Act Acting » Acting Studio » Constant speed prop question

Constant speed prop question

Question:

Be interesting to see a factual account. I’m not denying the Merlin car existed, I’ve seen photos in a magazine this year (though it certainly wasn’t a Rolls Royce car). I just think it would have burned hell and all fuel and not gone particularly fast.

Wasn’t this a converted Ford Capri? With stretched bonnet bearing Rolls Royce grille & Spirit of Ecstasy ornament? Sounds familiar. — Antony "Dop" Shepherd         | "I say the cheese is www.btinternet.com/~dop       |

Response:

– Hide quoted text — Show quoted text – Be interesting to see a factual account. I’m not denying the Merlin car existed, I’ve seen photos in a magazine this year (though it certainly wasn’t a Rolls Royce car). I just think it would have burned hell and all fuel and not gone particularly fast. Wasn’t this a converted Ford Capri? With stretched bonnet bearing Rolls Royce grille & Spirit of Ecstasy ornament? Sounds familiar. — Antony "Dop" Shepherd

There’s a picture of John Dodds one at :- http://www.ditto.mcmail.com/Weird/Weird066.htm There’s a write up on it at :- http://www.btinternet.com/~cbcox/beast/beast.htm which states "approx 750bhp at 2500 rpm" "cruises at 70mph doing 6-700rpm" and there’s a different one at :- http://www.ditto.mcmail.com/Weird/Weird072.htm which  (as of August 20000) is currently up for auction with an estimated price of $250,000. The stories surrounding the John Dodd car involve it being destroyed by fire on a German autobahn, being written off after being dropped into a ship’s hold after a Swedish drag-racing event and also the words ’sued by Rolls-Royce’ feature in a lot of the articles as well. If anyone else can find more info, then please post it and I’ll try and collate it onto my website ( I’m looking for some decent content). James…

Response:

Not only the engine efficiency, but also the propeller efficiency is greatly affected by something called advance ratio, which is related to the RPM and true airspeed.  Constant speed propeller makes it possible for the propeller running at better efficiency in a wide range of airspeed. –Mike – Hide quoted text — Show quoted text – In both the car and the plane, the engine is most efficient when it’s running in a certain RPM range.  To keep the RPMs within that narrow range, you need to adjust the transfer ratio in either case.  In the car, you do with with the gearshift.  You watch the tachometer (assuming you have one) and shift up when the RPMs get too high as your speed climbs.  This imposes a higher gear ratio between the crankshaft and the wheels, letting the engine run back down at a reasonable speed. In the plane, the difference is that the pitch of the prop blades is continuously variable, instead of stepwise variable like in a car. Also, built into the CS prop hub is a mechanical governor, which keeps changing the pitch to get the RPMs to be whatever you set with the blue knob. — Roy Smith, CFI-ASE-IA

Before you buy.

Response:

Not only the engine efficiency, but also the propeller efficiency is greatly affected by something called advance ratio, which is related to the RPM and true airspeed.

I don’t know if this is really correct (from a physics point of view), but here’s the way I look at that… The prop blade is just an airfoil, and as such, it has an AOA, just like a wing does.  And also like a wing, it produces lift (which shows up as thrust) and induced drag (some parasite drag too, but that’s probably pretty small).  Like a wing, there is a magic AOA which gives you the best L/D ratio. A controllable pitch prop (*) lets you keep the prop blades pitched to the best L/D AOA over a wide range of airspeeds and power settings. (*) A controllable pitch prop is one where you can change the blade pitch in flight.  A constant speed prop is a controllable prop which has the added feature of a RPM governor.  You can have a controllable pitch prop which is not constant speed, but I don’t believe any have been built like that in many years. — Roy Smith, CFI-ASE-IA

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… FWIW, I remember seeing a Merlin-powered "special" at the Brighton Speed Trials in the late 1950s. Does it still exist? Alan White … Aha. Found a picture of the said car: http://www.btinternet.com/~cbcox/beast/beast.htm

Not the one that was at Brighton! — Alan White in England’s Lake District. http://www.alan.lesley.ukgateway.net

Response:

