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| IL-2 Sturmovik The famous combat flight simulator. |
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Thread Tools | Display Modes |
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#1
06-22-2013, 09:35 PM
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Acceleration comparisons
As some of you may be aware, I recently more or less hijacked a thread in the Daidalos Team discussions; it had to do with the apparent sluggishness of the Corsair versus contemporary Japanese fighters, and kind of spread from there to the subject of acceleration and how it is affected by the trim model and the cockpit displays/indicators. I more or less volunteered to perform the tests and report my results, using a consistent test procedure and set of standards, since the latter part of my career in field engineering has been heavily involved with test procedures and validating the results.
I started testing various mid-war fighters’ accelerations at 10,000 ft or 3050m, from a starting speed of 270 kph IAS. Since most of us fly in the cockpit in-game (and frankly, because the Wonder Woman artificial horizon is more of a modern art project than a readable indicator, at least on my monitor), I decided to make my test runs in-cockpit, using the speedbar as my primary speed and altitude reference since it is the same for every aircraft. I use the Crimea map in the QMB with the time set for noon; no other (AI) aircraft are included and the conditions are set for clear weather. I set the altitude for as close to my test alt as possible, and then get over the sea and head due West (270°) with the radiators open to keep the engines as uniformly cool as possible before starting my test run. Ideally, I will settle into trimmed level flight at my desired altitude (I’ve mostly done the 3050m, but I’m starting a series of 100m tests), and maintaining a speed of 270-280kph indicated on the speedbar. I do NOT start recording at any point; the Save Track function after I exit the mission will save it in the more accurate ‘.TRK’ format. I simply close the rads, and shove the prop pitch and throttle to the firewall and engage WEP if that is an option. I then try to maintain level flight while the aircraft accelerates up to its top speed or it has been in ‘Overheat’ for 60 seconds or so (depending on the aircraft’s overheat model—some will take a while and some will spray oil over your windshield in 30 seconds or less). I then turn off WEP, back off on the throttle and prop pitch, and open the radiator or cowl gills to 100%, turn east and take about seven or eight minutes to return to my starting area before doing it all over again. I try to get at least three runs in before exiting the mission and usually four are possible; I’ve had a few occasions where I damaged the engine and had to end after two or three runs and had to make a new or extra track to get a full set of data. As I mentioned, I then select the Save Track option after exiting (I forgot a couple of times, which was enormously frustrating—usually a test track of four runs will take over 45 minutes). That’s the actual testing process. The hard part is extracting the data. It is not just a matter of running the track with a stopwatch in hand; you need to select the points that you will measure and compare, and from those average out your results into something understandable. I ended up creating two sets of tables, one for the basic data and one for analysis. I turned them into a pdf file, which is attached below. I quickly learned that accelerating into level flight is hard; the smaller and lighter aircraft will almost literally twist with the sudden torque, a lot like a good electric hand drill does when you apply power. This can put you three or four degrees off course, and usually heading sharply up; you have to anticipate it and apply stick and rudder as you shove the throttle forward. You then spend the next 15 or 20 seconds or so frantically applying elevator and rudder trim while you fight your stick’s springs or motor, and pray that you haven’t overdone the trim, because it usually isn’t felt until it’s too late and the nose starts dropping like an anvil. On the heavier aircraft, there is usually a bit of a nose-high angle at lower speeds and when full throttle is applied they will try to go almost straight up and in the direction of the torque / p-factor. Again, you will need to anticipate this a bit with stick and rudder or you will be a couple of hundred meters higher (and slower) than when you started. I mention this because it quickly became apparent that there were two big limiting factors to acceleration: weight and drag. Drag exerts itself at the higher end of the speed range, since it increases exponentially as speed increases; if your rudder and/or elevator are not properly centered, you can lose a lot of your potential speed increase. Weight is a constant and is obviously in play as you stray from level flight; climb too much, and you slow down or drop your nose just a little and you get an artificial boost (and it can be quite obvious, if you track altitude and bearing changes between intervals). I needed to at least factor in things like climb and dive between 10 kph intervals if I wanted to get something like valid results. So my tables include cells for not only time, but course and altitude for each interval. I start by measuring the time from 270 kph to 350 kph, then from 350 to 370, and then every 10 kph thereafter. I also note True Air Speed (TAS), overheat times and top level TAS achieved in each run. How quickly an engine overheats and the top maintainable level speed are