Flight Sim’s Obsession with POH Numbers

[mtr75 653]

13 minutes ago, Manny said:

POH numbers are critical. At TBO at around 2000 hours the Lycoming engine may be a tad inefficient but  we need to use the POH numbers, This is especially critical in IFR.  

From Wiki: "The Cessna 172S was introduced in 1998 and is powered by a Lycoming IO-360-L2A producing 180 horsepower (134 kW). The maximum engine rpm was increased from 2,400 rpm to 2,700 rpm resulting in a 20 hp (15 kW) increase over the "R" model. As a result, the maximum takeoff weight was increased to 2,550 lb (1,157 kg)."

Yes, the default 172 is so underpowered. I doubt it can match any of these "flight gaits" numbers

Performance = Altitude + Power

I'm sorry, but you are not getting rated horsepower from any 2000-hours engine in any context. Not a plane, not your car, not your lawnmower. Just ain't happenin'.

[Manny 1,293]

Just now, mtr75 said:

I'm sorry, but you are not getting rated horsepower from any 2000-hours engine in any context. Not a plane, not your car, not your lawnmower. Just ain't happenin'.

Thats why 2000hrs is TBO

[G550flyer 1,052]

Here my take on aircraft performance numbers in general. I am an aircraft performance guru because I love it. In my early days, other pilots hated calculating performance numbers and felt it was a job for the flight engineer. I on the other hand loved! It initially blew my mind that my old performance manual gave me numbers that my jet actually performed by. It was like magic. There is a reason for this. To grasp it, you first have to know how these numbers were created.

During design, engineers can predict what type of performance the aircraft will achieve. During flight tests, equipment is placed on the test aircraft and a series of maneuvers are performed. They will drag the tail during takeoff to find the unstick speed, reduce an engine to see if the behavior is expected etc. When the aircraft performs these maneuvers, climbs, descents, out of trim, manual controls, etc, this data is recorded. They then compare those numbers to predicted numbers. They check for variances. Maybe there is better performance than expected or worse and requires some changes. Once data is good and meets or exceed governing requirements, they move to find an average among the tests. Once they find the average number for a particular item, they may pad it or include a safety margin. For example, obstacle clearance criteria, they will add a pad to ensure you have a clearance margin. In this case, the actual aircraft should perform better than expected as long as it's applied and the aircraft is flown correctly. This pad also takes in account of normal degradation. Take brakes for example. They are worn down to a certain wear percentage and then the reject tests are performed. This ensures that as long as you stick with the manufacturers recommendation, those brakes will perform. This data is then used to create manuals, performance software and FMC software. I remember working with a company to convert our paper performance manual to an IPAD app. It was fun because I had to calculate the data manually and use the app while out flying for tests. Now that we have info on creation, lets look at practical situations.

In the flightsim arena, those numbers may not be important to you. But in the real world, it "should" be a big deal. I say should because I have flown with some peeps who don't take performance numbers as serious as I do. This is probably due to my military history. In that arena, I was in the max spectrum a lot. Routinely I found myself in scenarios where I needed every ounce of fuel and was limited by how much I can put on. Routinely, I would fly from DC direct Honolulu in the winter time. As you know, that jet stream dips low and kills you with head winds. I would takeoff at max gross weight and it would be questionable if I can get there with min fuel. I am talking about an island four hours out into the Pacific. Yes, we use ARINC and jetplan for planning, but things change even while you are flying. I am watching my flightplan, looking at winds/weather, looking at the box(FMS) and in the QRH running numbers against the box. When I choose to overfly Cali when I had scheduled a fuel stop, I need to be right! In all my years I have never seen the performance data wrong. I have always done just a little bit better than the performance data. I have seen the flightplan wrong due to winds or been totally off course due to build ups. I've also been in the middle of the Atlantic a little bit nervous back in my early days. Again, in the flightsim, may not be that important to you, but some like flying by those numbers for realism. When I predict that I will land with 4000 pounds while over the central USA and touch down Honolulu with 5000 pounds, imagine my feeling. I will now give you a good example of how good those charts are.