- Hide quoted text — Show quoted text – Be interesting to see a factual account. I’m not denying the Merlin car existed, I’ve seen photos in a magazine this year (though it certainly wasn’t a Rolls Royce car). I just think it would have burned hell and all fuel and not gone particularly fast. FWIW, I remember seeing a Merlin-powered "special" at the Brighton Speed Trials in the late 1950s. Does it still exist? Alan White I seem to recollect that he was a repairer of automatic transmissions for expensive cars and he used to advertise in the London Times. Name of Thomas or Tommy Dodd rings a bell, but haven’t been able to locate any further info!  Rolls-Royce hated him! seamus

I seem to remember it was the body of some other car (American?), of course heavily modified, with the Merlin engine and a RR radiator. The maker called it a Rolls Royce which RR didn’t like. In an article I saw about it, I seem to remember 2mpg being mentioned. Aha. Found a picture of the said car: http://www.btinternet.com/~cbcox/beast/beast.htm The chap’s name was John Dodd. Others using the Merlin exist, such as a wooden one. Apparently it is in the collection on one Mr Jay Leno, of "Tonight Show" fame. http://www.pathfinder.com/carculture/culture/woodies/04specials/speci… tml http://www.vuitton.com/va/cla/pagcla/clcona/zoom_2_98.html These people also want to put a Merlin in a car: http://www.bentleyboys.com/prod05.htm Paul

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I may be entirely wrong but a heavy vehicle (the engine alone is over half a ton) getting to Formula 1 speeds seems unlikely. A Merlin Spit was good for about 300mph TAS at low level but in a very streamlined package with only induced and parasitic drag to worry about. Any vehicle will have 4 fat tyres on the road generating infinitely more friction/drag. The transmission problems would have been enormous though, I guess, surmountable.

The engines typically weighed around 1800 lbs but put out 1400 to 1500 hp in supercharged form.  I too read the article but now cannot remember how the engine was fueled.  Even if it did not use the supercharger, it still would produce some 800 to 1000 horsepower and that would go a LONG way to producing a very high top end in a car, providing the gearbox was properly ratioed.  This would be the case even with the car weighing 4000 lbs.  For proof you need only look at the current crop of Nascar vehicals which easily top 200 mph on long tracks and could go much faster given long straights and proper gearing.  They typically develop 800 hp and are not lightweight cars although I do not know exactly what they do weigh. Corky Scott

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ha ha ha..what does merlin in a car have to do with piloting? – Hide quoted text — Show quoted text – Be interesting to see a factual account. I’m not denying the Merlin car existed, I’ve seen photos in a magazine this year (though it certainly wasn’t a Rolls Royce car). I just think it would have burned hell and all fuel and not gone particularly fast. FWIW, I remember seeing a Merlin-powered "special" at the Brighton Speed Trials in the late 1950s. Does it still exist? — Alan White in England’s Lake District. http://www.alan.lesley.ukgateway.net

Response:

– Hide quoted text — Show quoted text – Be interesting to see a factual account. I’m not denying the Merlin car existed, I’ve seen photos in a magazine this year (though it certainly wasn’t a Rolls Royce car). I just think it would have burned hell and all fuel and not gone particularly fast. FWIW, I remember seeing a Merlin-powered "special" at the Brighton Speed Trials in the late 1950s. Does it still exist? — Alan White in England’s Lake District. http://www.alan.lesley.ukgateway.net

I seem to recollect that he was a repairer of automatic transmissions for expensive cars and he used to advertise in the London Times. Name of Thomas or Tommy Dodd rings a bell, but haven’t been able to locate any further info!  Rolls-Royce hated him! seamus

Response:

I’m sorry my recollection of the article about this car seems vague. It was a while ago. He was evidently ticketed at some insane speed on the British Motorway, I forget the exact numbers. I wasn’t arguing about the fuel economy tho. I think you are probably accurate in your theoretical assesment of the fuel required. The engine burns roughly 75 gph at the mid range rpm and power setting, I can’t see it getting anything close to 16 mpg anymore that the fish I  nearly caught was as big as I claimed it was. As for the speeds, he could have attained say 150 mph with the horsepower he had, if he lightened the vehicle as much as possible, and with a clever tranny design. As you well know 150 mph can easily become 180, which is, what the heck, close enough to 200 to make the claim eh? —

