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Rucki

A noobs question about plane official data and sims

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I wonder why there´s always a difference in official plane performance data and what I can experience with flight simulators. I have only a little bit flight sim experience since xplane11 for VR and since MSFS is out im using that software, so no real life pilot experience at all, but I had often a look at the official performance data I could find online about the planes I´m flying and I wonder why the max speed in official data is always a little bit less than what I can experience in flight sim software.

Lets take f.e. the Diamond DA-62 in MSFS2020. Official Data says that maximum TAS is 192 knots @14.000 feet MCP (as I understand Maximum Continous Power = throttle lever = 100% @14.000 feet ?) https://en.wikipedia.org/wiki/Diamond_DA62  or https://www.diamondaircraft.com/en/private-pilots/aircraft/da62/tech-specs/ and with 20 knots wind from behind I could fly about 210 knots TAS @ 14.000 feet, so I thought that maybe the wind from behind was giving me too much speed, so I put the wind to 0 knots and I still could fly with 199 - 201 knots TAS.

Screenshot: MADzl7w.png

Is the default plane just too strong or what is the problem here ? Although Im pretty sure that I experienced pretty much the same with every plane xplane 11 and also in MSFS2020. It is like if find max real life TAS speed data online, then in the sim I can fly 5 to 10 knots faster at the given altitude. I understand the difference between cruise speed (most often something about 75% throttle for GA prop planes right ? With the goal to be as fast as possible but not to abuse the engine, like if you would driving a car with 7000 rpm all the time f.e. right ? ) and max tas speed and in many cases I even couldn´t find max speed @ 100% throttle @ best altitude. But in this case of the DA-62 I could find it and was still quite faster in the sim.

One additional question: why is the absolute max speed so rarely given in performance data ? Is flying @ 100% throttle with planes something like driving a car with max rpm ? I mean I would never ask Volkswagen how the performance is for the new Golf @ 6000 rpm, that would be a funny question for a street car, is it the the same for flying a plane @ 100% throttle ? I´m mostly asking about GA propeller planes btw.

 

 

 

Edited by Rucki
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You are correct, speed at cruise is typically given at a specified manifold pressure / RPM setting.

Like your automobile analogy, you do not usually fly with full throttle, both to protect the engine, and to conserve fuel.

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Bert

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1 hour ago, Rucki said:

One additional question: why is the absolute max speed so rarely given in performance data ? Is flying @ 100% throttle with planes something like driving a car with max rpm ? I mean I would never ask Volkswagen how the performance is for the new Golf @ 6000 rpm, that would be a funny question for a street car, is it the the same for flying a plane @ 100% throttle ? I´m mostly asking about GA propeller planes btw.

In piston engines, and really all engines, performance varies dramatically by altitude. This can be compensated for with turbochargers and turbo normalization in piston engines. Temperature and pressure also influence performance, as does weight and even CG. So 'max speed' is a bit elusive. In my RW plane I can get 2~3 extra knots by having a very aft CG (more weight to the back of the plane).

Normally aspirated engines lose power with altitude because the amount of oxygen they can ingest to combine with avgas in the cylinders decreases, so we have to decrease the amount of fuel a commensurate amount and, thus, the tiny explosions that occur in the cylinders that drive the engine get smaller and smaller with altitude. So, there's a sweet spot with every plane - the upside of thinning air is that there's less drag, but also less power from the engine. Somewhere, on a given day, there's an altitude that is the best middle ground between drag and thrust/power that gives a good cruise speed and economical fuel burn. In a turbo normalized or turbo charged engine, the reduction in oxygen at higher altitudes is offset by compressing, and thus delivering more, air to the engine. So in these turbo planes, we can get more power from the engine at a high altitude, and that's why they can produce much higher true airspeeds at high altitude, even though they actually perform a little worse at sea level (because they're heavier).

There's also a difference in how you operate a piston engine plane depending on whether you have a fixed pitch or constant speed propeller. You generally want to avoid running at max RPM - that's basically reserved for takeoff and initial climb, though in many fixed pitch prop planes you also climb at that setting - it varies by plane and engine. With a fixed pitch prop, like in the C152 for example, there is no prop control (blue) - only throttle and mixture. So, you take off with the throttle full forward, but you probably wouldn't cruise like that, you'd pull back a but to an appropriate cruise RPM setting.

