Aces High Bulletin Board
Help and Support Forums => Help and Training => Topic started by: Kenne on January 16, 2013, 10:10:54 PM
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is the throttle the one i have on my controls that increase MP?
if so, what does the control we use with the + and - key on the keypad.
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Throttle controls manifold pressure and +/- on the keypad controls RPM (blade pitch).
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Throttle controls manifold pressure and +/- on the keypad controls RPM (blade pitch).
so if i use the +- to decrease the rpm, im in fact increasing the pitch of the blades?
in a cruise enviroment, would this be like OD in a car?
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so if i use the +- to decrease the rpm, im in fact increasing the pitch of the blades?
in a cruise enviroment, would this be like OD in a car?
more like a neutral....decreasing your RPM extends the gas mileage...... :headscratch:
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Curious, is it modeled in AH that feathering the prop on a dead engine will decrease drag?
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Yes.
Wiley.
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Curious, is it modeled in AH that feathering the prop on a dead engine will decrease drag?
yes
singles don't fully feather though
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more like a neutral....decreasing your RPM extends the gas mileage...... :headscratch:
i understand that because i can see it in the ECB.
but if the +- decrease RPM it must be by increasing pitch.
so therefore i should reduce to max cruise by using the +- (prop pitch) instead of the throttle.
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i understand that because i can see it in the ECB.
but if the +- decrease RPM it must be by increasing pitch.
so therefore i should reduce to max cruise by using the +- (prop pitch) instead of the throttle.
yes :aok
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i understand that because i can see it in the ECB.
but if the +- decrease RPM it must be by increasing pitch.
so therefore i should reduce to max cruise by using the +- (prop pitch) instead of the throttle.
No. You should use both. Max cruise requires reductions in both manifold and rpm. E6B will give you the approximate settings.
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... so therefore i should reduce to max cruise by using the +- (prop pitch) instead of the throttle.
depends entirely on the engine. eg. for spits you should set full throttle and use the prop pitch to set the required boost level*. this method doesnt really work in the jug though where you need to set both pitch and throttle to the cruise settings.
your best bet is to grab a copy of the pilot notes for your favorite ride and do it by the book. this has worked for every aircraft Ive tried it on, which is pretty impressive - wtg HT!
* this is how you can climb to 25k in a spit XIV and still have 50mins cruise at 330mph :aok
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so if i use the +- to decrease the rpm, im in fact increasing the pitch of the blades?
in a cruise enviroment, would this be like OD in a car?
I always thought of rpm affecting pitch in terms of how hard the prop is digging in. So less rpm...less pitch....less digging....less load on the engine...less fuel consumption but also less speed.
I'm obviously not a pilot lol
Zaphod
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Where do I find the "pilots notes" for the planes? I've never heard of this.
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You may find a link to the POH in Wiki or AH Wiki when you look up the aircraft there or you can search the internet for "pilot operating handbook P-51" as an example. You won't find every POH and many sites are not free but you should be able to find most of the US and British aircraft.
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Pilot Notes for almost anything the RAF flew are available as cheap reprints on amazon, Operator's Handbooks or Operating Instructions for USN/USAAF birds the same. Axis handbooks not so easy to find, I have a couple for the Luftwaffe but good luck finding Russian or Jap stuff. www.scribd.com has quite a few viewable online otherwise google them, theres lots of PDF scans out there.
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No. You should use both. Max cruise requires reductions in both manifold and rpm. E6B will give you the approximate settings.
but with increased pitch, revs are reduced which reduces MP because more 'energy' from the eng is required to now turn the blade with increased pitch. So with no increase throttle input, the increase of pitch will reduce revs which are required to spin supercharger to PRODUCE MP.
yes?
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but with increased pitch, revs are reduced which reduces MP because more 'energy' from the eng is required to now turn the blade with increased pitch. So with no increase throttle input, the increase of pitch will reduce revs which are required to spin supercharger to PRODUCE MP.
yes?
Not sure about all that but I was also slightly wrong in the post you quoted. Always reduce rpm first then, if MP isn't at the right setting according to E6B adjust it down (or RPM up to raise MP slightly). This varies with altitude and again, while the E6B settings are close they're not always exact.
