If it has neither a turbocharger nor a supercharger, then the manifold pressure would actually be a vacuum, just like most gasoline powered passenger car engines. Turbochargers and superchargers both force air into the manifold, changing manifold pressure condition from a vacuum (negative pressure) to a positive pressure.
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The Staff of SOH
Engine power output and adjusting it.
- Thread starter anthony31
- Start date
Brett_Henderson
Charter Member
It's never really a vacuum (zero pressure), but it's always less than atmospheric (sans turbo/super charging), except when the engine isn't running. If your MP pressure gauge were more accurate, you could use it to set your altimeter, before starting the engine (adjusting for field elevation)... because it's really just a barometer for the inside of the intake manifold
Brett_Henderson
Charter Member
Side note on the Rotax.. My very first freeware model was a Europa-XS (FS2002). I gave up on trying to make it work.. and just put a 110 HP Lycoming in there.. And oddly enough, some years later, our club bought a Liberty-XL2 (essentially an factory built Europa), with an IO-240 in it.
Edit: so.. you could just us a standard engine (down-size and de-rate the engine out of the default C172), and then customize the tachometer to display the Rotax-like RPMs..
Edit: so.. you could just us a standard engine (down-size and de-rate the engine out of the default C172), and then customize the tachometer to display the Rotax-like RPMs..
I will try to build the Rotax engine as an exercise, but I need a couple more details:
What is the cruise and maximum manifold pressures?
What is the prop diameter and reduction gear ratio?
What is the pitch of the prop and is there a measured static thrust at a given power setting?
Is there a torque or HP to RPM graph that I can try to match?
- Ivan.
What is the cruise and maximum manifold pressures?
What is the prop diameter and reduction gear ratio?
What is the pitch of the prop and is there a measured static thrust at a given power setting?
Is there a torque or HP to RPM graph that I can try to match?
- Ivan.
Brett_Henderson
Charter Member
Manifold pressure is weather and altitude dependent.. I.E.. if you're at sea-level, and the atmospheric pressure is 29.92, your theoretical MP limit is 29.92 (with a theoretically perfect induction system). As you climb the available MP decreases. As for what YOU set MP at.. it's a non-issue with a fixed-pitch prop. The throttle is your only means for adjusting RPM, so you never get the most out of the available, atmospheric pressure. With a constant-speed prop you'd have the throttle wide-open at cruise altitudes, and would pull the RPMs back with the prop-control. You lose about an inch per 1000 feet, so at 8,000, a wide-open throttle would only get you ~22 inches of MP.
Just use published numbers for the prop diameter (double-checking so that a prop-strike doesn't happen with hard braking).
The gear-reduction is published too, but trust me, you'll be tinkering with that.
Same for prop-pitch..
Trying to match a realistic power curve (via a chart) is admirable, but it'll be problematic when you try to get realistic performance out of an engine that lives up to that graph (if you could even document that it does)(see the testing post itn the other thrust thread)
Hello Brett_Henderson,
I presume the "Other Thrust Thread" you are refering to is the one about Turbofans? I had not been paying any attention to that thread because I have no great interest in jets at the moment. There is plenty of things to read about old propeller driven planes for someone like me who is not an aeronautical engineer by trade. Jets may come later, but I believe they are just a temporary fad.
I am still messing around with aircraft from the WW2 era. I am a fan of big old piston engines. I have heard of the Rotax but know almost nothing about it. It's a bit too modern for me. I was hoping that someone here could provide some details because there is bound to be a difference betwen the version I find with a Google search and the one of interest here. Since this IS an exercise that isn't of great interest to me, I also don't want to spend a lot of time trying to research the topic. Thanks for the reminder about naturally aspirated engines. Last one I worked on for a plane was the rotary for a Fokker Eindecker that I will probably release eventually.
Although I agree with you that the engine modelling in MS FS or MS CFS is far from perfect (multispeed superchargers being the greatest example), I believe there is enough room to come fairly close to documented horsepower and thrust values for a given engine. My biggest difficulty is in finding the information on the original engine.
