Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Kweassa on August 30, 2003, 03:02:09 AM
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I have a question on the differences between constant speed props and variable air screws+automatic prop governer. From what I understand, the former was installed on most planes, and the latter were typical of Luftwaffe fighters..
As I was playing IL-2/FB ver1.1b, I noticed the change in prop/RPM features which implemented a noticeable difference between the two types.
With the constant speed props, in most occasions the prop can be set to 100%(meaning maximum fine), and the plane would be provided with maximum speed. But when the throttle is pulled back, the engine still tries to maintain the set RPM to a considerable extent - as a result, the plane decellerates pretty much slowly, and only when airspeed is pretty low, the RPM slowly falls behind(which I imagine, is due to the fact that the set RPM absolutely cannot be retained any longer at such speeds)
However, in the German planes such as the Bf109 or the Fw190, the throttle changes the set RPM also, and the result is a very responsive throttle system which, when the throttle is lowered, the plane decellerates very quick, and when the throttle is pushed forward, also accelerates very quickly - sometimes, often enough to blow the engine if for any reason the automatic governers are turned off, and the prop pitch is set to 100%.
Thus, in the game, the differences in two planes each give them a distinct 'double edge':
* the former, easy control and maintenance of engine RPM and continuous speed, but needs very quick adjustment of RPM when dangers of overshoot is near(since it can't decellerate fast enough unless prop pitch/RPM is lowered manually, quickly)
* the latter, speed control is very precise and responsive. But the system is more complex. Sometimes the automatic governers aren't always most efficient(for instance, drastic low-speed, prop-hanging appex of zoom climbs), and the consequences of meddling with throttle are also more dire - which usually means if for any reason the throttle is touched mistakenly, your plane will instantly begin feeling its effects.
..
In AH, usually when the throttle is pulled back, the set RPM on the constant speed props is also almost instantly pulled back.
If(just for sake of discussion :) ) that the depiction in IL-2/FB is closer to how it was, it would seem then the throttle/engine control of AH planes have the better aspects of each of the two differing RPM/prop management styles - stable and easy to manage as a normal constant speed prop, but RPM responding to throttle as quickly as the LW systems.
So historically, how is it different? Did those two systems work like it was depicted in FB?
I'm not implying how AH should be changed or anything - I'm just curious.
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adjusting the pitch in a 109 does just that adjusts the prop pitch. Any time you climb dive or adjust throttle rpms will change
a constant speed prop you set a desired rpm and prop pitch automatically adjusts to maintain a constant rpm. There are many variations in the actual mechanism. An oil-operated constant-speed propeller contains a mechanism to convert movement of the piston in the propeller dome into blade pitch changes. On planes with constant speed prop 100% pitch setting isnt necessarily Low (Fine) Pitch. Pitch is automatically adjusted to maintain set rpm.
Unless you go into a steep dive from alt or climb you dont need to adjust rpm except to conserve fuel. Any throttle adjustment you make rpms will stay constant. Speed is more a product of rpm adjustments. So when landing just throttling back wont necessarily slow you down fast enought to land because at 100 throttle if rpm is set to 2500 at 65% throttle you will will be at 2500 rpms.
A constant speed propeller always has a separate lever for the control of it. RPM is set by the propeller lever and the boost is set by the throttle. No matter what is done to the throttle or stick (climbing or diving) the RPM is maintained within a margin of the set value. The value e.g. 2500 RPM might vary 50 RPM up or down depending on how good the control unit is.
Variable Pitch Props
Like on the 109 you adjust the pitch angle of the prop blades directly, so at any given pitch setting, RPM still varied with power and airspeed.
They have an automatic setting which I believe is called aeromechanical. I not quite sure how the automatic adjustment worked but I have read where some props used counterweights to balance aerodynamic forces so that if the prop were under a load, the blades would flatten out automatically, increasing RPM and power. If the pilot increased the airspeed or reduced the power, the load would drop, the blade angle would increase, and the RPM would drop. Landing a 109 any throttle movement will directly effect rpm and speed.