useful information, IMHO. Also, I had a couple of aircraft fall just short of the next 10kph before I had to throttle back, and it didn’t seem fair to deprive them of the credit for that last 8-9 kph of IAS. I obtain my data from watching the track in Wonder Woman view with one finger poised over the P (Pause) key. I normally pause at start and at every key point or interval because there simply isn’t enough time to jot down the data between intervals. Some of the really fast aircraft take less than 2 seconds to go from on 10kph point to the next for several intervals and even the supposedly sluggish types take less than 4 seconds in the early phases. So you practically have to stop the track at every data marker. I normally speed up the track to 8X during the return to start phases to save time, but even so, a complete table normally takes a good 45 minutes to an hour. You get the raw time from the track’s lower right hand corner, the IAS from the speedbar and the precise altitude and course from the WW instrument displays. I then take the data from the Base Chart to fill out the Analysis Chart; this consists of breaking down the raw track time into the number of seconds between intervals and the meters up or down from the previous interval; these can vary quite a bit between runs, in part because you improve your trimming from one run to the next or because your attention wandered for a second just as the aircraft decided to suddenly drop or raise its nose (this is where the cockpit displays come in; some of the climb & dive, as well as the turn and bank indicators are decidedly off compared to others, and you can find yourself gaining or losing thirty or more meters in altitude in less than three seconds if you are not alert and steady on the stick). After filling in the data for each run, I then try to establish a mean or weighted average based on not only the times but also how much the alt varied over that interval. Most aircraft seem to blast through altitude variations early on, but at the far end of the speed range, a 10m variation can cost you a second or more (and remember that drag is increased in a climb or dive that you’re fighting against). From the Analysis Charts I then went to the Excel spreadsheet and created the charts attached below and in the following posts. I will be happy to answer questions or provide the charts showing direct comparisons of given types if I have tested them. cheers, horseback 
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#4
06-22-2013, 11:03 PM
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Interesting. Slap some water-injection engine in that F6F, and its acceleration performance gets close to Fw190s/P-38s.
Maybe the F6F is not that total dog it seemed before, but the 'sair just shines too bright in that department. Seems though that with the F6F-5 you can easily leave Zeros+Ki-61 wheezing and puffing behind you.
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#5
06-22-2013, 11:27 PM
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Quote:
__________________
Find my missions and much more at Mission4Today.com
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#6
06-23-2013, 12:20 AM
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This became very obvious when I started doing my tests at 100m, where I had hoped to use the horizon line for a reference instead of staring at the climb or altitude indicators and slowly going cross-eyed. The P-47 and Hellcat are much worse than the Corsair, while the IJN and IJA fighters don't seem to have this problem at all. In comparison to the US inventory, the Japanese fighters, while a bit slow, can be whipped around with a great deal of precision and confidence without burning up a lot of energy. If you get your American birds out of trim, you can't be sure where you'll end up, but you can be sure that you will have bled off a lot of your E in the process. cheers horseback
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#7
06-23-2013, 01:24 AM
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Quote:
__________________
Find my missions and much more at Mission4Today.com
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#8
06-23-2013, 04:20 AM
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Horseback, since you were on the WW view, you weren't checking the slip ball by chance? The higher the engine power, the more important it is.
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#9
06-23-2013, 05:44 AM
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Regarding trim, both the F4U and the F6F as tested by NACA required loads of elevator trim. Stick force changes were in the range of 30lb between 200 and 400 mph in level flight. Trimmed for neutral stick forces at low speed, pilots would sometimes not be able to keep the aircraft in a dive.
Trim as suchs seems to have been a bit more effective than it is in game.
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#10
06-23-2013, 09:00 AM
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Is giving it full throttle and RPM going to give the best acceleration especially to heavy planes with huge power?
Most prop speed controls will try and maintain prop RPM which at lower speeds may be course enough to stall the prop just keeping it from over-rev. Manual 100% RPM will waste full power but at least be a bit better than prop AOA too high. Either way is "spinning wheels" without smoke and noise to show it. Getting the most out of a plane isn't 'simply' anything. If you want accurate data, get a devicelink data logger like UDPSpeed and ditch the stopwatch.
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Linear Mode