With a plane that has a constant speed prop, you have the (usually blue) prop control added to the throttle and mixture controls. The prop control controls RPM by adjusting the angle of the blades on the prop. In this case, the throttle is controlling the actual "power" output of the engine (measured as manifold pressure), and a governor in the propeller hub keeps the RPM constant (hence the name constant speed) as you adjust throttle by changing the angle of the prop blades. As you change the position of the prop control, that target RPM changes. Because of this, in a normally aspirated (no turbo) piston power plane with a constant speed propeller, especially at higher altitudes, you would cruise with full throttle, but you'd adjust your prop RPM to an appropriate setting to keep the actual RPM lower. At different altitudes, different settings work better or worse, and there's also the fact that the slower the prop spins, the quieter it is in the plane.

This gets even more complex when we add mixture (the red handle) "into the mix" - generally speaking, there's the concept of running 'rich of peak' or 'lean of peak' and you can find many a debate among piston pilots about what to do, but if you do run lean of peak, which is much more fuel efficient, you have a third lever to control power output. Of course, this isn't simulated in MSFS, so is moot in this case.

As to your question about speed in the sim, yes, it is pretty hard to model a plane to perform exactly on target at all altitudes, speeds, temperatures, pressures, etc. They get it as good as they can. These default planes could definitely be better, but we can probably never expect them to be exactly representative of the real world planes in all conditions. That said, if you ask and 5 pilots of the same model plane, they'll each tell you a different top speed for their plane...there are so many variables that influence how fast a plane can go on any given day...

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Brochure figures for aeroplanes are quite often misleading; sometimes that is because the manufacturer is deliberately trying to be misleading to make things sound cool, but very often it isn't for any reasons which are intended to mislead anyone at all. To explain a bit more about that...

Much depends on what actually was tested when the aeroplane was being developed and the brochure being written; quite often what actually can be tested has limitations. As an example, the maximum demonstrated crosswind limit for most Boeing 737 models, is somewhere around 30 knots if you look in the manuals for the thing. But this is not because the 737 is incapable of handling a stronger crosswind than that, it is because when the 737 was being certified, the worst crosswinds they could find during the tests which the Federal Aviation Administration were observing, were around 30 knots, and so Boeing could only document what they had demonstrated.

Similarly, you'll find that pretty much every jet fighter developed since the late 1950s has been capable of flying at Mach 2 if you look at the specification sheet for one, but what that doesn't tell you is that most of them would be so low on fuel by the time they actually managed to accelerate to that speed, that they'd pretty much have to head straight home, or meet up with an aerial tanker. Likewise, many could not make it up to those kind of speeds with lots of the weapons they are able to carry, and even if they can, they can't maneuver very well whilst carrying them. So putting in the brochure that your new shiny MiG can do Mach 2.5 might not be an actual lie, but it might also not be a really practical proposition either.

Much of this kind of thing is true with GA aeroplanes too. GA aeroplanes don't fly at mach 2 or carry bombs and missiles of course, but they do fly in very varied temperatures around the world, with slightly different fuel grades on occasion and with different payloads as well. Since GA aeroplanes have to be trimmable and controllable with quite a varied load, from solo flight up to carrying several passengers and their luggage, what you can find is that they might be nearer the brochure speed figure at a very specific payload distribution configuration, since a different payload may shift the centre of gravity to a point where the aeroplane might fly a bit better than it does when only carrying a solo pilot with enough fuel for a local flight or whatever. Certainly when you fly a GA aeroplane solo, you will notice it handles better than when there is another person sat alongside you adding 200 lbs of weight to the thing.

And it doesn't stop there either. Almost every pilot who has flown a few of the same type of aeroplane, will tell you that one they flew was better on the controls than another one, or easier to trim than another, or whatever, even though the things looked identical and were fitted with the exact same equipment. Often there is no easy way to explain this, other than perhaps to suppose that minor differences in panel alignment when the thing was constructed or some such, is probably affecting the aerodynamics a little bit.

 

Edited by Chock
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28 minutes ago, cwburnett said:

In piston engines, and really all engines, performance varies dramatically by altitude. This can be compensated for with turbochargers and turbo normalization in piston engines. Temperature and pressure also influence performance, as does weight and even CG. So 'max speed' is a bit elusive. In my RW plane I can get 2~3 extra knots by having a very aft CG (more weight to the back of the plane).