The trick to flying these in manual is to find the correct powerband (comination of throttle and pitch setting)
Adjusting throttle and pitch in flight should be as follows.
lower throttle 1st then prop pitch
increase prop pitch first then throttle
when increasing throttle after prop pitch watch your rpm guage so you dont over rev the eng.
A variable pitch propeller also has a separate control lever or button. A certain pitch is set and the RPM varies with throttle movement and speed changes.
The 190s
190as had what was called a "Kommandogerät". It was an advancedpower control system where by the pilot just set his throttle to a given positon then rpms and prop pitch adjusted to preset optimum postions to maintain optimum power. The Komandogerate system was a BMW speciality which combined engine control functions within one control unit. You just put the throttle at xx position for takeoff/climb/combat/cruise. It reduced the pilot workload.
So by simply adjusting the throttle should directly control speed.
The d series mostly used the JUMO213A and in the Jumo handbook it states the the VS111 Airscrew was "vollautomatische Verstellluftschraube" (translation I think means fully automatic, adjustable propeller).
Some one with more knowledge then my self will need to explain that one.
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Originally posted by Batz
You just put the throttle at xx position for takeoff/climb/combat/cruise. It reduced the pilot workload.
I may be wrong, but these seem to be ranges, not just positions.
190A-8 throttle handle (http://www.clubhyper.com/reference/fw190cockpit/throttle.html)
Automatic propellor pitch may be manually overrinden also from the throttle handle. There was also a switch to set prop pitch to auto or manual.
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The way I understand it the prop could be overidden if the Kommandogerät linkages were damaged. The pilot would then go into manual to adjust prop pitch. Normal operation would be in automatic.
Maybe some one with the 190a8 manual can clear that up.
yes they were ranges not fixed positions.
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I'm getting confused.
So in typical constant speed props, adjusting the throttle will less effect the overall speed of the plane, compared to the throttle/RPM/Prop systems as in the 109/190s?
The advantages of reduced work load seems pretty clear on the constant speed props, but it sounds like what the 109/190s had, were as much efficient and easy-to-use for the pilot.
Then, what are the advantages of the constant speed props in the first place? It seems like a more disadvantageous system in that the fixed RPM rate will result in less effective speed control than compared to the systems that directly influence the RPM with the throttle.
Or, am I understanding something wrong? :confused:
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American test pilots were amazed with kommandogerat once they tested it on captured planes. Automatic prop-pitch, automatic fuel mixture, auto rpm setting based on throttle, and ammo counters were some of the wonders they found on these planes.
kommandogerat was, in fact, a small and primitive electro-mechanical computer. More than problably, a uncommon technology for its time.
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Originally posted by MANDOBLE
More than problably, a uncommon technology for its time.
Can you fawn a little more blatantly?
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Originally posted by Kweassa
I'm getting confused.
So in typical constant speed props, adjusting the throttle will less effect the overall speed of the plane, compared to the throttle/RPM/Prop systems as in the 109/190s?
The advantages of reduced work load seems pretty clear on the constant speed props, but it sounds like what the 109/190s had, were as much efficient and easy-to-use for the pilot.
Then, what are the advantages of the constant speed props in the first place? It seems like a more disadvantageous system in that the fixed RPM rate will result in less effective speed control than compared to the systems that directly influence the RPM with the throttle.
Or, am I understanding something wrong? :confused:
I think you are confused here. If you have a fixed pitch prop, or an adjustable one that has two or three settings it's very limiting. With a setup like this you can't always get the MAP you want (which is the real indication of how much torque the engine is producing) because you have to use the throttle to keep the engine from overrevving. Then other times when you want full power (which is generally going to occur at max RPM and max MAP) you can't get all the RPMs you want because of the prop pitch. Basically, with these type of prop setups you are losing a lot of efficiency at all but a couple power settings that they are optimized for.
In a Constant Speed setup you have a throttle control that adjusts manifold pressure, and then a prop control that adjusts rpm (by chaning the prop pitch). With a setup like this you can maintain the MAP and the RPM you want all the time. Essentially you have a much wider range where you are getting maximum efficiency. The reason they are called "constant speed' is because if you set it for a certain rpm, you can then adjust the throttle(MAP) and it will stay at that rpm. The FW190s system almost certainly was a constant speed prop, it was just controlled by the Kommandogerat and not directly by the pilot.
