Web Moore, being overjoyed at this finding. We were always puzzled when magazine writers criticized a transient nose-up motion with initial flap extension. To Cessna pilots it was practically unnoticeable, probably because we extended the flaps smoothly and evenly. Jerking the flap handle upward would produce the momentary nose-up motion. However, the overall result was far superior ot other airplanes of that era (and this era.)"

[mit]

don't forget that a 185 has stab trim, not trim tab.

[Jr.CubBuilder]

True, I hadn't thought about that, but it's the pitching tendancy after putting the flaps down that was puzzling me. I tied down yesterday afternoon next to a 152, and it did look like the stab. sits a little higher up in relation to the cord line of the wing. I'll have to eyeball it next time I'm parked beside a 172.

Just out of curiosity does the A model tend to pitch down with flaps on to a much greater extent than a 172?

I also noticed that it appears that my wings have more washout than either the 152 or 172. I'm just starting to notice more differences than similiarities. A year ago I thought a 170 was just an early 172 without a nose-wheel.

[GAHorn]

It depends upon which year model 172 you're looking at. The 56 170 and 56 172 were just as you described...different gear/square tail. The 172 continued to evolve over the years, with current production 172's quite different than earlier versions.

[zero.one.victor]

As you pull on more flaps with a ragwing, the nose pitches down. Trimming for desired approach speed, I land three-point with full nose-up trim, & not quite full-nose-up trim for a wheel landing. This makes a go-around pretty exciting, as you're trying to drastically re-trim nose-down, ease down the flaps, AND keep the yoke musculed forward--all at the same time. A departure stall would be pretty easy to accomplish during this procedure. I would think landing with the trim in a more neutral position would be a good thing.
I always thought the nose-up pitching of a B as you pull on more flaps makes things easier--as you come into the pattern, start easing off on the power. As the nose (& speed) starts to come down, add a notch of flaps on downwind & trim for around 80. You can fly a 80-70-60mph pattern just by adding a notch of flaps base & on final, instead of re-trimming. You end up on final at 60, with 3 notches of flaps hanging out, and the trim set not too far from take-off position in case of a go-around.
Of course, I don't have much time in a B to prove out my theory, but it sure sounds good. Works real well in the hangar!

Eric

[Jr.CubBuilder]

Actually it has been working out pretty much as you describe, and I've been amazed with how little trim it needs although I suppose that might change if I start carrying passengers or cargo. I would be really curious now to fly one of the early 172s to see how it compares. The rental that I was flying before was a 1976 172 and it just seemed to fly like it was big and heavy.

[zero.one.victor]

That's because it was, at least the heavy part. If you look at the spec's for various Cessna's, you'll see the empty weight of the later version models get heavier & heavier. Check the factory weights for both the 180 & 182, early versus late.
Re: trim, I've had friends tell me they never have to re-trim at all, from cruise flight all the way into & thru their landing. I don't know how different their cruise trim is from take-off trim setting, which is where you'd want to be for a go-around.

Eric

[GAHorn]

The early 172's were given squared tailfeathers for several reasons. The nosegear required greater vert.stab./rudder authority to overcome the short turning moment of the nosegear, especially in gusty winds while manuevering on the ground. This had a happy bonus in lessening the requirement for coordinated use of rudder in turning flight. (Ever notice how the 170 taildragger seems less stabil direcitonally and requires more energetic use of rudder? The larger squared vert/rudder relieves some of that need for pilot input.)
The nosegear also needed a more authoritative elevator to lift the nosegear out of a muddy takeoff, so the increased size of squared elevators were the answer to that as well.
If you've ever flown the early 172/175 series of airplanes you'll notice how very sensitive they are especially in pitch. This is due to the cable/push-pull/bellcrank geometry carried over from the 170 with it's small rounded tailfeathers being almost too much for the larger tailfeathers. This contributed to overcontrolling in the hands of inexperienced pilots.
Cessna's answer was to change that geometry to "stiffen" up the elevator inputs. This made the elevators in the later aircraft stiffer and easier to maintain a given airspeed in gusty air, but it is also consequently less responsive than the earlier aircraft in control feel. This control "responsiveness" is what so many pilots admire in the 170 as compared to the heavy, stiff feel of later aircraft.

[N1478D]

aaaaaaahhhhhhh! If one was to paraphrase you George, you are saying that the older girls feel better! If you wait long enough for them to get in to their second childhood then maybe everything they get in their hands they will want to put in their mouths.

Seriously, not doing the A vs B thing - that stiffer feeling in the 172 is very undesireable after enjoying the A, and there is some difference in the B, a little more stiff than the A on the pedals.

More trim needed in the A because of not having counter balances like the B has. Flaps are so small, that if you increment a notch at a time you can keep the nose from ever moving down, passenger only knows you reached down and grabbed a big lever but never feels anything. But, put em in all at once and the nose drops, but that is hard on the system so we don't do that.

[GAHorn]

Joe, there should be no difference in the A vs B models as far as rudder feel goes. What you may have experienced is likely an individual airframe difference rather than a model difference. (In flight, I've been unable to detect any difference between the early vs late B models, even with the tailwheel steering changes.)

[N170BP]

If you set the rudder cable tension per the book (tighten turnbuckles
until rudder pedals are neutral, 6" aft of the firewall, measuring to the
hingeline of the brake pedal), I think there is sufficient differences
in airframes (worn rudder / brake pedal linkage / components, people
measuring 6" to the aft of the insulation they've applied to the firewall,
etc.) that some aircraft will feel different than others. Not to mention
those folks who have either incorrectly rigged the rudder pedal tension
and/or used incorrect (el-cheapo, whatever they could find) rudder
return springs.

[zero.one.victor]

Excuse me! I use nothing but El Cheapo-Aero brand parts, genuine FAA-PMA quality. And proud of it!

[Bruce Fenstermacher]

Hey hey did I miss something. Where can I get this El Cheapo brand of aeromotive parts.

[Bruce Fenstermacher]

Hey hey did I miss something. Where can I get this El Cheapo brand of aeromotive parts.