Here is an example of what can be accomplished with engine tuning for a Japanese A6M2 Zero:
(2550 RPM)
Altitude Power Speed Manifold Pressure
500 915 295 37.8
2500 931 304 37.8
5000 952 310 37.8
7500 973 317 37.8
10000 995 324 37.8
12500 1001 336 37.3
15000 906 330 33.7
17500 820 326 30.5
20000 735 321 27.5
22500 656 315 24.7
25000 583 310 22.2
27500 517 304 19.9
30000 452 295 17.7
32500 392 277 15.8
35000 339 247 14.1
Service Ceiling (100 fpm) is 36,100 feet.
Initial Climb Rate is around 3400 fpm
Best Climb Rate is 3580 fpm
Because it was flown with an autopilot during most of the testing, I expect that under manual control, its service ceiling would be a couple thousand feet lower.
The engine power is slightly low at 20,000 feet and presumably at higher altitudes but that was done as I described earlier to lower the service ceiling.
Performance and Engine power versus altitude are pretty much where I want them to be. They don't match any particular flight test report of an actual aircraft but aren't too far off any of the reports either. The engine power isn't too far off manufacturer's specs at lower altitudes but are noticeably off by 20,000 feet.
- Ivan.
I presume the "Other Thrust Thread" you are refering to is the one about Turbofans? I had not been paying any attention to that thread because I have no great interest in jets at the moment. There is plenty of things to read about old propeller driven planes for someone like me who is not an aeronautical engineer by trade. Jets may come later, but I believe they are just a temporary fad.
I am still messing around with aircraft from the WW2 era. I am a fan of big old piston engines. I have heard of the Rotax but know almost nothing about it. It's a bit too modern for me. I was hoping that someone here could provide some details because there is bound to be a difference betwen the version I find with a Google search and the one of interest here. Since this IS an exercise that isn't of great interest to me, I also don't want to spend a lot of time trying to research the topic. Thanks for the reminder about naturally aspirated engines. Last one I worked on for a plane was the rotary for a Fokker Eindecker that I will probably release eventually.
Although I agree with you that the engine modelling in MS FS or MS CFS is far from perfect (multispeed superchargers being the greatest example), I believe there is enough room to come fairly close to documented horsepower and thrust values for a given engine. My biggest difficulty is in finding the information on the original engine.
Here is an example of what can be accomplished with engine tuning for a Japanese A6M2 Zero:
(2550 RPM)
Altitude Power Speed Manifold Pressure
500 915 295 37.8
2500 931 304 37.8
5000 952 310 37.8
7500 973 317 37.8
10000 995 324 37.8
12500 1001 336 37.3
15000 906 330 33.7
17500 820 326 30.5
20000 735 321 27.5
22500 656 315 24.7
25000 583 310 22.2
27500 517 304 19.9
30000 452 295 17.7
32500 392 277 15.8
35000 339 247 14.1
Service Ceiling (100 fpm) is 36,100 feet.
Initial Climb Rate is around 3400 fpm
Best Climb Rate is 3580 fpm
Because it was flown with an autopilot during most of the testing, I expect that under manual control, its service ceiling would be a couple thousand feet lower.
The engine power is slightly low at 20,000 feet and presumably at higher altitudes but that was done as I described earlier to lower the service ceiling.
Performance and Engine power versus altitude are pretty much where I want them to be. They don't match any particular flight test report of an actual aircraft but aren't too far off any of the reports either. The engine power isn't too far off manufacturer's specs at lower altitudes but are noticeably off by 20,000 feet.
- Ivan.
Brett_Henderson
Charter Member
Neat stuff ..
The high altitude numbers do look a bit odd. The airpseeds look close, but not for the amount of MP in use.
Even with the handicap of no, multi-speed super-chargers.. you DO have some very handy (outside the engine) parameters to play with.. that could help "realize" that performance table. Namely; the critical-altitude, and max/min blade pitch... combined with, wing-efficiency/prop-thrust/induced-drag/parasitic-drag.
Like mentioned in the other thread.. first get it to where takeoff airspeed and lift are reached after using up a realistic length of runway (per the load).. Then work on cruise performance at altitude.. go back and forth until those extremes are close enough to satisfy you.. then work on climb-performance, obviously returning to the original tests as you progress to make sure they're still close.
A model might very well have a realistic engine performance curve.. but if it leaps off the runway too easily, or flies with jets at 35,000msl (or uses too much runway, and doesn't climb well), the in-game experience suffers.
The high altitude numbers do look a bit odd. The airpseeds look close, but not for the amount of MP in use.