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I agree with everything Andy said but would expand a few areas. The most efficient cruise setting is low RPM (less friction, less mechanical stress, etc) and high MAP (wide open throttle so optimum intake efficiency), if this combination gives MAP higher than RPM it’s usually called "oversquare". Providing you keep within the flight manual restrictions oversquare settings will not lead to excessive cylinder pressures, damaging detonation or loss of fertility (despite some bar stories). It follows that the optimum altitude to fly at (from the powerplant efficiency viewpoint) is that which allows you to achieve the percentage power that you want with a combination of full throttle and (nearly) the lowest legal RPM setting. There is a trap here for those who don’t like reading flight manuals. As an example, the Bulldog and Arrow have ostensibly the same engine yet the Bulldog manual allows the MAP to exceed the RPM by up to 4.5 but the Arrow manual only allows a lower number. 25/25 may be the correct climb setting for certain aircraft but in others a wider throttle (higher MAP) may give a richer mixture and better cooling. The reason that the levers must be moved in the right order is simply that if you do it in the opposite sequence you may move the engine into a condition where the cylinder pressures are too high and cause detonation. The simplest way of seeing this is to sketch a graph of MAP (y axis) and RPM, mark out a triangular "avoid" area in the top left corner, visualise how you would get between a high power point (top right corner) and a low power point (bottom left corner) without entering the "avoid" area. I’ve seen a Lycoming bulletin (can’t recall which) that said that routinely selecting high RPM with lowish MAP (eg downwind) can lead to excessive engine wear. So, there are both financial and noise incentives to stay at cruise RPM in the circuit and to only go to the max RPM setting late on finals (RBG checks) ready for a go-around. Instructors should explain systems and operation as part of a type conversion but if anyone flies a CS prop and isn’t sure about this stuff, firstly RTFM and if still unsure ask a suitably qualified instructor. Dave Sawdon

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I just have to ask…  HOW!? I’ve seen a Spitfire engine; things the size of a large coffee table or desk, how the hell did he fit it in a car?! Not necessarily. I recall a blurb in Mechanix Illustrated back in the mid 60s about a fellow that did this. He found a Merlin in a salvage yard and had it installed in his Silver Cloud. He said he got the same mileage (about 16 mpg) before and after. George Patterson,  N3162Q.

– Mike O’Malley                   University of Illinois          home:   (708)479-1879   Urbana-Champaign              cell:   (708)217-3622 Institute of Aviation           school: (217)278-0527 low." ICQ 32677796                    "You can land anywhere once!" AIM omalmi

Response:

Andy, This is so useful..thanks. It all applies to the Bulldog that I fly, and although I do what I am told, it is excellent to have it explained so well. Christopher One think that bugs me is the "prop fully forward (high RPM) downwind" brigade, all they do is make a lot of noise and annoy the neighbours best way is to set the prop lever to the same RPM as used for the climb out (2400/2500), then on final (3/400′) REDS – BLUES – GREENS Regards

–  Kind Regards, Christopher  Christopher Jarman  Web Site: http://www.argonet.co.uk/users/quilljar/ _

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- Hide quoted text — Show quoted text – If you put a Spitfire engine in a car then you probably won’t need much of a gearbox… but you will pay a heavy price in fuel consumption. Not necessarily. I recall a blurb in Mechanix Illustrated back in the mid 60s about a fellow that did this. He found a Merlin in a salvage yard and had it installed in his Silver Cloud. He said he got the same mileage (about 16 mpg) before and after. Yes, that was in the 1970s. The "vehicle" caught fire eventually, I vaguely recall. It was a hybrid made up of various different vehicle parts, to carry the very heavy engine. Peter. —

I seem to recall that the engine was in fact a Griffon, which seems even more incredible. I think that Rolls Royce were not impressed, taking legal action to prevent the vehicle being named a Rolls Royce and also to stop the use of the square radiator grill and Spirit of Ecstasy  statuette. Graham Salt

Response:

Be interesting to see a factual account. I’m not denying the Merlin car existed, I’ve seen photos in a magazine this year (though it certainly wasn’t a Rolls Royce car). I just think it would have burned hell and all fuel and not gone particularly fast.

FWIW, I remember seeing a Merlin-powered "special" at the Brighton Speed Trials in the late 1950s. Does it still exist? — Alan White in England’s Lake District. http://www.alan.lesley.ukgateway.net

Response:

What I’m really trying to ask is for a comprehensive guide of what the blue lever and the black lever really do to your aircraft.