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Yes, but don`t forget that Constant speed props didn`t work just like setting them to a desired RPM, and then forget about the whole thing. The engine had certain limitations, the RPM and MAP had to be matched in a certain range, otherwise overrevving/overboosting would occur, and damage the engine. That`s why constant speed prop plane manuals list the optimum(?) boost+rpm figures for pilots to follow, and the exact order which to apply first, boost or RPM... now since the prop pitch pretty much functions as a gear like in cars, I believe you always have to use the optimum RPM and MAP combination.
Now where I believe the difference lay between 109/190 and allied CS systems, was that the Allied pilots had to set the optimum RPM as well, which was kept by the CS unit regardless speed changes , whereas Germans only choosed the desired MAP, and the opti RPM was automatically adjusted and kept by some governing unit, which AFAIK was of some different nature than CS props...
Certainly some solid insight would be great.
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Kweassa sorry I wasnt to clear,
My mention of speed was in reply to your question
as a result, the plane decellerates pretty much slowly, and only when airspeed is pretty low, the RPM slowly falls behind
When u reduce throttle you will see the rpms drop then return to set rpm. This is because the prop blade is adjusting. As the pitch angle of the blade adjusts it takes less of a bite out of the air and you decellerate.
When you increase throttle you will see rpms climb then settle back on set rpm. The pitch angle of the blade adjusts to take a larger bite of air.
In a 109 as you move the throttle rpms climb and drops while the prop blade angle stays the same. Atleast in FB you can feel and see the plane decellerate/accellerate faster.
You will notice a 10% adjustment on the 109 throttle more then you would on the p47.
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The propeller of the Bf 109 worked as a constant speed propeller at any given throttle setting but rpm varied according to the throttle. I don't know if there was any noticeable difference in the terms of braking, for example in the P-47 and some british planes it was possible to join throttle and propeller setting together so the net result was about same; when throttle was decreased also rpm decreased.
gripen
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Thanks for the answers.
gripen, in that case I'm getting confused again. According to what people have taught me so far in this thread, the term "Constant Speed" seems to indicate a constant rotational speed of the engine - ie. the engine RPM is set, and the propeller pitches automatically vary to maintain the set speed.
Then, in the case of P-47s, if the RPM varies with the throttle, would that mean it would essentially be a same system as the German planes had? In that case, the CSP and VPP are again confusing for me to understand :eek:
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Kweassa:
I Think a lot of confusion happens do to terms CSP & VPP because the Pitch will vary with a constant speed prop.
Changing maninfold pressure does not change the RPM of the Prop as long as the plane is in certain speed ranges.(other than a very short spike) This is because the Prop govener will maintain the RPM as long as the blades are not up against there stops for more or less pitch.
There are 2 conditions when in normal operation you must move both RPM and throttle, one before the other.
When changing from a cruise RPM setting and going to mill power. The RPM must be set to MAX first before the throttle is brought forward. This is do to the fact that an engine produces more torque at lower RPM, hence could cause an over torque situation at low RPM and High manifold settings.
The Reverse is also true when changing to cruise from MIL. Manifold pressure first then RPM. Once again to prevent High MAN, Low RPM.
Prop over speed happens when in a high speed dive the Prop is up against the MAX Pitch stop. Hence it can no longer control the RPM And will act just like a fixed pitch prop. To prevent the over rev in high speed dives, the Man pressure must be retarded,since lowering the RPM will have no effect do to the max pitch stop.
And to my knowldge all WWII fighters had constant speed props that all worked the same. The only difference was the controls to those props where some had an throttle that controled both RMP and MAN to easy pilot load.
HiTech
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Thanks for the answer!
.. which brings more questions :)
I've heard the term 'aeromechanical' screw, referred to as something different from a prop governer system in normal CSP systems - is that true then?
If I recall, the AMS is to adjust prop pitch automatically via air pressure working on the propeller blades - is that something different in function compared to the prop governers CSP systems had, as in USAAF/VVS planes?