Even with the handicap of no, multi-speed super-chargers.. you DO have some very handy (outside the engine) parameters to play with.. that could help "realize" that performance table. Namely; the critical-altitude, and max/min blade pitch... combined with, wing-efficiency/prop-thrust/induced-drag/parasitic-drag.
Like mentioned in the other thread.. first get it to where takeoff airspeed and lift are reached after using up a realistic length of runway (per the load).. Then work on cruise performance at altitude.. go back and forth until those extremes are close enough to satisfy you.. then work on climb-performance, obviously returning to the original tests as you progress to make sure they're still close.
A model might very well have a realistic engine performance curve.. but if it leaps off the runway too easily, or flies with jets at 35,000msl (or uses too much runway, and doesn't climb well), the in-game experience suffers.
Hi Brett_Henderson,
Why do the high altitude numbers look odd? This is a supercharged 1700 cubic inch 14 cylinder engine.
In CFS1, there aren't the CFG parameters to play with. So far, from what I can tell in reading the Microsoft SDK just about all the variables we have been tweaking line up with the variables the SDK describes.
The variables you suggest tweaking are well documented for the real aircraft and should not be tweaked in my opinion. Why should we change the propeller pitch range for example when the actual numbers are documented in USN and TAIC reports? I bellieve we should stay as close to real numbers as possible.
As I see it, when you start changing other numbers to compensate for inaccurate engine output, other less than obvious performance parameters get screwed up. Engine output is relatively easy to adjust and doesn't depend all that much on other airframe variables, so I believe it should be the FIRST thing to be tuned. I don't believe your aversion to virtual engine tuning makes sense.
A FW 190D I worked on for a friend a couple years ago had engine output that was way too low. To compensate for this, the induced and parasitic drag were much too low. This plane would glide just about forever with the engine off which isn't very realistic.
Another series of aircraft I tested a few years back had engine power that was about 50% higher than it should have been. The aircraft flew well enough with power on, but were way too easy to land because throttling back the engine was about equivalent to deploying air brakes (which did not exist).
- Ivan.
Why do the high altitude numbers look odd? This is a supercharged 1700 cubic inch 14 cylinder engine.
In CFS1, there aren't the CFG parameters to play with. So far, from what I can tell in reading the Microsoft SDK just about all the variables we have been tweaking line up with the variables the SDK describes.
The variables you suggest tweaking are well documented for the real aircraft and should not be tweaked in my opinion. Why should we change the propeller pitch range for example when the actual numbers are documented in USN and TAIC reports? I bellieve we should stay as close to real numbers as possible.
As I see it, when you start changing other numbers to compensate for inaccurate engine output, other less than obvious performance parameters get screwed up. Engine output is relatively easy to adjust and doesn't depend all that much on other airframe variables, so I believe it should be the FIRST thing to be tuned. I don't believe your aversion to virtual engine tuning makes sense.
A FW 190D I worked on for a friend a couple years ago had engine output that was way too low. To compensate for this, the induced and parasitic drag were much too low. This plane would glide just about forever with the engine off which isn't very realistic.
Another series of aircraft I tested a few years back had engine power that was about 50% higher than it should have been. The aircraft flew well enough with power on, but were way too easy to land because throttling back the engine was about equivalent to deploying air brakes (which did not exist).
- Ivan.
Brett_Henderson
Charter Member
Odd in that you can get 247kias (over 400tas @ 35000msl) with only 14" of manifold pressure.
I don't believe my method makes sense either.. lol But the simplicity of the MSFS model makes all of this an experiment, at best.
One way or another, if you're concerned about accurate in-flight , you're gonna have to start tweaking for it. Knowing that your engine itself is accurate, but the model won't perform realistically, defeats the whole purpose. Standing on a stack of data that proves the engine power curve is good means nothing if the airplane can't get airborne, AT the proper airspeed, using up the proper amount of runway.. or worse.. leaps off the ground well before it should at something other than V3.. or climbs ridiculously fast (or slow), or is capable of jet-like airspeed at cruise (or is too slow)and if your engine gauges aren't returning accurate data for what your asking the airplane to do, it gets even messier.
I've found, that if you don't play around "inside" of the engine, the other stuff is more achievable... especially gauge readings (ala MP).