Hope that answers it.
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but with increased pitch, revs are reduced which reduces MP because more 'energy' from the eng is required to now turn the blade with increased pitch. So with no increase throttle input, the increase of pitch will reduce revs which are required to spin supercharger to PRODUCE MP.
Depends. In real life on normally aspirated engines reducing RPM will cause the MP to increase.
With boosted engines reducing RPM will decrease MP since by reducing RPM you are also slowing the turbo/supercharger.
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Pull up the clipboard, click on e6b, look at max cruise.
For the spit its 2400 rpm, and 4 boost. Hit the - to 2400 rpm and adjust throttle to 4 boost.
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neg, for cruising in spits (and other merlin/griffon/sabre engines) set full throttle, then reduce rpms until you show 4 boost. if you still get more than 4 boost at finest pitch (~1800 rpm), then reduce the throttle.
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Just tested your theory....neg
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Just tested it on a Spit16 in the Training Arena (which means fuel burn of 1.0). 100% plus slipper. Took it to 10k alt. Full Mil power at level off. Tried to back it down to +4 boost with just the - key but it wouldnt get there. Pulled back throttle until it did get to +4. Within a minute I was at 2,000 rpm and ever so slightly less than +4 boost. I noticed that I now was showing over 550 miles of range! However, I don't know that this was sustainable. It appeared that speed was dropping off very gradually and the GPH keep dropping down to 61. I pushed up throttle until it would hold exactly +4 boost. At that point the GPH went to 63-64 and range dropped to 538. Probably more sustainable and still very impressive! But it does appear to me that on the Spit16 at least, you don't need a full 2400 rpm. I will try to test sometime on a longer flight to see what happens at the lower rpm.
Tex78
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Curious, is it modeled in AH that feathering the prop on a dead engine will decrease drag?
*note, you must feather (as much as it will let you possibley do in AH) a "dead" engine before it completely seizes along with the prop. (So after the last loud *Ka-Chung!* but before it completely stops spinning.)
I always thought of rpm affecting pitch in terms of how hard the prop is digging in. So less rpm...less pitch....less digging....less load on the engine...less fuel consumption but also less speed.
I'm obviously not a pilot lol
Zaphod
Less RPM is more digging.
Ever canoe or row a boat or paddle acoss a pond on a dingy? It can be confusing thinking in terms of less/more pitch - lets instead refer to pitch as "grab" or "resistance". So, back to the boats, if you paddle with your paddle grabbing the most water (a 90-degree pitch) you're getting more resistance and taking longer strokes into the water than if you turned the paddle 45 degrees. At 45 degrees you're gettign less resistance/grab and youre (capable of) stroking at a higher rate (higher RPM).
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Just tested it on a Spit16 in the Training Arena (which means fuel burn of 1.0). 100% plus slipper. Took it to 10k alt. Full Mil power at level off. Tried to back it down to +4 boost with just the - key but it wouldnt get there. Pulled back throttle until it did get to +4. Within a minute I was at 2,000 rpm and ever so slightly less than +4 boost. I noticed that I now was showing over 550 miles of range! However, I don't know that this was sustainable. It appeared that speed was dropping off very gradually and the GPH keep dropping down to 61. I pushed up throttle until it would hold exactly +4 boost. At that point the GPH went to 63-64 and range dropped to 538. Probably more sustainable and still very impressive! But it does appear to me that on the Spit16 at least, you don't need a full 2400 rpm. I will try to test sometime on a longer flight to see what happens at the lower rpm.
Tex78
you'll also find that its very alt-dependent, I assume it works best at FTH although I havent verified it. certainly the XIV cant maintain a 2000rpm cruise at reasonable speed in the flat-spot (~20-22k) but at 22-23k 50mins cruise at ~330mph TAS is about right. one the rare occasions I fly the XIV I'll leave the pitch fine until the engine gets bogged down, or I need to stop overspeeding in a dive.
I keep mentioning the XIV because the effect is more pronounced on the griffon than the merlin, presumably because its greater capacity means it generates more torque at lower rpms than the merlin. :headscratch:
edit: Its very useful in the mossie too, extends that 50% loadout which never seems quite enough fuel to use up all that lovely 20mm :aok
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so if i use the +- to decrease the rpm, im in fact increasing the pitch of the blades?
in a cruise enviroment, would this be like OD in a car?
