Hi,

Is rudder travel the same on 180 vrs. 185?

Yes, sorta. It's 24 degrees right and 24 degrees left. The only differences lie in the amount of variation allowed. The early 180s were 24, period. Later were 24 + or - 1 degree. The 185s were 24 plus 0, minus 1 degree.

You can find this kind of information easily by simply doing a search for the applicable Type Certificate Data Sheets (TCDS). In the case of these airplanes, they're found under Textron. Univair also has links to these TCs, btw.

Those TCs offer all sorts of information on approved equipment, CG range, etc, etc. For example, here's the link for the C-180: https://rgl.faa.gov/Regulatory_and_Guid ... Rev_68.pdf

MTV

Tks for the good info!

If I remember correctly the dorsal fin on the 185 had to be enlarged mainly to prevent rudder lock.

What made it more vulnerable vrs. the 180?

MTV can verify, but I believe it was a yaw stability issue on floats.

I'm not sure why the 185 had a larger dorsal from the git go. Early 180s did not, but they were all much lower gross weight.

The 185 started out as a 3350 pound gross airplane. When the Kenmore STC was developed to increase the gross weight on the 180s, it also required the 185/late 180 style dorsal fin.

So, I always assumed the size of the fin was dictated by gross weight. Now, specifically why, I can't tell you.

As to dorsal fin on 180s on floats, it totally depends on the floats installed. My 66 H model was on EDO 2870s, and did not require added vertical fin. A 180 on 2960s I believe did, in the form of a small ventral fin.

Bigger floats generally require more fin area, can be dorsal or more often ventral.

MTV

Tks, just emailed Kenmore with the Q.

Will post if they explain.

I can't recall off the top of my head, but the larger dorsal may have had something to do with CG range as well.

FWIW, Most Skywagons I work on for the first time have improperly rigged rudders in terms of travel.

Let me tell you all about a little test called the directional stability test... Something the FAA has implemented since the early days of their existence..

Here is the low down of the test in question with a hypothetical airplane... say 2000 lbs GW with a CG range of 10 to 24 inches.. this aircraft is powered by a Lycoming 0-360 with constant speed prop and has a stall speed of about 55-56 mph..

The test requires the aircraft to be loaded with ballast to the max gross weight with the CG all the way aft loaded.. so airplane at 2000 with fuel and pilot with CG at 24 inches.. the airplane is flown to altitude and slowed down to an air speed of 1.2 above stall, so around 60 MPH... now the pilot gradually increases the power to full continuous power setting while pulling the nose back to remain at that 60 MPH speed... you can imagine how high the nose is while you are not gaining altitude... now the pilot gradually fully cross controls the airplane... say full right aileron with full left rudder... once the aircraft is fully within this cross control situation, let go of the rudder and put your feet on the floor.. the aircraft will immediately start turning the direction of the ailerons.. BUT, the test REQUIRES that the wings remain level to no more than 10 degrees roll and the turning must either come to a complete stop or slow significantly...

We were required to pass this exact test on our STC to install an 0-360 Lycoming with constant speed prop on the PA-22/20 series aircraft... on that airplane the left test would slow, right would stop... BUT.. we did not pass it the first time around...

There are a few things that can be done to modify the aircraft to pass... increase the DORSAL fin area (180 to 185, early Maule to later Maule, etc).. Add a rudder trim with take off setting... Derate the continuous power rating with a take off power limit.. how many airplanes do we know that have a Take off power setting for 5 minutes..??

We eventually passed the test by derating the continuous HP with a takeoff power rating for 5 minutes and including our already approved rudder trim STC with a setting marked for take off..

Brian


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Brian
Thanks for the education. I often see stuff on aircraft and in the POH and wonder why it is set up that way. I figure there was a reason someone went to the trouble of making a point of it so I tend to follow what instructions say.
DENNY

Brian-StevesAircraft wrote:Let me tell you all about a little test called the directional stability test... Something the FAA has implemented since the early days of their existence..

Here is the low down of the test in question with a hypothetical airplane... say 2000 lbs GW with a CG range of 10 to 24 inches.. this aircraft is powered by a Lycoming 0-360 with constant speed prop and has a stall speed of about 55-56 mph..

The test requires the aircraft to be loaded with ballast to the max gross weight with the CG all the way aft loaded.. so airplane at 2000 with fuel and pilot with CG at 24 inches.. the airplane is flown to altitude and slowed down to an air speed of 1.2 above stall, so around 60 MPH... now the pilot gradually increases the power to full continuous power setting while pulling the nose back to remain at that 60 MPH speed... you can imagine how high the nose is while you are not gaining altitude... now the pilot gradually fully cross controls the airplane... say full right aileron with full left rudder... once the aircraft is fully within this cross control situation, let go of the rudder and put your feet on the floor.. the aircraft will immediately start turning the direction of the ailerons.. BUT, the test REQUIRES that the wings remain level to no more than 10 degrees roll and the turning must either come to a complete stop or slow significantly...

We were required to pass this exact test on our STC to install an 0-360 Lycoming with constant speed prop on the PA-22/20 series aircraft... on that airplane the left test would slow, right would stop... BUT.. we did not pass it the first time around...

There are a few things that can be done to modify the aircraft to pass... increase the DORSAL fin area (180 to 185, early Maule to later Maule, etc).. Add a rudder trim with take off setting... Derate the continuous power rating with a take off power limit.. how many airplanes do we know that have a Take off power setting for 5 minutes..??

We eventually passed the test by derating the continuous HP with a takeoff power rating for 5 minutes and including our already approved rudder trim STC with a setting marked for take off..