Sure, you want to start with something close, "inside" the engine, but if you venture into air-file tables in search of an accurate engine, you're making the actual performance envelope harder to achieve.
I wish you didn't have to play with stuff like wing-effiency, prop-pitch, drags, etc.. when trying to mate takeoff performance to cruise performance (and everything in between), but you really have no choice. It would be wonderful if you could compartmentalize it all.. make a good engine.. make a geometrically/aerodynamically accurate air-frame and just "bolt" them together, and get predictable results.. but that aint gonna happen. I advise you to keep the engine "stock" air-file wise, until you get the performance right.. then (if you really wanna re-open the tweaking cycle), try to "realize" the engine in its own right. Just know that you'll have quite a task getting the takeoff/climb/cruise numbers back to where you want them, not to mention major skewing in gauge readings...
I'm just sharing hard-earned experrience, and truly do respect those willing to strive for realism.. ...and I'm realizing that these topics can get argumentative.. so I'll step aside unless asked for input.
Some interesting things occur in altitude performance of supercharged engines. Sometimes the data does not lead one to the proper conclusions. The way FS is set up, the critical altitudes for ADI (Water Meth Systems) and Mil power are the same. In practical applications, the lower MP of the Mil power can occur at a higher altitude than the superchargers ability to provide the maximum manifold pressure acceptable for ADI use. The overall true airspeeds may not actually be that much different, but occur at different altitudes.
In FS maximum available MP and the power delivered will not change with altitude up to the critical altitude. In real engines, especially ones with several supercharger stages and gearing, the maximum power available at the high blower will be less than low blower due to the very high power consumption of the blower, which is subtracted from what the engine can deliver to the prop.
For high altitude operation of fast aircraft, mach drag must be taken into consideration. For WII aircraft this may show up as the plane being faster than book values at max speed at altitude.
Lots of stuff going on all at once!
Cheers: T
In FS maximum available MP and the power delivered will not change with altitude up to the critical altitude. In real engines, especially ones with several supercharger stages and gearing, the maximum power available at the high blower will be less than low blower due to the very high power consumption of the blower, which is subtracted from what the engine can deliver to the prop.
For high altitude operation of fast aircraft, mach drag must be taken into consideration. For WII aircraft this may show up as the plane being faster than book values at max speed at altitude.
Lots of stuff going on all at once!
Cheers: T
Hi Brett_Henderson,
I guess the reason that the numbers don't look right is because what you are reading as Knots IAS is really MPH TAS.
Pity to sign off. This discussion was getting pretty interesting considering the radically different approaches we seem to have to basically the same problem.
- Ivan.
I guess the reason that the numbers don't look right is because what you are reading as Knots IAS is really MPH TAS.
Pity to sign off. This discussion was getting pretty interesting considering the radically different approaches we seem to have to basically the same problem.
- Ivan.
Brett_Henderson
Charter Member
That's kinda like asking for input.. :salute:
OK.. if it's 247 TRUE mph, then it's 215KTAS, wich means the indicated airspeed wouldn't be much more than 110 knots.. I'm no Zero expert, but that can't be right... though it probably is right for 14" of MP.
I wonder what Vs is at 35,000msl.. ? I'm thinking it should have long since stalled..
(I'll do some Zero research)..
Edit: That does indeed look pretty good, especially within the MSFS model. I'm not sure qbout the 14" of MP, but at this altitude extreme.. it's realistic enough for simming. Now, if your takeoff numbers are good, and the runway required for takeoff/landing is close... and it doesn't climb like a jet with after-burners (lol).. and your MP/RPM readings satisfy the discriminating sim-pilot..you've nailed the primary flight model.
OK.. if it's 247 TRUE mph, then it's 215KTAS, wich means the indicated airspeed wouldn't be much more than 110 knots.. I'm no Zero expert, but that can't be right... though it probably is right for 14" of MP.
I wonder what Vs is at 35,000msl.. ? I'm thinking it should have long since stalled..
(I'll do some Zero research)..
Edit: That does indeed look pretty good, especially within the MSFS model. I'm not sure qbout the 14" of MP, but at this altitude extreme.. it's realistic enough for simming. Now, if your takeoff numbers are good, and the runway required for takeoff/landing is close... and it doesn't climb like a jet with after-burners (lol).. and your MP/RPM readings satisfy the discriminating sim-pilot..you've nailed the primary flight model.