Yes exactly.
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I've noticed often the E6B will tell me settings that I can't attain, because MP winds up too low. I assume that's due to alt. Would I always need to lose altitude to get to be able to use those settings?
If the above is true, by matching the E6B's RPM at the higher alt, but only able to attain the lower MP, am I better or worse off?
Wiley.
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*note, you must feather (as much as it will let you possibley do in AH) a "dead" engine before it completely seizes along with the prop. (So after the last loud *Ka-Chung!* but before it completely stops spinning.)
Less RPM is more digging.
Ever canoe or row a boat or paddle acoss a pond on a dingy? It can be confusing thinking in terms of less/more pitch - lets instead refer to pitch as "grab" or "resistance". So, back to the boats, if you paddle with your paddle grabbing the most water (a 90-degree pitch) you're getting more resistance and taking longer strokes into the water than if you turned the paddle 45 degrees. At 45 degrees you're gettign less resistance/grab and youre (capable of) stroking at a higher rate (higher RPM).
That makes sense. One more question.
What does manifold pressure indicate exactly? In other words I want to think of the throttle on the plane just as I would the throttle on a vehicle. So if I increase throttle, engine rpm increases and the car goes faster assuming no downshift occurs. In a turbocharged engine the intake pressure also increases with rpm increase but it isn't linear. So on my desiel truck (manual...no shifts) I can give lots of throttle and see the pressure really increase but the rpm increases bit slower....hard acceleration gives a large spike in pressure, moderate accel gives less of a pressure spike but in both cases pressure lowers and settles down once the acceleration (or heavier workload such as climbing a hill) is over.
When I look at the manifold setting on the plane I tend to see that as a 1 to 1 relation with the throttle or how much fuel I'm giving the engine. Is the manifold pressure a measure of intake manifold pressure? At full throttle are we running around at max allowed (governed) manifold pressure? For example a gen 3 5.9l cummins had a factory max allowable pressure of 30 psi. You can ask all ya want of the engine, it will not (under factory settings) give more than 30 psi. With decreased workload (pitch or rpm setting) on the engine then the pressure reduces..similar to my truck? Why do our manifold pressures not spike under heavier workload such as in a climb vs diving? Does the prop go through a gearbox....or is it bolted directly to the engine crankshaft?
That's actually alot of questions..sorry lol. I am just curious as I really don't have a good understanding of this stuff and I find it interesting. Sorry for the truck analogy but that is something that I understand and sort of how I try to understand the plane engines. :salute
Zaphod
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I've noticed often the E6B will tell me settings that I can't attain, because MP winds up too low. I assume that's due to alt. Would I always need to lose altitude to get to be able to use those settings?
If the above is true, by matching the E6B's RPM at the higher alt, but only able to attain the lower MP, am I better or worse off?
Wiley.
Great question. I think about this a lot. I know the engines will be at optimal efficiency between 1.0 and 1.1 atmospheres, or 32-35psi boost. Anything under 1.0 produces pumping losses due to vacuum, and anything over produces losses to produce Boost by the compressors. However, then you have to factor in Prop efficiency, and drag losses. It would seem the best combo would vary by altitude. So the settings listed in E6B must be the best settings, but they don't say at what alt they are best for. I would think it's more efficient to fly faster at high alt, then low alt. I would think flying 200 mph produces much lower drag losses than flying 300 mph. But the E6B setting often produce speeds around 300 mph. If I lug the engine down by dropping prop RPM, and closing the throttle to maintain 1.0atm, I can fly at (say) 200 mph. The only factor that would stop that setting from being more efficient is prop efficiency. At higher angles, Thrust:Torque ratio must be dropping quite a bit, or E6B settings wouldn't be so high. So what I end up doing is climbing at the E6B settings because I can count the glide as "range". A bf-109 will maintain a 150mph glide at a descent rate of 1.3K/minute. So the plane will travel 2.5 miles for every 1300 feet of alt. so if I'm 6k I can glide for 11 miles. If I'm climbing, I assume the plane is storing more E than when cruising. So at the E6B settings, I pick a climb angle to maintain about 225 airspeed. If that climb ends up being 1200 ft/min, and I have 5 minutes of fuel left, I will reach 6K when I run out of gas and will have travelled 16.47 miles (ground distance). If I then glide at 150 mph at a descent rate of 1300 ft/m I will travel another 11.5 miles before touchdown. That's a total of 28 miles. compare that to 300 mph for 5 minutes at level cruise which ends up being 25 miles. The glide ratio for the 109 is about 10:1. These numbers are swags from my experience in the MA. I have not run a controlled experiment. Results would vary based on the glide ratio of the plane. The glide ratio for the 109 is about 10:1. Fw-190s are worse, in which case I don't think this method would be better for them vs level cruise. Again I should run an experiment. :salute
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That makes sense. One more question.