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Umm, HT? They didn't all work the same. The result was always the same (a change in pitch/RPM) but the control systems used were very different. Some had an oil/spring arrangement in the prop dome, others used only oil, and the Curtiss had an electric motor. Overspeeding a Curtiss prop basically rendered the whole prop useless because the electric motor controlling RPM/pitch would burn out under high strain. Hamilton-type props, during overspeed, could possibly blow oil lines due to the high pressure involved. Oil and spring props are the best; they don't need oil pressure to set minimum pitch so you don't run the risk of blowing oil lines during overpseed.
Plus, the three adjusted at different speeds. A Curtiss, being driven by an electric motor, would allow max RPM to be hit as soon as the engine was making enough power to set it. John Deakin has flown C-46's with both Curtiss and Ham-Standards fitted. He prefered the Curtiss because it gave him max RPM almost instantly on takeoff. The Ham-Standard needed high oil pressure supplied by an engine-driven pump before the prop got up to speed. Since neither oil pressure nor engine speed at idle is enough to allow full prop control, it took longer to reach max RPM on takeoff. Oil & spring props had the same problem as the Ham-Standard.
To quote Deak...
(HS = Hamilton, CE = Curtiss)
Very simply described, it's just an electric motor in/on the prop hub, geared way down to have a LOT of power, which twists the blades as needed by the governor. Power comes through a slip ring and brushes, which really need to be well-maintained and kept clean. Those that were not well-maintained contributed too the somewhat evil reputation enjoyed by this wonderful prop.
We operated them on the C-46 in SEAsia, alongside the more common Ham Standards. I liked the CEs better, as they gave better takeoff performance. They'd go to full redline RPM at the start of the takeoff roll, while the HSs would not, you wouldn't get full RPM until about 40 knots or so.
Most installations have some variant of what we had for control, a toggle switch that was forward for "Auto," centered for "Off," pull down and to the left for "Decrease RPM," or down and to the right for "Increase." Spring loaded to "Off" from "Inc" and "Dec."
In AUTO, RPM was controlled by any of a variety of methods, from the old flyweight system that simply made electrical contacts instead of porting oil, to more sophisticated systems with master tach generators and slaves, auto-synch, etc.
Prop governors either used a flyweight system attached to an oil valve, or like Deak said above it made an electrical contact. Flyweight governors were spun by the engine and counter-acted by a spring on the flyweight shaft. If the flyweights weren't spinning fast enough, they'd leave the oil valve full open (calling for higher RPM). When the flyweights were spinning a tad too fast, it would open another valve to reduce RPM. A pretty simple arrangement that worked very well. On a Curtiss rig, the flyweights would open or close electrical contacts to change RPM.
I'm not entirely sure, but I think Aeromech props worked backwards from normal ones. Prop pitch might always be set at full high (redline RPM) and increases in airpseed command the works to lower the RPM. Don't quote me on it though!
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Flakbait [Delta6]
Delta Six's Flight School (http://www.worldaccessnet.com/~delta6)
Put the P-61B in Aces High
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Originally posted by Kweassa
Then, in the case of P-47s, if the RPM varies with the throttle, would that mean it would essentially be a same system as the German planes had? In that case, the CSP and VPP are again confusing for me to understand :eek:
No, it`s different than German systems. Bf109, Fw190 has one lever: it controls boosts, and an automation sets the optimal RPM to that boost level. You move only one lever.
On Spit/P-47 etc., there are two levers (at least...:p ) : one for RPM, another for Boost. You have to use them in the manner Gripen descriped. However, the trouble of handling two levers made Allied designers to make it simplier, i.e. they designed the levers shape in a way that the TWO levers could be GRABBED at once, and moved together. It was a simple solution, but unlike German system, it meant that it produced the optimal Boost/RPM values only at max. power, at say, cruise it was only about the optimal boost/rpm pair values.
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Hi Flakbait,
>I'm not entirely sure, but I think Aeromech props worked backwards from normal ones. Prop pitch might always be set at full high (redline RPM) and increases in airpseed command the works to lower the RPM. Don't quote me on it though!