At their service ceiling most plane are pretty wimpy handlers and need a very light touch. About the only use for really high altitude is the high fast corridor which might keep the competition form ever quite getting up to you, as with a recon bird, or as a perch with which to dive on prey.
The Power off IAS speed at which the Zero would stall should remain about independent of altitude though the reduced thrust would also increase the power on stall speed. 110 knots (or maybe a bit slower) IAS might be somewhere around the min thrust speed for this plane, which is the speed you would creep toward at high altitude to get any extra thrust that might allow for some more climb.
The rate of climb figures are the hardest to come up with in your own test and probably the most unreliable real test data out there. If the thrust and drag (and weights) are correct the ROC should fall into line.
Cheers: T
The Power off IAS speed at which the Zero would stall should remain about independent of altitude though the reduced thrust would also increase the power on stall speed. 110 knots (or maybe a bit slower) IAS might be somewhere around the min thrust speed for this plane, which is the speed you would creep toward at high altitude to get any extra thrust that might allow for some more climb.
The rate of climb figures are the hardest to come up with in your own test and probably the most unreliable real test data out there. If the thrust and drag (and weights) are correct the ROC should fall into line.
Cheers: T
Brett_Henderson
Charter Member
Yeah.. rate-of-climb numbers are tough to nail in the sim.
The next step for this flight model, is to see how it takes off. Weren't they often flown from carriers ? So.. load it up with fuel and payloads it would see in carrier service.. and see if it can get up and flying from a carrier-equivelant length of runway.. then check the glide-ratio AT published best-glide.. and then see if it can be controllably landed at a carrier approach speed/configuration.
The next step for this flight model, is to see how it takes off. Weren't they often flown from carriers ? So.. load it up with fuel and payloads it would see in carrier service.. and see if it can get up and flying from a carrier-equivelant length of runway.. then check the glide-ratio AT published best-glide.. and then see if it can be controllably landed at a carrier approach speed/configuration.
The A6M Zero is a very light aircraft (about 5500 pounds fully loaded) with a fairly large wing: 39 foot wingspan and 240 square feet of area. There is nothing particularly weird about the airfoil. It has a couple degrees of washout. Basically its reputation for agility is because of the very low wing loading.
The stall speed I am getting with this aircraft is 68 mph clean at an altitude of 500 feet which is in the ballpark. This is by reducing power and speed and maintaining altitude until the aircraft stalls. If I throttle back to idle, the aircraft slows down so fast that I can't always note the speed at stall.
Longitudinal stability is VERY poor near the service ceiling. On manual controls instead of autopilot, I am lucky to come within 2000 feet of what the autopilot can do. The test for service and absolute ceiling (about 800 feet higher) was done with about 75% fuel. With only 50% fuel, it will go about 1000 feet higher. Service ceiling should only be about 33,000 feet.
Documented climb rates are all over the place for this plane: anywhere from just under 3000 fpm to 4500 fpm depending on the source.
Takeoff performance and low speed acceleration were rather mediocre until I tuned the power curves to increase the HP below max rpm. It still peaks at 2550 rpm, but HP doesn't drop off very fast as rpm is reduced. I start off with the stock P-51D flight model but it seems like the propeller tables don't really fit this plane. I have been experimenting with swapping in the stock Hurricane propeller tables.
With a plane like this, I don't know of a source for Glide Ratios or even cruise throttle settings. The max throttle settings are a compromise. Research documents an "Overboost" setting, but the sim doesn't really support increased MP AND RPM with WEP.
Climb tests are a pain to conduct and I don't know that I really got those right, but they are my best effort.
I am not so sure ROC really falls into place so easily because it depends on a propeller efficiency at an advance ratio that is very far from where max speed is achieved. Way back with the 714th was around, there was a lot of discussion about tuning propeller efficiencies to adjust climb rates without affecting max speed. I also have a Spitfire Mk.IX in the works that is using P-51D propeller tables that accelerates way too well and climbs at 4900 fpm while the real ROC should be about 4100 fpm. The engine between the two planes is almost exactly the same but it seems that in real life, the Spitfire had a much worse propeller.
- Ivan.
