Tech Questions from current Buyer of 337

[Centered Power]

Thanks for threads I previously saw, but.. actually I referred in my question to the fact that my query is NOT addressed in these threads.

I am not confused about Knots vs mph. I am not asking about the pre 73 vs the post 73 and am not asking about the distinction beween gear extension / retraction vs flying with gear fully extended down and locked ---- these are all quite clear in the various POH's and in the threads herein. -----NO the question --And my apologies if it wasn't apparent is specifically about the difference between 1969 POH and 1971 POH --140mph vs 160mph. All the other verbiage I included was my feeble attempt to show I was already aware of all the matters in the two threads referred to. (Also sme of the confusion addressed in these threads may have been exacerbated by the former change to GAMA "standard" POH.

Back to specific question:

WHY is 1969 POH different from 1971 POH: 140mph vs 160 mph ?

Both of these are pre 73 planes and pre GAMA POH format!

IS there a structural / Parts difference ?

Or was there a recalculation of mechanical stresses by Cessna ?

Or something else?

Thanks again -- also there is question 2 about Robertson above.

[hharney]

Cessna never made the same Skymaster throughout it's manufacturing process. That's why there is Straight, A's, B's, C's etc. models. They continued to make changes to try to market the airplane. The gross weights changed throughout the years also, it increased as the models increased. My personal understanding is because of marketing. They may have had to spend some money to add these extras (changes in certification) but it must have made sense to them to do it. The 1971 F model has the highest gross weight and therefore useful load too. It was the last of the swing door models and therefore had the latest and greatest stuff. Structurally, was it the same as the D as you asked? I am sure there must have been something to convince the FAA that the gross weight could be increased, what those are I am not 100% sure. But something justified it. The F model is the one to get if you can find one, rare birds. They only made 83 F models in 1971 and 64 F models in 1972. That is if you want a swing door model, but they were lighter planes and had the luggage door too. The 1973 and after got heavier and they eliminated the luggage door. There are some other differences to like the front prop blades are 2 inches longer than the rear prop blades. Why? To even the score on the single engine climb and ceiling for each engine. The rear was always better than the front so the solution was to put a longer blade on the front. Too bad because it really made the blade clearance close to the ground.

I have never seen any angle/pitch info on the Robertson STOL mod. Not sure it would be eas to do with all the models and differences that were made over the years. AOA would have helped back then but no one used those much for testing.

Hope that helps too

[JAG]

Centered Power,
My two cents worth - looking at my IPC (1965 thru 1969 Skymaster) you will clearly see there are differences in the structural components on the main landing gear bulkhead assembly (additional doublers after S/N 0526 for the higher gross weights, and even some different bulkhead channel part numbers after S/N 0610). Perhaps there are some more structural differences from after 1969 to 1971 that raised this gear speed number. To the normal eye (and even some trained mechanics) the structure looks identical, but Cessna, like all OEMs, made small changes to structure with small doublers or thicker webs and channels that strengthen up the structure. You would need to study the IPC between those model years. Sometimes you find some weird changes in the models based on marketing, however you have to be cautious when it comes to some things associated with limitations like speeds and weights - as there were subtle differences in the engineering to justify these changes such as small structural differences or flight control rigging differences to compensate for the different limitations.
Regards,
Jeff