For my faa cpl checkride I had to make the calculations for take off distance and verify. It was very acuratewhen applying the short field take off techniques on which those values are based.
However .. In real life I never calculate. My ISA takeoff distances is approximately 450 meters. I have set minimums of 700 for concrete and 900 meters for grass. Else I simply do not go there. Being turbocharged does give me the benefit that density altitude is less of an issue .. And allmost all runways I go to in europe are near sealevel. However if I would leave from a higher runway and or the temperatures are clearly a lot higher than isa than I would do the math.
I do a book calculation for each flight. But thats because I have access to about 3 different C172 varients with slightly different short field techniques. When I fly a new plane, new to me, I double the book figure for margin and quickly bring this down as I see what the aircraft will actually do.
What I have noticed is that older aircraft perform better than book on take-off run, but climb slower and have reduced speed in the cruise. The flight I did at the weekend had book values of 450m ground roll and flight time 1:02. I achieved about 300m ground roll and a flight time of 1:13.
Someone said to me that it is to do with a fatiguing propellor flattening over time. I will be interested to see if the performance changes when we get a new prop in a few years.
The flight I did at the weekend had book values of 450m ground roll and flight time 1:02. I achieved about 300m ground roll and a flight time of 1:13.
Of course it could just have been that 15kt headwind! ;)
Whether one makes calculations for every flight and the safety factor questions are separate in my mind. For each aircraft I fly, I do
Simple weight and balance limits that always keep me in the envelope, assuming me as pilot and full tanks departure and empty tanks arrival. For the typical four-seater: (1) heaviest front-seat passenger with one pax (2) heaviest rear-row total load [typically C of G limited] and (3) heaviest total load.
Performance calculation for density altitudes 2,500 and 5,000ft to give me a minimum runway length I am happy to operate out of.
A simple glance at these figures tells me whether I can fly or not. Anything outside these conditions, I do an individual calculation.
The envelope exploration for the W&B takes a bit of time to make sure that there are no interim conditions that could see you outside, but really cuts down the hassle factor.
fatiguing propellor flattening over time
I need that little PPRuNe face symbol with the two really big eyes for "oh my god!". If you are thinking "fatigue" and propeller in the same paragraph, much less that same sentence, you're not flying the plane until the prop is changed! Extremely few GA propellers have any fatigue concerns any more, the couple which did I'm sure are long gone by now (and they were never on 172's).
What is much more likely is that during the life of the plane, someone has replaced with a different pitch, or repitched the prop. Within a range, both of these are entirely legal to do, but rarely invoke what should be new performance tables. So a fixed pitch prop plane which had had the standard prop replaced with a climb prop, will get up and out better, but fly slower at the same power setting.
Some fixed pitch propeller aircraft, and I think that 172 is among them, state on their type certificate data sheet (TCDS, their bible) that they must achieve a minimum runup static RPM. This is a casual check of power and pitch. Logically, the engine redline would be the maximum. If you could get redline RPM static on the ground, you really could not fly the plane, as much of the engine power will never be extracted, (unless you oversped the engine). On the other hand, if you cannot make redline, either the engine is tired, or it's got the wrong pitch prop.
The performance tables for fixed pitch aircraft often overlook the effects on performance of changing the pitch within the permitted range. Pilots should allow for this when using the tables. You could look in he aircraft records to determine what pitch prop is installed, though even if you know, unlikely there is any data as to how that relates to the performance - and there sure should be!
I once had to ferry back a Piper Tomahawk, which I knew had the wrong prop on it (long story). It was my first time flying a Tomahawk, and I had to check myself out, so I was unsure what to expect. Static run up was 1900 RPM, and takeoff was eternal, but in cruise, it was a rocket. Once I got it home, and they replaced the prop with the correct one, it was much more as one would expect, and we kept it for a few years in a 550M grass runway, which had wires at one end, and never had a problem.
symbol with the two really big eyes for "oh my god!"
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I need that little PPRuNe face symbol with the two really big eyes for "oh my god!". If you are thinking "fatigue" and propeller in the same paragraph, much less that same sentence, you're not flying the plane until the prop is changed! Extremely few GA propellers have any fatigue concerns any more, the couple which did I'm sure are long gone by now (and they were never on 172's).
Next time I am at the flying club I will look back through the logs on that one. The problem that I think you always face is that there are so many "experts" and fewer experts in the aviation field and slowly, over much time, you come across the evidence which can reveal so much.
The explanation that was given to me is that over time a fixed pitch cruise prop will flatten, i.e. slowly become a climb prop; offering lightening runway performance and doggish cruise performance. This I must say, being a mechanical engineer, made a bit of sense to me due to the forces and the angles of action. I am in a club with access to multiple aircraft for the first time of my career, and the older the aircraft the more it seems to back this up. However I did not think about the prop being changed for another model with differing pitch angles. As mentioned I will look at this, next time I go flying.
The explanation that was given to me is that over time a fixed pitch cruise prop will flatten, i.e. slowly become a climb prop; offering lightening runway performance and doggish cruise performance.
I suppose the point is that such deformation (if indeed it does occur) would not necessarily indicate fatigue....which is related to cyclic stress....(I'm an engineer too!)
A 1951 Super Cub with a vintage set of drum brakes means that under most circumstances you can indeed take off from any landing strip you have landed in. With other G-aircraft I use the CAA AFM charts which have the public factors applied - plus the safety sense factors with respect to grass, gradient, etc
With the SC and some headwind I tend to use 500 yards, with the factored length for a no wind grass runway being 440 yards. Shorter strips I would go in lighter, with a headwind. The SC with average skill and minimal braking will get into 400 yards, but having a margin brings peace of mind. A light SC will take off and clear 50 feet in 250 yards. If your flying is mainly into grass of around 600 yards, I would suggest a farm strip aircraft and not try and shoe horn a complex, faster aircraft.
I suppose the point is that such deformation (if indeed it does occur) would not necessarily indicate fatigue....which is related to cyclic stress....(I'm an engineer too!)
Let's take a bigger step back.... Um, no, the aerodynamic characteristics of a certified fixed pitch prop will not change, unless acted upon by an outside force. Air, and rotation in the intended plane, will never be those forces.
Propellers are type certified, and required to maintain the characteristics of their certification as long as they wish to remain type certified. They will not change in normal use at all.
Now, for comparison sake, many rotor blades are subject to deterioration from fatigue, and other forces. They were born with life limitations, which when followed, will assure that they do not change their characteristics during their "safe life".
Those airworthiness limitations are "approved", and thereby mandatory, unlike manufacturer's "recommended" limitations, like an engine TBO.
So, an airworthy prop (means it does conform to its type design), on an airworthy engine, in an airworthy plane, will assure that the plane will have its certified performance, no matter the age of the plane.
If propeller/engine/airframe condition are a detectable factor in takeoff distance factors, one or more of them is not airworthy.