The stall speed I am getting with this aircraft is 68 mph clean at an altitude of 500 feet which is in the ballpark. This is by reducing power and speed and maintaining altitude until the aircraft stalls. If I throttle back to idle, the aircraft slows down so fast that I can't always note the speed at stall.
Longitudinal stability is VERY poor near the service ceiling. On manual controls instead of autopilot, I am lucky to come within 2000 feet of what the autopilot can do. The test for service and absolute ceiling (about 800 feet higher) was done with about 75% fuel. With only 50% fuel, it will go about 1000 feet higher. Service ceiling should only be about 33,000 feet.
Documented climb rates are all over the place for this plane: anywhere from just under 3000 fpm to 4500 fpm depending on the source.
Takeoff performance and low speed acceleration were rather mediocre until I tuned the power curves to increase the HP below max rpm. It still peaks at 2550 rpm, but HP doesn't drop off very fast as rpm is reduced. I start off with the stock P-51D flight model but it seems like the propeller tables don't really fit this plane. I have been experimenting with swapping in the stock Hurricane propeller tables.
With a plane like this, I don't know of a source for Glide Ratios or even cruise throttle settings. The max throttle settings are a compromise. Research documents an "Overboost" setting, but the sim doesn't really support increased MP AND RPM with WEP.
Climb tests are a pain to conduct and I don't know that I really got those right, but they are my best effort.
I am not so sure ROC really falls into place so easily because it depends on a propeller efficiency at an advance ratio that is very far from where max speed is achieved. Way back with the 714th was around, there was a lot of discussion about tuning propeller efficiencies to adjust climb rates without affecting max speed. I also have a Spitfire Mk.IX in the works that is using P-51D propeller tables that accelerates way too well and climbs at 4900 fpm while the real ROC should be about 4100 fpm. The engine between the two planes is almost exactly the same but it seems that in real life, the Spitfire had a much worse propeller.
- Ivan.
Brett_Henderson
Charter Member
When I was poking around, I remember seeing something like 70knots as a clean stall speed. That seems low for an airplane of that size and power, but would fit the low wing-loading that you mention. Your 68mph translates to 59knots and is too far off (16%) for an accurate model, but well within tweaking, and certainy admirable accomplishment going at it from your methods.
Edit: Make that 70mph .. so your Vs is dead-on.. ignore the 16% references.. but keep in mind checking the key airpeeds (I've really got to stop thinking exclusively in knots.. *sigh* )
When best-glide is unpublished, something between Vx and Vy works, or if those are unknown, something near Vref is close.. and if that's not known, just go with "1.3 X Vs", as Vref.
The actual glide-ratio itself isn't as important, as getting a decent glide ratio AT best-glide airspeed. Not so much for flying an engine-out ducumentation, but as a target for putting the whole flight-profile together... especially aprroaches..
This is where we see a small problem cascade into several erroneous performance figures. If Vs is off by 16%, you really can't come up with a useable Vref, and the Vx/Vy stuff is probably off by more than 16%.... and as you try to correct all of this while working under an already highly tweaked engine, power curve; your adjustments are less predicable for performance at high power settings... For example; the first thing I'd try to do about the stall-speed, is to increase wing efficiency (and counter it with drag adjustments) , and that will no doubt wreak havoc on your cruise speeds, and high-altitude performance...
Your airspeed/power/altitude numbers are too good for me be critical, other than to re-mention that for the whole envelope, it works out better when you go after the "inside engine" stuff, after you get the in-flight stuff right, using the brute force power/thrust scalars. But of course, that's just my preference..
Is this a CFS model ? Meaning that I'll never get to play with it in FSX if you ever upload it ?
On the theory of using as realistic numbers as is possible (and how the MSFS model makes that nearly impossible) .. When I set to make an air-file/cfg set for a model... I get all of the coordinates right out of the modeling software (Gmax)... Obviuosly that works well for contact-points, and gear-compression animation.. but for wing/stabilizer/control-surface locations, not so much. When I built my Cessna 310; accurate location, area, and deflections for the elevators made it virtually un-flyable. The compromises there are ridiculous, but the important goal (ultimately) is for accurate, in-flight performance. My V-tail Bonanza was the ultimate in compromise, as I had to "simulate" ruddervators within a flight model that only reckognizes rudders and elvators. The animation though, worked out quite well.. how they respond to both pitch and yaw inputs from the pilot..