Hmmm … here goes … A normal fixed pitch prop works like an aircraft wing, it has, lift, drag and an "angle of attack", but unlike an A/C wing the angle of attack is fixed at a compromise angle. (Not quite correct but it will do for now) The disadvantages of a fixed pitch prop are:- 1)      During the early stages of take-off the angle of attack is too         large giving slow acceleration and a longer take-off run.         (Just pulling away in your car in 2nd or 3rd gear) 2)      During high speed flight the angle of attack is too small         causing the prop to be inefficient.         (Just like driving down a motorway in 2nd or 3rd gear) These problems were overcome by the introduction of, firstly the variable pitch prop, and then the constant speed prop (the two *are* different). The introduction of the variable pitch prop gave the pilot another lever to play with, which directly controlled the blade pitch angle. This was difficult to use and was soon replaced by the constant speed prop. The constant speed prop also gave the pilot another lever but this time it controlled the prop R.P.M via a governor system which controls the blade angle by oil pressure With a fixed pitch prop Power and R.P.M. were considered to be the same thing, after a little thought it will be obvious that they are not (a car going up hill with full throttle and low R.P.M.) (a car going down hill with throttle closed and high R.P.M) If the prop lever now controls R.P.M. we need another gauge to tell us how much power is being applied. This gauge is Manifold Pressure. It is calibrated in inches of mercury and normally goes from 0 to over 30, atmospheric pressure is approximately 30" (OK, for the pedants, International Standard Atmosphere 29.92") Prop Lever controls …… R.P.M. We now enter a new environment with more emphasis on setting numbers and less on "seat of the pants" TAKE-OFF 1)      All levers fully forward (Throttle, Prop and Mixture)         (this does *not* apply to turbo charged a/c) 2)      After the aircraft has been cleaned up in the climb (Gear and         Flaps up) reduce power and R.P.M. to 25" and 2500 R.P.M.         in that order (Throttle back then Prop back)         This 25/25 setting is typical of a light aircraft like the         Cherokee Arrow 200 and is known as "Max Continuous Power" 3)      For cruise settings refer to the aircraft manual but as a         general rule don’t set the manifold pressure higher than the         R.P.M. (I know, I know … many a/c manuals say that the manifold pressure can be higher than the RPM under certain circumstances, but, as I said … READ THE F***ING MANUAL) Order for use of Power, Prop and Mixture:- Power up …… Mixture up, Prop up, Throttle up                 (Right to Left) Power down …. Throttle back, Prop back, Mixture back                 (Left to Right) "Rev up – Throttle back" One think that bugs me is the "prop fully forward (high RPM) downwind" brigade, all they do is make a lot of noise and annoy the neighbours best way is to set the prop lever to the same RPM as used for the climb out (2400/2500), then on final (3/400′) REDS – BLUES – GREENS Regards — Andy Hart

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The governor is mounted on the engine and not in or on the prop hub.  The prop control sets the spring pressure on the fly-weights inside the governor which will take a neutral position at the set rpm.  If the rpm varies the fly-weights will move the pilot valve in the governor to either send oil to or allow oil to drain from the prop which maintains the desired rpm. Changing spring pressure on the pilot valve by moving the control will set a new rpm. The prop uses oil pressurized by the pump in the governor to move a piston connected to the prop blades by cams or other linkage, there maybe counterweights, springs and pneumatic charges used to assist the prop in operation.  Single-engine airplanes will got to max. rpm/flat pitch with loss of oil pressure and most multiengine planes go to feather with loss of oil pressure. Higher rpm makes the flyweights in the governor move outward which raises the pilot valve, draining oil on a multiengine aircraft and sending oil to the dome on a single. There are many forces acting on the prop, the governor with assistance from counterweights and springs controls the prop. – Hide quoted text — Show quoted text – Say you want to decrease speed whilst remaining straight and level. I know that in the PA23 I select 18"/2300 (down from 22"/2300) and maintain altitude by pitching up. I know that because I’ve been taught it by the venerable George Orive, CFI. But why do I do that? What’s the theory behind the reduction in power? The prop control is essentially the same as the gearshift in a car, except that 1) it works backwards from how a car works, and 2) it’s continuously variable, whereas a car typically has 3, 4, or 5 discrete gear ratios that you can pick from. In a car, you have two controls; the gas pedal and the gearshift.  The gas pedal is exactly the same as the throttle on a plane; it simply is connected to the throttle plate in the carburator by linkages.  Push down on the gas pedal, or push forward on the throttle, and the plate opens up more, allowing in more fuel/air mixture (minor handwave here when some annoying twerp in the studio audience asks about fuel injection). In both the car and the plane, the engine is most efficient when it’s running in a certain RPM range.  To keep the RPMs within that narrow range, you need to adjust the transfer ratio in either case.  In the car, you do with with the gearshift.  You watch the tachometer (assuming you have one) and shift up when the RPMs get too high as your speed climbs.  This imposes a higher gear ratio between the crankshaft and the wheels, letting the engine run back down at a reasonable speed. In the plane, the difference is that the pitch of the prop blades is continuously variable, instead of stepwise variable like in a car. Also, built into the CS prop hub is a mechanical governor, which keeps changing the pitch to get the RPMs to be whatever you set with the blue knob. — Roy Smith, CFI-ASE-IA