I was in a C-141B hauling cargo from Norfolk NAS on the way to Ramstein AB Germany. Back then, the runway was 7000ft, a bit short for my likes in a heavy jet. Our weight was limited by runway available meaning we were max performing. The engineer passed up the numbers, pax and cargo loaded and we started. Rolling down the runway was standard and the end of the runway came close, which was normal for Norfolk. The AC(guy in charge) rotated and we climbed away leaving faint black smoke behind. The engineer will calculate climb data, initial level off settings, 3 engine altitude for engine loss and next time to climb. They give us a EPR setting for cruise and a cruise speed. You set that cruise EPR and the jet would magically accel to cruise speed after level off. After some discussions on getting our oceanic clearance and getting the plotting chart ready, noticed, the jet is still at climb speed. We asked the engineer to re-check his numbers. He gives us the exact same numbers. Why aren't we accelerating? He even checked it again, same number. The other engineer ran them and again, same numbers. I then advised the engineers to run the same EPR and use our current speed backwards to calculate our performance weight. They both came back with a weight 10,000 pounds heavier than what we should have been. I called ATC and let them know we wanted to return to origin. Upon landing, we had the Navy load team take off the pallets, open the cargo bins and weigh them. Apparently we had some bins with huge safes in them and loaded with all kinds of word not allowed. The Bin weights were all off to a total of 10,000 pound. That's the Navy thinking you can load a plane like a ship lol. Just imagine if we had lost an engine at takeoff. Our critical field length was equal to runway available. That means if we had chose to stop at V1, chances were that we would have departed the runway. Be nice if flightsim could simulate those conditions where data becomes a factor.

Rick       

[Manny 1,293]

For real pilots these performance numbers are life and death.

[mtr75 653]

Just now, Manny said:

Thats why 2000hrs is TBO

False. Find that for me in the FAR's. It ain't in there. TBO is not regulatory. It is actuarial. That's what the insurance company actuaries established as the most likely time your engine will word not allowed out. You can run an engine for 10,0000 hours if the compressions are okay and you're comfortable doing so.

I've been involved in two engine overhauls in GA aircraft, and the overhauled engine in one case got a 30% improvement in fuel usage per hour for the same speeds. And the engine it replaced had gotten a top overhaul at 1,400 hours and we ran it until past 2,100 hours.

So no, that's not why TBO is 2000 HRS, and an engine at 1,999 hours is not performing anywhere near how a new engine performs.

[mtr75 653]

Just now, Manny said:

For real pilots these performance numbers are life and death.

For real pilots, adding percentages to these performance numbers are life. Following the performance numbers can and has resulted in death. If you think you're going to nail the performance numbers on a takeoff over an obstacle, you will be a lonely pilot. Because nobody will fly with you.

There are old pilots and there are bold pilots. There are no old, bold pilots.

[Manny 1,293]

I agree, it atrophies over a period of time, the general guide for the Lycoming engines on the 172 is 2000 hours, which flight schools (at least the good ones) use. My flight school was willing to sell me a 5 year old Cessna SP (the last few before G1000) here in Texas for $120K and I was told that I may have to knock down the engine and rebuild? or put a new Engine and it could cost me another $20K They were honest about it to me.

[Los 416]

10 minutes ago, G550flyer said:

Be nice if flightsim could simulate those conditions where data becomes a factor.

Rick 

First off, thank you for your service on this Veterans Day here in the US!

I don’t disagree with anything you said. I am not qualified to do so anyway. I know my personal minimums and they are low! 😃

However, in transport category aircraft I can imagine the tolerances being much smaller, 2-3%, as noted by the poster before, but in GA - where after all it’s a select group from the general public, but non-the less the general public, and there’s no dedicated group of engineers and technicians to keep the airplanes within those tolerances, such as in the military or airline, that there would be much more variance as noted by other GA pilots on here?

