C172 Performance Numbers

[aardvark2zz]

Here are some results of C172 performance calculations I did a while ago since I found that the POH had some key numbers lacking.

What is interesting are the very differing speed numbers that also differ from the POH. Especially the glide velocities and best climb velocity.

I guess Cessna had some good reasons for presenting their numbers; for which I do not know except to maybe simplify things, avoid stall, and to avoid a too nose up lack of view condition.

Presenting the numbers as an image simplified the formatting a lot.

If you have better numbers or spot errors do pls post them !!

http://www.airdisaster.com/user-uploads/Cessna172planeAircraft.gif

[Genghis the Engineer]

How did you calculate these values? Without that information, it's very hard to usefully offer a comment about them.

I confess BTW that in about 13 years of doing aeroplane performance analysis as part of my job, I've never seen any particularly good reasons to trust purely calculated values, not firmly checked against carefully obtained flight test data. I'd be equally certain that FAA would only have signed off the C172 flight manual when confirmed by flight test data.

G

[flyboyike]

This is the second forum he posts this uselessness on.

[aardvark2zz]

This is the second forum he posts this uselessness on.

I could say the same thing about you, "uselessness" :yuk:

You seem to be a glass half empty kinda guy :( too bad, what a waste.

With the thousands of posts per day on this great forum, and considering that you're stalking me here and there, read this "uselessness", and especially took some of your precious time and effort to reply to this "uselessness" I am amazed that you would give me all that attention. :D

I am truely touched :E

What is it about your life (or lack-of) that you would waste your time on such negative useless behavior. Pls look at your life and reassess what it was, is, and what it will be. Choose good.

PLEASE PLEASE don't apply that snotty destructive attitude around the cockpit of a CRJ100 in Minneapolis !!!!

With such an Id as "flyboy..." I wouldn't be bragging much; especially after seeing your picture. :eek:

Take this as constructive criticism to move on in life in a proper and better direction and not dwell on the attitude of can't, shouldn't, wouldn't, etc...... :ok:

[aardvark2zz]

How did you calculate these values? Without that information, it's very hard to usefully offer a comment about them.
I confess BTW that in about 13 years of doing aeroplane performance analysis as part of my job, I've never seen any particularly good reasons to trust purely calculated values, not firmly checked against carefully obtained flight test data. I'd be equally certain that FAA would only have signed off the C172 flight manual when confirmed by flight test data.
G

This is not taken from only a few operating points in flight.

The POH does effectively contain "some" test data.

Of course any flight models have to be cross-checked with extensive flight data.

I mainly used the flight data in the POH cruise performance of velocity, and fuel flow vs altitude, power setting, and temperature. Wing area, density altitude, etc..... there is approximately 100 operating conditions.

I also used the slow-flight regime of stall speed, climb rate vs altitude, glide performance. Prop efficiency variability.

Using numerical methods from university I fit the lift, and drag equations to determine the coefficients: Cl, Cd, Cdo, K, etc..

I cross checked the resultant perf calculations with the known published cruise performance data and my experience flying; especially hot TO :}

Satisfied that the fit was adequate (e.g. stall speed, stall angle, Vmax cruise, horiz and vert velocities vs altitude during climbs, etc.... ) I then calculated the special conditions above.

As for the FAA, I'm sure they do have all the data in their files. The problem is that some basic perf data is not in the POH !!!!:ugh: :mad: :yuk: :{

[cessna l plate]

That's all fine, and indeed a good idea for ab-inition students to have something to give them a reference.

But, and here is a good but, what exactly are these figures based on?
Take a brand new engine in a clean airframe with minimal kit and they probably hold good. But what about a filtyhy airframe (we've all seen them) that can cost a couple of knots, and what if the engine is getting tired, what about if the owners have installed every conceivable toy in the instrument panel. The whole thing could weight 100/200 kilos more, or worse than that.

As with cars, the manufacturer might say it can do 150 mph, but that is the book speed, in perfect conditions, with a highly trained driver, on a test track. The reality is take 2 C class mercs from about 1998 (like mine). Mine is in the garage for regular service, isn't thrashed, any little thing that goes wrong is fixed, I am sure it will do the book speed, or damn close to it. Compare that to one that hasn't been near a service centre for 3 years, has infrequent oil changes and so-on. I will pay money to see someone try and get the book speed out of it.

What I am trying to say, is that although there might be a standard set of figures, each individual aircraft is different, only by a couple of knots here and there, but that can be critical in some circumstances, best to fly to a standard set of numbers that exceed what the book says by a few knots, for the wife and kids if you like.

[Genghis the Engineer]

aardvark2zz

Please be a lot more specific if you want any meaningful feedback.

What data, how determined, what tests have you flown, what are the equations you are using to standardise your data (and where obtained / derived), what are your concerns about the validity of the data in the POH? None of this is at-all clear.

G

[pistongone]

Data is similar to statistics, you can prove/disprove whatever you want with a little manipulation. Seeing as everything in flying is what you think the aircraft can do then add 30% for safety, it all makes calculations to 3 places after the decimal rather academic doesnt it? In reality you will get a given speed for a given power setting and usually it will be somewhere near what you expected. If it is way out then you do an extra FREDA t's and P's check etc to make sure you havnt left the flaps or wheels down or the prop in fine.
And to be honest is anyone going to re-calc for the actual temp and pressure of the given day, assuming you have calibrated instruments able to measure the parameters to the required accuracy? So as i said, take the book figure, add 30% and go flying:ok:

[Genghis the Engineer]

Data is similar to statistics, you can prove/disprove whatever you want with a little manipulation. Seeing as everything in flying is what you think the aircraft can do then add 30% for safety, it all makes calculations to 3 places after the decimal rather academic doesnt it? In reality you will get a given speed for a given power setting and usually it will be somewhere near what you expected. If it is way out then you do an extra FREDA t's and P's check etc to make sure you havnt left the flaps or wheels down or the prop in fine.
And to be honest is anyone going to re-calc for the actual temp and pressure of the given day, assuming you have calibrated instruments able to measure the parameters to the required accuracy? So as i said, take the book figure, add 30% and go flying:ok:

Ah, but I'm often the bloke who wrote the book with the figures in.

G

[pistongone]

Its a tough job Genghis and someones gotta do it:ugh: :ugh: Its like engineering, we have a datum line which we work from. But that is all it is, a datum, everything is either above or below it and in the real world we allways build in a margin for correction. That, as i understand it, is the pilots job!
Nice to make your aquiantance Genghis any how:ok:

[Genghis the Engineer]

True enough, although it may surprise you to know that often a huge amount of work also goes into deciding the value of those safety factors - to give the pilot the maximum flexibility without crossing the safety line.

However, none of this actually answers the real question right now - which is what on earth is aardvark2zz going on about.

G

(Also a pilot)

[flyboyike]

PLEASE PLEASE don't apply that snotty destructive attitude around the cockpit of a CRJ100 in Minneapolis !!!!



It's a CRJ200. If you're gonna yell at me, at least get your facts straight.

[aardvark2zz]

It's a CRJ200. If you're gonna yell at me, at least get your facts straight.

I got it from your Id profile :E

[flyboyike]

I got it from your Id profile :E

Good catch. Profile fixed.

[Sensible]

I do like a good laugh first thing in the morning :} :} :}