 

Thank you a lot for your time to explain it for me 🙂
So I understand it now that the official data is more or less made with an average temperature and altitude and if we change that (like we can easily in flight sims) there will be always some slight difference like in real life.

So that would mean If I would buy an Diamond DA-62 in real life, i could expect about 5 to 10 knots more (as stated in official performance charts) in european winter time, if I would fly the plane at 100% throttle @14.000 feet, right ? (that was pretty much what I did on the Screenshot). And of course a little bit less in summer time ?

Is the difference between 100% throttle and f.e. 75%  throttle in an plane in real life really that high like f.e. between an car engine @6000rpm or @2500 rpm ? So if I would own an plane I would pretty much never want to fly that plane @100% throttle, besides the first few feet in climb after start, because I could decrease the lifetime of that engine a lot down ? How much would that be ? I mean I wouldnt expect an car engine, driving by an mad man 18 year old need for speed driver, to last more than (if it would be new) 80k to 100k kilometres. How much would be the difference in an Cessna 172S between someone who is always @100 throttle and and some who is always at cruise speed throttle 75% ?

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19 minutes ago, Chock said:

Much depends on what actually was tested when the aeroplane was being developed and the brochure being written; quite often what actually can be tested has limitations. As an example, the maximum demonstrated crosswind limit for most Boeing 737 models, is somewhere around 30 knots if you look in the manuals for the thing. But this is not because the 737 is incapable of handling a stronger crosswind than that, it is because when the 737 was being certified, the worst crosswinds they could find during the tests which the Federal Aviation Administration were observing, were around 30 knots, and so Boeing could only document what they had demonstrated.

 

So there´s no minimum test condidition for aeroplanes ? 30 knots crosswinds sound incredible low for me ( i repeat, I have no real live experience, only sim) but I experienced good 60 to 70 knots winds behind,front and cross in xplane11 flights between europe and the usa (through southern greenland) @30.000 to 40.000 feet. with the 737 zibo

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56 minutes ago, Rucki said:

So that would mean If I would buy an Diamond DA-62 in real life, i could expect about 5 to 10 knots more (as stated in official performance charts) in european winter time, if I would fly the plane at 100% throttle @14.000 feet, right ? (that was pretty much what I did on the Screenshot). And of course a little bit less in summer time ?

I wouldn't say that, no. The performance charts vary by plane, and since mine is so very old, it may have fewer (or more?) charts than a the POH for a DA62. That said, the charts tend to account for all these variables. Here's the cruise chart for a Seneca III that I did my initial multi engine training in.

gSsv58N.png 

56 minutes ago, Rucki said:

Is the difference between 100% throttle and f.e. 75%  throttle in an plane in real life really that high like f.e. between an car engine @6000rpm or @2500 rpm ? So if I would own an plane I would pretty much never want to fly that plane @100% throttle, besides the first few feet in climb after start, because I could decrease the lifetime of that engine a lot down ? How much would that be ? I mean I wouldnt expect an car engine, driving by an mad man 18 year old need for speed driver, to last more than (if it would be new) 80k to 100k kilometres. How much would be the difference in an Cessna 172S between someone who is always @100 throttle and and some who is always at cruise speed throttle 75% ?

So, the thing about 100% throttle is that doesn't equate to 100% power except at sea level and standard temperature (15C). For example, if you look above at the chart, see the 100% curve? It isn't a straight line because the higher you get, and the thinner the air, the less power the engine produces. So in this plane, which IS turbocharged, 65% power is MAX power at about 17,000' +/- depending on temperature. In my plane, which isn't turbocharged, that curve happens much lower, so by the time I'm at 12,000', full throttle is only 65% power or so.  There's no (additional) risk to running an engine at 65% power.

So, if you had a 172S and flew it at sea level at full power, you'd wear it out faster than someone flying at only 75% power. But if you're at 10,000' and at full throttle, you're not running at 100% power, so you're not really wearing it out faster.