Edit: Make that 70mph .. so your Vs is dead-on.. ignore the 16% references.. but keep in mind checking the key airpeeds (I've really got to stop thinking exclusively in knots.. *sigh* )
When best-glide is unpublished, something between Vx and Vy works, or if those are unknown, something near Vref is close.. and if that's not known, just go with "1.3 X Vs", as Vref.
The actual glide-ratio itself isn't as important, as getting a decent glide ratio AT best-glide airspeed. Not so much for flying an engine-out ducumentation, but as a target for putting the whole flight-profile together... especially aprroaches..
This is where we see a small problem cascade into several erroneous performance figures. If Vs is off by 16%, you really can't come up with a useable Vref, and the Vx/Vy stuff is probably off by more than 16%.... and as you try to correct all of this while working under an already highly tweaked engine, power curve; your adjustments are less predicable for performance at high power settings... For example; the first thing I'd try to do about the stall-speed, is to increase wing efficiency (and counter it with drag adjustments) , and that will no doubt wreak havoc on your cruise speeds, and high-altitude performance...
Your airspeed/power/altitude numbers are too good for me be critical, other than to re-mention that for the whole envelope, it works out better when you go after the "inside engine" stuff, after you get the in-flight stuff right, using the brute force power/thrust scalars. But of course, that's just my preference..
Is this a CFS model ? Meaning that I'll never get to play with it in FSX if you ever upload it ?
On the theory of using as realistic numbers as is possible (and how the MSFS model makes that nearly impossible) .. When I set to make an air-file/cfg set for a model... I get all of the coordinates right out of the modeling software (Gmax)... Obviuosly that works well for contact-points, and gear-compression animation.. but for wing/stabilizer/control-surface locations, not so much. When I built my Cessna 310; accurate location, area, and deflections for the elevators made it virtually un-flyable. The compromises there are ridiculous, but the important goal (ultimately) is for accurate, in-flight performance. My V-tail Bonanza was the ultimate in compromise, as I had to "simulate" ruddervators within a flight model that only reckognizes rudders and elvators. The animation though, worked out quite well.. how they respond to both pitch and yaw inputs from the pilot..
Hi Brett_Henderson,
I thought the stall speed was pretty close also so when you mentioned 70 knots, I had to go check:
From: Information Intelligence Summary No. 85 (December 1942)
(Probably a summary of flight testing of Tadeyoshi Koga's A6M2 recovered from Akutan)
Stall Speeds were the following
Gear Up, Power On ==> 64 knots
Gear Up, Power Off ==> 68 knots
Gear Down, Power On ==> 53 knots
Gear Down, Power Off ==> 60 knots
Climb Rate at 30,000 feet was 850 fpm
Climb Rate at Sea Level is stated as 2750 fpm
Climb Rate at 15,000 feet is stated as 2380 fpm
My plane climbs at 3580 fpm at 12,000 feet.
My plane also achieves best climb at 170 mph which is about 30 mph higher than it should be.
Ceiling is stated as 38,500 feet but I believe this is way too high. Other sources state 10,000 Meters or 32,800 feet.
BTW, If you want to check if this plane for CFS will be useable for any more modern sim, I have a A6M5 (and a bunch of other stuff) hosted here. It is a very old visual model and flight model, but should let you know really quickly if the newer plane built with the same methods will work.
I still don't think I understand how getting the basic flight performance numbers correct with the scalars outside the AIR file and THEN tuning the engine parameters works out in the long run. Once you tune the engine power, you would cause all kinds of changes to the basic flight performance so you would need to adjust the scalars yet again?!?
- Ivan.
I thought the stall speed was pretty close also so when you mentioned 70 knots, I had to go check:
From: Information Intelligence Summary No. 85 (December 1942)
(Probably a summary of flight testing of Tadeyoshi Koga's A6M2 recovered from Akutan)
Stall Speeds were the following
Gear Up, Power On ==> 64 knots
Gear Up, Power Off ==> 68 knots
Gear Down, Power On ==> 53 knots
Gear Down, Power Off ==> 60 knots
Climb Rate at 30,000 feet was 850 fpm
Climb Rate at Sea Level is stated as 2750 fpm
Climb Rate at 15,000 feet is stated as 2380 fpm
My plane climbs at 3580 fpm at 12,000 feet.