Again, I’m not questioning any one’s knowledge as I probably have the least here.

This question is meant in the Socratic sense to gain more insight!

Edited November 11, 2020 by Cmcollazo71

[Manny 1,293]

3 minutes ago, mtr75 said:

For real pilots, adding percentages to these performance numbers are life. Following the performance numbers can and has resulted in death. If you think you're going to nail the performance numbers on a takeoff over an obstacle, you will be a lonely pilot. Because nobody will fly with you.

There are old pilots and there are bold pilots. There are no old, bold pilots.

so you ignore the numbers and fly your hunches eh! and fly IFR from the seat of your pants too 🙂

Edited November 11, 2020 by Manny

[mtr75 653]

Just now, Manny said:

so you ignore the numbers and fly your hunches eh! 🙂

Nope, I take the numbers and add 50%. 🙂 At least for the takeoff and landing distances. For airspeeds (approach speeds, VX/VY, etc.) I fly 'em how they are written. I guess I should be specific, I am talking (in particular) about takeoff distances.

The other thing that I think is important is range. It doesn't surprise me at all that people can't get the book fuel flows and ranges from the POH. Happens in real life as well, for example, wind is what you expected it to be, etc. Gotta stop and get more go juice!

Don't mean to be argumentative with you, Manny. Sorry if I was.

[Manny 1,293]

OK those your personal minimums and they are good to have. I too have mine. But the starting point has to be those numbers. 

Anyway, it all started with should the sim aircraft fly by those numbers? I think it should, it should be close if not on the dot. This is a sim after all. The default 172 is so out of whack.

[Chock 17,769]

Anyone who has flown several models of the same aeroplane will tell you that they all handle slightly differently from one another. For example, at a flying school and you end up with one aeroplane as opposed to another, of the same model, you either smile or groan a bit when you see which one you will be using that day. The book says they are identical, but they rarely are.

[robert young 2,897]

I loathe and hate POHs. They are exaggerated in both directions. For example Diamond tends to be very conservative about performance values. They probably do not want to be challenged in case one of their aircraft doesn't climb as well as expected. Others exaggerate climb and cruise speeds in ideal conditions, which never really exist.

I don't particularly care if one or other sim aircraft doesn't exactly match POH values. MUCH more important is whether it flies in a convincing and believable way. But OCD is not just an isolated disease. It seems to attract people who like simulators. And their collective influence forces addon producers to churn out hundreds of pages of frankly tedious stuff, just so you know they are being diligent. But they are not being diligent at all. They are just frightened in case some tiresome nerd contacts them and tells them that the rivet close to the wing root is too big or in the wrong position.

I wish sim pilots would understand that in aviation PR is king. Everything is presented as though it is a flawless production line. Anyone who has flown a variety of, say, C172 aircraft knows that some fly beautifully and are a pleasure to be in, and others fly like bricks.

 

Edited November 11, 2020 by robert young

[Los 416]

7 minutes ago, robert young said:

I don't particularly care if one or other sim aircraft doesn't exactly match POH values. MUCH more important is whether it flies in a convincing and believable way.

Straight from the horse’s mouth in my opinion! I mean that as a complement and with respect!

9 minutes ago, Chock said:

Anyone who has flown several models of the same aeroplane will tell you that they all handle slightly differently from one another.

Woohoo! You know you have a good post when Chock is in the mix...

That’s it! I got Robert and Chock commenting... if I get Bert... I can never post on Avsim again and die happy! 🙏🏽😜😃😆🤣

Edited November 11, 2020 by Cmcollazo71

[Sylvenzo 48]

2 hours ago, SUNDR1V3R said:

I guess 90% of the new simmers don’t even know what a POH means.. They just want to fly the 787 thru Eiffel Tower or stuff like that..

- I am a new simmer

- I don’t fly 787 thru the Eiffel Tower(The wingspan is too wide)

- Why don’t you take your superior knowledge and educate my on that!!

-Everybody in this forum as been a new simmer at some point including you!!