Airplane engines have what's called a TBO (time between overhaul). On my plane, it is 1,600 hours. This is the recommended time when the engine should be taken apart and overhauled. But, engines are finicky and can require 'top overhauls' in half that time if not kept in good condition. The thing about engines, though, is that if I had to pick between an engine that was always run at full throttle and redline RPM, but flown every week, or an engine that was never used about 75% power, but had been sitting for 6 months inactive, I'd take the redline engine any day. The best thing you can do for an engine is run it and change the oil regularly. 😉 But we're really getting off in the weeds here.

Bottom line is you want to keep RPM under control, but that doesn't mean 75%. For my plane, redline is 2,700 RPM, climb is at 2,600 RPM and typical cruise is at 2,500 RPM (or lower if I want a quieter ride). But at 12,000', with 100% throttle and 2,500 RPM, I'm only getting 65% power, so no risk to the engine.

This stuff can get pretty complicated and there's the whole concept of the 'red box' when thinking about what conditions can damage an engine. Here's an interesting article on it if you're so inclined.

https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-12_red-box-red-fin.pdf

But, lastly, remember, the DA62 runs on diesel (jet A) and is a bit of a different animal from what I'm describing here. The DA62 also has an electronic engine control (not sure if it is officially FADEC, but close enough). This means you actually only have the power levers and it controls prop pitch and mixture itself. For these planes, you typically don't cruise at 100% throttle, because that equates to max RPM also, so you probably run them in real life at like 90% throttle. Usually the last inch of throttle movement is only prop control and not manifold pressure. So, this plane is basically doing all the hard work for you when it comes to engine management, and I'm sure the POH specifies various cruise power settings that can be matched on the MFD.

Edited by cwburnett
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30 minutes ago, Rucki said:

So there´s no minimum test condidition for aeroplanes ? 30 knots crosswinds sound incredible low for me ( i repeat, I have no real live experience, only sim) but I experienced good 60 to 70 knots winds behind,front and cross in xplane11 flights between europe and the usa (through southern greenland) @30.000 to 40.000 feet. with the 737 zibo

Those are takeoff and landing values when they test for crosswind. You'll certainly run into higher winds aloft. In the case Chock described, they're stating a maximum crosswind value for takeoff or landing that was demonstrated during testing as being controllable by an average pilot.


Chris Burnett, Working Title
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There are a lot of factors. Typically brochure figures are for a 'standard day', which is (measured at sea level) a density of 1.229 kg/m³, a pressure of 1013.25 hPa and a temperature of 15 degrees C (good luck finding a day which has those exact figures at sea level).

Obviously when it comes to testing aeroplanes, you want to push them as much as safely possible so you can test what their capabilities are, but even if you can find the most evil crosswind on the planet at your test airfield on the day you are testing, engineers and test pilots will always err on the side of caution when putting recommendations for what a plane can do in the manual.

There is a world of difference between what an experienced test pilot who is mentally prepared for making a test flight can manage on a plane loaded with water ballast, and a line pilot with quite probably less piloting experience than that test pilot, in addition to having other things to think about, such as passenger comfort, safety and serviceability whilst making their third commercial flight of the day. That test pilot could probably get that 737 they've been test flying for months down in a 45 knot crosswind just fine, but that doesn't mean they'll be recommending that this is what gets put in the manual as something Joe Average airline pilot can easily be expected to pull off.

Back with GA aeroplanes and the original point of the topic, when it comes to running them to best advantage, we can do stuff in flight sims which you probably would not want to do in the real things. For example, run your real-life Cessna at full throttle for two hours with the fuel leaned off to the point where considerably less fuel and more air is going into those cylinders, reducing the effect of cooling which fuel has before it ignites, then throttle all the way back and descend quickly, and you can fully expect to be risking some shock cooling on your engine on the real thing and probably some seriously fouled plugs when you throttle back up as well. But in your sim you can do that all day every day without worrying about any of that stuff (unless it is an A2A add-on aeroplane which does factor in that stuff).

If however, you want to be a bit more realistic, have a look at the article linked below, because whilst throttling back and leaning off the fuel could give you more range or endurance on your sim aeroplane, there are other things to consider in terms of wear and tear even at reduced throttle settings, in particular, the amount of stuff which gets deposited on the internal bits of your engine such as the valves and cylinder heads. It's worth a read:

https://www.cessnaflyer.org/flighttraining/item/798-learning-lessons-the-science-of-operating-your-airplane-engine.html

Edited by Chock
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Alan Bradbury

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