There's yet another (probably similar) system relying on a finned spinner that somehow interacts with the main propeller to set the desired propeller speed.
I have no idea how it works in detail, though. I used to think the finned spinner changed the main propeller pitch via planetary gears or something similar, but from observing parked aircraft, the finned part seems to spin freely.
This principle was used on the Bf 108 as well as on Pilatus and Fiat trainers.
Regards,
Henning (HoHun)
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Isegrim,
Please consult manuals of the Bf 109 and DB 605. At any given power setting (say 1,3ata 2600rpm or what ever) below full throttle height the propeller of the Bf 109 worked as a constant speed propeller ie it keeped 2600 if it was possible within pitch adjusting range of the propeller (as hitech noted, in dive adjusting range might be too little). In the Bf 109, there was a single lever which adjusted rpm and MAP simultaneously. In the P-47 there was one lever for the propeller (rpm) and another for the MAP but these could be connected together with a simple mechanical device, not as sophisticated as automatic system in the Bf 109 and Fw 190 but the from pilot's viewpoint the result was about same in the combat situation ie a single lever for engine controll.
The propeller rarely worked at optimal rpm, generally at low altitude automatic settings were too slow and too fast at high altitude. And during WWII there was certainly not technology to build a device which could have adjusted prop rpm to optimium at any altitude.
gripen
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flakbait: I was not refering to the mechanics of how the prop was controled. Just that they all worked with the same principle.
My RV8 happens to work with OIL to fine pitch/ counter weight to MAX pitch. (suited for acro incase of motor or oil loss prop goes to max pitch i.e. least drag)
Most HARTZELS are OIL to Max pitch/spring to fine pitch. Some use compressed gas instead of spring.
HiTech
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Hi again,
>There's yet another (probably similar) system relying on a finned spinner that somehow interacts with the main propeller to set the desired propeller speed.
Here's a photograph.
http://www.x-plane.org/users/hohun/argus.jpg
Regards,
Henning (HoHun)
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I thought you might have been refering to that, HT, but you weren't too precise in your earlier post. So I figured I'd cover the bases just for info sharing sake.
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Flakbait [Delta6]
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Put the P-61B in Aces High
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to clarify any confusion, a constant speed prop is going to allow for the fastest acceleration/deceleration out of any system. With RPM set to maximum, the engine is operating at its peak horsepower for max acceleration, and with the power back to idle and RPM set to max, the propeller acts like a brake to slow down quickly.
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The propeller would act as a brake, which will eventually drag the engine RPM down and lower airspeed - is this right?
..but I'm curious - would that offer faster decceleration than a system that directly influences RPM itself?
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No thats not right Kweassa.
With Throttle to ideal and MAX RPM. The drag will not lower the eng RPM. The DRAG is produced by the HIGH engine RPM but now power comming from the Prop is being USED by the engine.
Think of it this way, somthing is turning the engine fast, and it's not the gas burning.
There is also increased drag effects on the prop as it turns faster .
HiTech
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Originally posted by Karnak
Can you fawn a little more blatantly?
HAHAHAH!!! WTG Karnak!
You are making great progress. I think it is safe to say that you are no longer in the Ted Kaczynski stage.
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Wow, some of you guys have some long winded posts....LOL, mine will be too and probably more info than you're looking for. Ahh....hitech's post regarding which lever to move first brings back many a memory from A&P school......Oh those were the days.....tearing down and rebuilding R-2800's, puttin the Ham-standard props on....runnin em up......aaahhhhhh....that was the life. Real oil, not this synthetic stuff I work with now....yuk!
Now, given a, let's say, round engine....the 2800 for example. there are 4 controls. 1. Throttle 2. Prop control 3. mixture control. 4. Supercharger control. All four are important and to avoid a jug leaving home, ya gotta do what hitech said in his earlier post. Now for grins let me say this: (hope this helps a bit to straighten out the confusion)
1. Throttle: Primarily controls manifold pressure (with a couple exceptions)
2. Prop Control: Primarily controls the prop position through a governer of some kind. Since we're talking constant speed here, the prop control has three positions full low pitch (increase rpm), full high pitch (decrease rpm) and everywhere inbetween, which is the constant speed range.