Brian


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Good information, Brian. I'm sure that'd explain the requirement for the big fin on early 180s when the gross weight increases.

MTV

Yep... increase gross weight enough and do that same aft loaded CG test at that higher weight and the need to modify to the bigger dorsal fin would come into play..

What amazes me is there are a few STC’s out there where I wonder how or if they had to go through that same test as we did and not modify the airplane accordingly...

Brian


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DENNY wrote:Brian
Thanks for the education. I often see stuff on aircraft and in the POH and wonder why it is set up that way. I figure there was a reason someone went to the trouble of making a point of it so I tend to follow what instructions say.
DENNY

That STC was quite the education to us too... took over 2 years to get approval..

Brian


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Brian-StevesAircraft wrote:Yep... increase gross weight enough and do that same aft loaded CG test at that higher weight and the need to modify to the bigger dorsal fin would come into play..

What amazes me is there are a few STC’s out there where I wonder how or if they had to go through that same test as we did and not modify the airplane accordingly...

Brian


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Indeed, I’ve flown a couple of seaplanes that were just as happy to fly sideways as straight ahead. My 170 on PeeKay B 2300s was high on that list. I doubt it’d have passed that test, even with the optional ventral fin.

But then I happen to have known the gent who signed off that STC.....and that explains it.

MTV

Excellent information Gents and sorry to come back late.

Bryan, when you mention,

...say full right aileron with full left rudder... once the aircraft is fully within this cross control situation, let go of the rudder and put your feet on the floor.. the aircraft will immediately start turning the direction of the ailerons.. BUT, the test REQUIRES that the wings remain level to no more than 10 degrees roll and the turning must either come to a complete stop or slow significantly...

You only state to let go on the rudder but you also let go on the aileron correct?

I can imagine the negative effect of more HP and directional stability, could you explain the relevance of limiting power to 5min please?
Just to shorten the time frame under that critical contributing factor?

The negative effect of weight in directional stability comes into play because of the increased moment and a higher risk of reaching a greater yaw angle?

Scary stuff. Takeoff fast. Don't be a test pilot.

[email protected] wrote:Excellent information Gents and sorry to come back late.

Bryan, when you mention,

...say full right aileron with full left rudder... once the aircraft is fully within this cross control situation, let go of the rudder and put your feet on the floor.. the aircraft will immediately start turning the direction of the ailerons.. BUT, the test REQUIRES that the wings remain level to no more than 10 degrees roll and the turning must either come to a complete stop or slow significantly...

You only state to let go on the rudder but you also let go on the aileron correct?

I can imagine the negative effect of more HP and directional stability, could you explain the relevance of limiting power to 5min please?
Just to shorten the time frame under that critical contributing factor?

The negative effect of weight in directional stability comes into play because of the increased moment and a higher risk of reaching a greater yaw angle?

Only the rudder is released... ailerons stay at full deflection... The test is conducted at maximum continuous horsepower.. When we did it we set our continuous horsepower to 160, thus having 180 HP set for the first 5 minutes.. the idea of this is at the max continuous setting there is less torque being produced so the less the aircraft will have a tendency to roll during the test..

Hope that makes sense..

Brian


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Just making sure I was not missing someting with your 5 min restriction comment.

Bill Thompson worked for Cessna for 28 years as an engineering test pilot and later as the Manager of Flight Test and Aerodynamics. He wrote the excellent book, Cessna Wings for the World, which explains the development of the Cessna single engine models. According to this source:

“The most distinctive feature of the C-185 is the large dorsal fin which was needed for preventing rudder lock (from extreme angles of yaw in power-on directional stability tests) in the floatplane. Since the tailwheel steering system had been perfected in the C-180 landplane, we believed that the increased weathervaning tendencies due to the large dorsal fin could be controlled in long taxi runs in strong crosswinds in the C-185 landplane. This belief was confirmed, and, in fact, the same dorsal fin was adopted in late model C-180 landplanes for commonality on the production line.”

“This large dorsal fin also made the pilot hold greater rudder deflection in cross-wind-landing drift correction. This, of course, would cause the steering chains to hold the tailwheel in a misaligned position at touchdown, promoting a violent shimmy in some cases. Thus it was necessary to install a manual tail wheel lock as an anti-swivel device. The locking lever, located on the cabin floor console, controlled a spring-loaded locking lug on the tailwheel assembly. This device was used, as desired, before each landing. Typically, it was used mainly in crosswind landings to keep the tail wheel aligned straight ahead while the rudder was used independently for directional control in the early phase of the landing roll.”

“Bruce Barret recalls: “…the agonies we went through making the camber-lift 180-185 floatplanes comply with contemporary hardline FAA requirements for (primarily) directional stability and control. The first 180J/A185F floatplanes had the big dorsal added to the 180, a ventral fin, and some rudder centering springs. This was not received well in the field…”

“…As a result of all the field antagonism and confusion, additional development was accomplished through a lengthy flight test program. Ultimately the ventral addition was eliminated…”

The above confirms Manuel’s belief that the 185 had the large dorsal fin to prevent rudder lock. It also confirms Cannon’s and Brian’s comments about yaw in the directional stability tests. Previously missing in the above discussion, however, is the timing of the large dorsal fin on the 180 floatplanes concurrent with the introduction of the cuffed leading edge as well as the simple explanation of using the large dorsal on late model 180 landplanes for commonality of production. It also shed’s light on the ventral fin discussion in the Ramp mummies’ thread. I feel your pain, Kurt.

Ross

Thanks for sharing such valuable information Ross.

Must say I flew more than 40 years before knowing what a "rudder lock" was.