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I know how it works (sort of!), and I know the settings to use in the PA23 Apache, but how do you really use the constant speed/ variable pitch prop? What does it do (apart from the obvious) that the fixed pitch one doesn’t?

    Better takeoff and climb performance, and improved fuel burn in cruise.  Take a look at:         http://www.avweb.com/articles/pelperch/pistonlinks.html Three related articles on Manifold pressure, props, and mixture.  Very good intro/review, IMHO.

Response:

I know how it works (sort of!), and I know the settings to use in the PA23 Apache, but how do you really use the constant speed/ variable pitch prop? What does it do (apart from the obvious) that the fixed pitch one doesn’t? Say you want to decrease speed whilst remaining straight and level. I know that in the PA23 I select 18"/2300 (down from 22"/2300) and maintain altitude by pitching up. I know that because I’ve been taught it by the venerable George Orive, CFI. But why do I do that? What’s the theory behind the reduction in power? What I’m really trying to ask is for a comprehensive guide of what the blue lever and the black lever really do to your aircraft.

You want to know how manifold pressure and pitch relate to power, don’t you? First of all it is important to understand that "manifold pressure" has nothing to do with the variable pitch prop – you have that with any piston engine. It is the pressure whith which air/fuel mixture enters the cylinders. Higher the pressure means more air entering the cylinder means more power. With a fixed pitch prop, higher power always results in higher RPM. Hence the RPM gauge is all you need to know how much power you are generating. A MP gauge is not needed. Now enter the VP prop. The "black lever" does exactly the same as it does on a fixed prop – it opens the throttle, hence increases manifold pressure and power. With the "blue lever" full forward there is no difference between a VP and a fixed prop. More power, higher RPM. You just have the additional MP gauge to look at and impress passengers with. When you reduce power, let’s say to a 22/23 cruise, this is what happens:  - you pull back the black lever — manifold decreases — less power    If this is all you do, you essentially fly it like a fixed pitch    prop at very high RPM.  - you pull back the blue lever — this tells the prop governor to    reduce RPM — prop governor increases pitch of the prop —    prop has higher angle of attack — higher resistance —    RPM decreases — even less power Why bother? The decreased prop pitch will result in a more efficient operation at cruise speed. Even better – by setting a specific RPM rather than having a lever to directly control the pitch the prop adjusts itself to higher cruise speed. You accelerate – prop moves through air more quickly – prop angle of attack decreases – prop governor will reduce pitch to keep prop speed constant — pitch more appropriate for the higher speed. This is similar to an automatic gearbox changing gears as you speed up, just smoother. When you want to fly at 65% power there are normally several settings to achieve this – e.g. 21/24 or 22/22. Remember, higher RPM – higher power, higher manifold – higher power – you can reduce one and increase the other and still have the same power output. Now back to your deceleration. You reduce MP to 18". The prop governor will decrease pitch to maintain RPM, but you still have reduced the engine power. Its just the governor making it impossible to notice this on engine RPM so you need that MP gauge. You could reduce RPM aggressively to the same effect. But if you do that you will probably blow the Engine up – this is equivalent to trying to accelerate from 10 to 20 mph in fifth gear with full throttle – the power output is reduced by brutally squeezing the RPM down on an engine that "wants" to produce more power. Phew, this has gotten rather long. I hope that this is understandable. Maybe somebody can explain to me why maneouvering speed (Va) increases with higher weight – this is something I *never* understood. Keep curious! Christof PS. I assumed that you know the relationship between power, attitude, speed and rate of climb/descent – if not I have just made a fool of myself for replying to an obvious troll… ;-)