What does manifold pressure indicate exactly? In other words I want to think of the throttle on the plane just as I would the throttle on a vehicle. So if I increase throttle, engine rpm increases and the car goes faster assuming no downshift occurs. In a turbocharged engine the intake pressure also increases with rpm increase but it isn't linear. So on my desiel truck (manual...no shifts) I can give lots of throttle and see the pressure really increase but the rpm increases bit slower....hard acceleration gives a large spike in pressure, moderate accel gives less of a pressure spike but in both cases pressure lowers and settles down once the acceleration (or heavier workload such as climbing a hill) is over.
When I look at the manifold setting on the plane I tend to see that as a 1 to 1 relation with the throttle or how much fuel I'm giving the engine. Is the manifold pressure a measure of intake manifold pressure? At full throttle are we running around at max allowed (governed) manifold pressure? For example a gen 3 5.9l cummins had a factory max allowable pressure of 30 psi. You can ask all ya want of the engine, it will not (under factory settings) give more than 30 psi. With decreased workload (pitch or rpm setting) on the engine then the pressure reduces..similar to my truck? Why do our manifold pressures not spike under heavier workload such as in a climb vs diving? Does the prop go through a gearbox....or is it bolted directly to the engine crankshaft?
That's actually alot of questions..sorry lol. I am just curious as I really don't have a good understanding of this stuff and I find it interesting. Sorry for the truck analogy but that is something that I understand and sort of how I try to understand the plane engines. :salute
Zaphod
I'll break it into two parts and hope I don't confound things.
First, in reference to gearboxes with your truck and planes. Most prop planes have a single/single-set gear reduction. From memory, but I would need to look it up to be certain (and coincidently I will be looking those sections up hopefuly soon when I get three prop blades back from Texas anydaynowortwoweeks... :bhead ), a Packard built Merlin mustang has a 1:2 fixed ration with the prop, meaning for every two full circular roations of the prop, then engine goes through one cycle/rotation.... prop strikes are very bad because at best you maybe bend a prop and sheared some of the reduction gearing before bending some rods or worse, and on the opposite side of the spectrum you can overspeed in a steep dive - unpleasantries avoided by your land-loving truck thanks to neutral and the ability to shift up to 5/6 gear while pushing mach 75+ on the highway.
Manifold pressure... uh-boy, lets give it a try.... In a nutshell, it's measuring atmospheric/air vaccum "pressure" the engine is intaking/consuming/sucking-into-the-carb. Mind you, with engine off on the ground you'll get a reading of the neutral/natural-atmospheric/barometric pressure (~28.6" or whatever the weatherman says it is at the moment). While at idle, you're starving an engine of something to "idle" it back (usualy air with the throttle back), so normal/average reading at idle would be ~10-15". Throttle is one of three (the other two being prop pitch control and your fuel mixture control) controls and governs how much air/atmosphere you're letting into the engine - MP tells you how much air your throttle setting is letting into the engine which can vary with a number of circumstances, so it is not accurate to describe it as a "throttle position indicator".... Confusing enough yet???.... here's a great in debth explanation: http://www.askacfi.com/421/what-is-manifold-pressure.htm
Take the good information provided by Vinkman and it starts to make sence why you wont get much more or less than mid 30"s and how the german 0.9/1.0/1.1 ata converts.... now, I msut admit, clueless am I about how precisely the british boost system works but I'm assuming it has something to do with their metric "bars" for pressure measurements.
