The whole “over square” business is obviously nonsense – because there is no physical relationship between RPM (revs/minute) and MP (inches).
It’s like saying you are not allowed to drive a car at a speed (in km/h) exceeding the width of the road (in feet).
The reason this stuff hangs around for so long is a lack of engineering education.
Some engines have specific limits e.g. here you see that the MP should not go above a certain value (27.2") unless the RPM is above a certain value (2300). The reason is unknown but it has been speculated it is to do with the stress at the base of the propeller flange.
Peter wrote:
Some engines have specific limits e.g. here you see that the MP should not go above a certain value unless the RPM is above a certain value. The reason is unknown but it has been speculated it is to do with the stress at the base of the propeller flange.
In the specific case shown in the figure you posted it’s noted as a propeller limitation, but in general the reason is fuel octane limitations, no different than opening full throttle on a motorcycle at 1500 rpm. Knock, knock, knock is often the result of either and the piston doesn’t like it. Low rpm gives more time for the fuel to break down and become unstable before its burned smoothly, and higher cylinder pressure accelerates the process of it doing so. Manifold pressure correlates to cylinder pressure.
The under square and over square thing is just happenstance. For larger engines ‘square’ operation in conventional units is a conservative limit of manifold/cylinder pressure to rpm that avoids detonation, so it’s a useful point of reference. Most engines and in particularly most smaller cylinder engines can almost invariably be ‘lugged’ more than ‘square’ without exceeding fuel octane limitations. If they aren’t supercharged sometimes you can do anything you want.
Funny things happen by coincidence with units. My favorite is this: steel elastic modulus 28 million, steel density 0.28 lb/cu in, aluminum modulus 11 million, aluminum density 0.11 lb/cu in, titanium modulus 16 million, titanium density 0.16 lb/cu in. I have no idea why that’s the case but it’s useful for those with limited memory!
Martin wrote:
Martin 22-Nov-15 15:16 #88
C210_Flyer wrote:
Got smart and now have security tape as well as a filtered funnel for fueling.
Filter I understand, but tape? To see whether someone opened the caps?
Yes because of the quantity of the FM (foreign matter; sand like particles) unfortunately it was not saved otherwise I would have looked under a scope.
There are only two possibilities 1. Pumped bad fuel. Ive been to these pumps a number of times with no problems. 2. Someone dumped a handful of dirt into the tanks.
Now Im hoping it was #1 and not #2. Even though the probability is low it cant be ruled out. So its a precautionary measure.
Funny things happen by coincidence with units. My favorite is this: steel elastic modulus 28 million, steel density 0.28 lb/cu in, aluminum modulus 11 million, aluminum density 0.11 lb/cu in, titanium modulus 16 million, titanium density 0.16 lb/cu in. I have no idea why that’s the case but it’s useful for those with limited memory!
If you google for
number patterns
you can fill your boots 
Someone dumped a handful of dirt into the tanks.
There is a pilot who occassionally pops in here who apparently got sand put in his PA46 fuel tank in Turkey. It amazes me this doesn’t happen much more often.
Peter wrote:
Someone dumped a handful of dirt into the tanks.
There is a pilot who occassionally pops in here who apparently got sand put in his PA46 fuel tank in Turkey. It amazes me this doesn’t happen much more often.
One reason I didnt put the American Flag on the tail. Its like waving a red flag in front of a Bull with some people.
Alexis – Since you summarily classified my >100 Annual Inspections experience on every Cirrus type out there, as " BS ", I genuinely thought you would have some DATA to share with us.
Surely you can do better than just your “facts” are based on 2 years ownership of a G2 SR-22 ?
C210_Flyer wrote:
One reason I didnt put the American Flag on the tail. Its like waving a red flag in front of a Bull with some people.
D-regs are legally required to display two German flags on the tail. Kind of strange because we normally hide our flags as much as we can whereas the Americans put 3 flag poles in every garden and decorate it with “support the troops” billboards 
Michael wrote:
Surely you can do better than just your “facts” are based on 2 years ownership of a G2 SR-22 ?
Owning and reguarly flying a particular aircraft is not quite the same thing as doing annual inspections?
stevelup wrote:
Owning and reguarly flying a particular aircraft is not quite the same thing as doing annual inspections?
Correct.
Inspecting dozens of different aircraft and their logs, gives a very complete overview of the fleet at large.
OTH, owner’s often believe that THEIR experience is representative of the entire fleet, and that of course is inaccurate.
If you REALLY want to learn about a particular type of aircraft, go out and find the shop that has the largest volume under maintenance contract, then take the lead mechanic out to lunch and ask lots’ of open ended questions …
achimha wrote:
achimha 23-Nov-15 16:09 #97
C210_Flyer wrote:
One reason I didnt put the American Flag on the tail. Its like waving a red flag in front of a Bull with some people.
D-regs are legally required to display two German flags on the tail. Kind of strange because we normally hide our flags as much as we can whereas the Americans put 3 flag poles in every garden and decorate it with “support the troops” billboards
Dont forget the yellow ribbons.