3. Mixture Control (don't have in AH) controls the fuel/air mixture for best performance/economy. Also shuts down the engine...No, you don't use the magneto switch to shut the engine off.
4. Supercharger control (not in AH): Controls the amount of boost given to the cylinders.
Everything sinking in alright? I think most will agree on those. Now, when we say increase/decrease RPM we're referring to the engine, not the prop. I think you're straight on that. Now on to the next thing....building blocks of information....gotta love it! LOL
When you start a high powered recip. engine, like the 2800, you'll start it with the mixture control in idle cutoff, both mags on. You've primed it with the primer and as soon as the engine fires and starts to run, you move the mixture to "full rich"....this is how the engine is run before takeoff. It keeps the engine cooler using full rich. Now, the engine is running....rumble, burp, rumble, rumble, burp......(valve overlap) R2800 doesn't like to idle.....it's a runner! Now, the prop control should be at the "full increase rpm" (low pitch) stop. If you look at your manifold gauge, you should see a pressure which is way under atmospheric.... (no boost) Next we set the engine rpm to 2200 with the throttle (one of the exceptions mentioned above) and do our magneto checks.....75-150 rpm drop per mag.....not more than a 50 rpm split between em. Ok, thats' normal...now we check the constant speed range...drop the throttle to around 1400-1800 rpm and let it stable out. move the prop control out of the "full increase rpm" position. The further aft you move it, the higher the prop pitch will become and the engine will start to "load" dropping rpm.... Now if you leave the prop control in that position (other than the two stops)...moving the throttle will change the prop pitch to either a higher or lower blade angle depending on which way you move the throttle.....increase throttle=increase prop pitch (constant speed remember?) decrease throttle=decrease prop pitch. During this time, if you look at the engine RPM gauge, it'll be constant (it'll move slightly with abrupt changes in throttle setting, but return to it's selected rpm) BUT, if you look at the manifold gauge you'll see it move up and downas you move the throttle. Forward, increase prop pitch, your manifold pressure will rise accordingly. (don't worry, there's a red arc where you're not supposed to exceed). As you move the throttle back toward idle, prop pitch decrease, the manifold pressure will decrease. Now if you grab the prop control and jerk it back toward you to the "full decrease rpm" stop, you've just blown the jugs off your engine and oil's going all over the damn place! The engine is making all kinds of bad noises which it shouldn't. The mechanics are PISSED! However, if you do it slowly and watch the manifold pressure, you may be able to get it to the full high pitch stop without overboosting the engine. If not, you'll have to lower the throttle before going any further. Any clearer? I hope it is....
That's how a constant speed prop works...
There are three basic kinds (excluding the exotic models like the one bladed prop) of propellers in this world. They are:
Fixed pitch
Ground Adjustable
Variable Pitch
In the variable pitch category you have the constant speed prop, and the two position prop. The only difference between the two is what makes them work. the constant speed uses a prop goverener and the two position prop has what's called a three-way valve.
We won't get into the mixture control or the supercharger control as hitech didn't model them in the game. Suffice to say that throttling back in the game has the same effect as moving the mixture control to "auto-lean". and presing the E key after landing has the same effect as moving the mixture into "idle-cutoff". In AH we have some really super performng engines...They run at full power all day long and produce sea level peformance at 30k! I like it :)
FWIW
EDO;)
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I think that the best way to understand this braking issue is to think engine output. When you throttle down you actually reduce engine output and to keep rpm constant the constant speed propeller will change to lower pitch. Note that propeller will produce thrust all the time if the pitch adjusting range of the propeller allows this (and engine still does some output) . So deacceleration depends how much you reduce engine output which is not allways same as reducing rpm; if engine produces same output at lower rpm a constant speed propeller will just change to even higher pitch (if available).
There might be some real braking effect by propeller if pitch adjusting range of the propeller is too little, then engine would overrev. Another issue is negative pitch, there were some experiments with negative pitch for shortening landing run, but AFAIK these were with experimental propellers.
gripen