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I always thought of rpm affecting pitch in terms of how hard the prop is digging in. So less rpm...less pitch....less digging....less load on the engine...less fuel consumption but also less speed.
I'm obviously not a pilot lol
Zaphod
Less RPM, means the prop blades are cutting deeper into the air (imagine taking larger gulps of it less often). Lower RPM means a finer pitch (less gulps but more often). Even if the pitch could be lowered to a 0 degree situation the engine is still running and burning up fuel. Imagine sitting in neutral with the accelerator floored, you aren't going anywhere but you still burning up the gas.
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I'll break it into two parts and hope I don't confound things.
First, in reference to gearboxes with your truck and planes. Most prop planes have a single/single-set gear reduction. From memory, but I would need to look it up to be certain (and coincidently I will be looking those sections up hopefuly soon when I get three prop blades back from Texas anydaynowortwoweeks... :bhead ), a Packard built Merlin mustang has a 1:2 fixed ration with the prop, meaning for every two full circular roations of the prop, then engine goes through one cycle/rotation.... prop strikes are very bad because at best you maybe bend a prop and sheared some of the reduction gearing before bending some rods or worse, and on the opposite side of the spectrum you can overspeed in a steep dive - unpleasantries avoided by your land-loving truck thanks to neutral and the ability to shift up to 5/6 gear while pushing mach 75+ on the highway.
Manifold pressure... uh-boy, lets give it a try.... In a nutshell, it's measuring atmospheric/air vaccum "pressure" the engine is intaking/consuming/sucking-into-the-carb. Mind you, with engine off on the ground you'll get a reading of the neutral/natural-atmospheric/barometric pressure (~28.6" or whatever the weatherman says it is at the moment). While at idle, you're starving an engine of something to "idle" it back (usualy air with the throttle back), so normal/average reading at idle would be ~10-15". Throttle is one of three (the other two being prop pitch control and your fuel mixture control) controls and governs how much air/atmosphere you're letting into the engine - MP tells you how much air your throttle setting is letting into the engine which can vary with a number of circumstances, so it is not accurate to describe it as a "throttle position indicator".... Confusing enough yet???.... here's a great in debth explanation: http://www.askacfi.com/421/what-is-manifold-pressure.htm
Take the good information provided by Vinkman and it starts to make sence why you wont get much more or less than mid 30"s and how the german 0.9/1.0/1.1 ata converts.... now, I msut admit, clueless am I about how precisely the british boost system works but I'm assuming it has something to do with their metric "bars" for pressure measurements.
Thanks, actually that does make sense. Vinkman's post and your post answered another question regarding the meaning of ata settings vs pressures in US rides. I assumed as you said that british readings were in bars also. I didn't however know what ata was...which appears to be a relative measurement to atmospheric pressures if I understand correctly rather than a more direct indication given in US and british planes.
The manifold pressure indication was confusing to me because I never realized that in aircraft you're able to control more things vs just throttle. If I got this right then in a modern vehicle I control throttle...which is fuel and air. The modern vehicle controls the ratios and all I see is engine rpm increasing or decreasing. With an aircraft I also control those things but I have instrumentation telling me how much air I'm giving the engine....manifold pressure...which changes according to alt. because at higher alt.s the air is less dense therefore for a given amount I'm letting in then pressure decreases for the same throttle settings. I had forgotten about mixture control...something I don't directly control in a modern vehicle or in AH. Does AH then vary mixture automatically? Do you have to mess with mixture much in a real plane or is that something you rarely adjust?
Thanks all :salute
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I've noticed often the E6B will tell me settings that I can't attain, because MP winds up too low. I assume that's due to alt. Would I always need to lose altitude to get to be able to use those settings?
If the above is true, by matching the E6B's RPM at the higher alt, but only able to attain the lower MP, am I better or worse off?
Wiley.
If you look at the speed chart you'll see what the max speed is for your altitude. Then you can play with RPM to see if a fuel burn decrease is worth the speed decrease given your current objectives or if you need to change altitude to meet your speed/range goals.