My plane also achieves best climb at 170 mph which is about 30 mph higher than it should be.
Ceiling is stated as 38,500 feet but I believe this is way too high. Other sources state 10,000 Meters or 32,800 feet.
BTW, If you want to check if this plane for CFS will be useable for any more modern sim, I have a A6M5 (and a bunch of other stuff) hosted here. It is a very old visual model and flight model, but should let you know really quickly if the newer plane built with the same methods will work.
I still don't think I understand how getting the basic flight performance numbers correct with the scalars outside the AIR file and THEN tuning the engine parameters works out in the long run. Once you tune the engine power, you would cause all kinds of changes to the basic flight performance so you would need to adjust the scalars yet again?!?
- Ivan.
Brett_Henderson
Charter Member
It is dead on.. I caught my mistake and edited the post (in red)... I'll start working in MPH, instead of knots, so that we're on the same page
As for the other numbers.. Having a Vy that's 30mph off means that approach speeds will be way off too. Three numbers (Vy / Vref / Best-glide) should be pretty close as initial references when making a flight model. The amount of power from the engine (by power-curve or throttle position) will effect the ROC, but should NOT effect Vy. However, the MSFS model WILL alter Vy by power. If you adjust the power curve to match a ROC (level flight is also a ROC that exactly counters gravity), you'll be chasing Vy all over the place. This points out the first, major compromise. We have to assume that any time you'd want Vy, you'd be at high (if not full) power.. and on an approach (Vref) you'd want low (if not idle) power.
So... adjusting the power curve for airspeed per altitude first, means throwing important V-speeds out the window (ala a Vy that's 30mph off).. and if you DO go back and chase V-speed accuracy by airframe tweaks (wing-efficiency, drag, etc), your power curve (airspeed per altitude), and ROC (including level-flight) goes whacky.
In summary... if you're trying for in-flight accuracy, you'll find that what the MSFS model already does to power/altitude, makes a realistic flight model more achievable. Obviously you'll want to go back and forth (power-curve / airframe) for finishing touches, but starting off with an air-file-ized power-curve (as oppose to just brute-force power scalars and accepting the built-in power-curve), makes it way more complicated than it needs to be. Chances are that when all is said and done; the air-file tweaks will be all but negated.
It is salient that the best rate of climb figure is so far off on the fast side. That may speak to the thrust being off quite a bit at lower speeds. This would also show up in poor takeoff performance. The zero at a typical weight should get off in 500 ft or so in zero wind. Even the monster F7F I am working on at the moment has a zero wind takeoff in the order of 850 ft.
The other possibility has to do with the lift drag curve being off by a considerable ammount. If this were the case one would have quite a sense of being in the area of reverse command at any sort of lower airspeed.
I don't know if you have used Airwrench at all. One of the features that Jerry has in there are very useful curves from the calculated data. I haven't used it in quite a while, but Jerry is constantly updating and improving it's capabilites. Certainly a good place to look to see if something is wierd and from whence it might issue.
Just a guess, but a plane such as the Zero shuld have a L/D ratio somewhere around 12:1. AFSD generates a stream of real time data on engines, thrust, drag, AOA, LD ratio etc. Using the FS playback feature (not available in CF2?) one can evaluate the goings on quite well, with a lot of test flying and hard work.
Good Luck, sounds like a great project. If all elese fails, steal a prop table from a similar plane of HP, Speed and performance, such as F4F.
Cheers: T
The other possibility has to do with the lift drag curve being off by a considerable ammount. If this were the case one would have quite a sense of being in the area of reverse command at any sort of lower airspeed.
I don't know if you have used Airwrench at all. One of the features that Jerry has in there are very useful curves from the calculated data. I haven't used it in quite a while, but Jerry is constantly updating and improving it's capabilites. Certainly a good place to look to see if something is wierd and from whence it might issue.
Just a guess, but a plane such as the Zero shuld have a L/D ratio somewhere around 12:1. AFSD generates a stream of real time data on engines, thrust, drag, AOA, LD ratio etc. Using the FS playback feature (not available in CF2?) one can evaluate the goings on quite well, with a lot of test flying and hard work.
Good Luck, sounds like a great project. If all elese fails, steal a prop table from a similar plane of HP, Speed and performance, such as F4F.
Cheers: T