Response:

I know how it works (sort of!), and I know the settings to use in the PA23 Apache, but how do you really use the constant speed/ variable pitch prop? What does it do (apart from the obvious) that the fixed pitch one doesn’t? Say you want to decrease speed whilst remaining straight and level. I know that in the PA23 I select 18"/2300 (down from 22"/2300) and maintain altitude by pitching up. I know that because I’ve been taught it by the venerable George Orive, CFI. But why do I do that? What’s the theory behind the reduction in power? What I’m really trying to ask is for a comprehensive guide of what the blue lever and the black lever really do to your aircraft.

A fixed pitch prop gives you thrust only as a function of RPM.  A constant speed prop gives you thrust as a function of both RPM and propeller pitch. Relative to a fixed pitch prop, the constant speed propeller gives you better static RPM for takeoff, and allows lower cruise RPM. You set the blue lever for 2300 RPM.  Reducing MP from 22" to 18" doesn’t change the RPM, thanks to the governor, however at the lower power setting the only way the governor can maintain 2300 RPM is to flatten the propeller pitch.  So the pitch flattens and the propeller doesn’t take as big a bite out of the air and you get less thrust, thus you either descend or slow down. Was that your question?  How you can get a power reduction without changing the propeller RPM?

Response:

Say you want to decrease speed whilst remaining straight and level. I know that in the PA23 I select 18"/2300 (down from 22"/2300) and maintain altitude by pitching up. I know that because I’ve been taught it by the venerable George Orive, CFI. But why do I do that? What’s the theory behind the reduction in power?

The prop control is essentially the same as the gearshift in a car, except that 1) it works backwards from how a car works, and 2) it’s continuously variable, whereas a car typically has 3, 4, or 5 discrete gear ratios that you can pick from. In a car, you have two controls; the gas pedal and the gearshift.  The gas pedal is exactly the same as the throttle on a plane; it simply is connected to the throttle plate in the carburator by linkages.  Push down on the gas pedal, or push forward on the throttle, and the plate opens up more, allowing in more fuel/air mixture (minor handwave here when some annoying twerp in the studio audience asks about fuel injection). In both the car and the plane, the engine is most efficient when it’s running in a certain RPM range.  To keep the RPMs within that narrow range, you need to adjust the transfer ratio in either case.  In the car, you do with with the gearshift.  You watch the tachometer (assuming you have one) and shift up when the RPMs get too high as your speed climbs.  This imposes a higher gear ratio between the crankshaft and the wheels, letting the engine run back down at a reasonable speed. In the plane, the difference is that the pitch of the prop blades is continuously variable, instead of stepwise variable like in a car.   Also, built into the CS prop hub is a mechanical governor, which keeps changing the pitch to get the RPMs to be whatever you set with the blue knob. — Roy Smith, CFI-ASE-IA

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On your last question, maneuvering speed Va increases with weight because it’s defined as the speed at which the banked stall curve crossed the (usually +3.8g) load factor limit. The stall speed curve moves to higher speeds as the airplane weighs more. And so the intersection with the load factor limit also moves up in speed. John. — John T. Lowry, PhD Flight Physics; 724 Alderson Ave.; Billings MT 59101 Voice: 406-248-2606 Web site: http://www.mcn.net/~jlowry

– Hide quoted text — Show quoted text – I know how it works (sort of!), and I know the settings to use in the PA23 Apache, but how do you really use the constant speed/ variable pitch prop? What does it do (apart from the obvious) that the fixed pitch one doesn’t? Say you want to decrease speed whilst remaining straight and level. I know that in the PA23 I select 18"/2300 (down from 22"/2300) and maintain altitude by pitching up. I know that because I’ve been taught it by the venerable George Orive, CFI. But why do I do that? What’s the theory behind the reduction in power? What I’m really trying to ask is for a comprehensive guide of what the blue lever and the black lever really do to your aircraft. You want to know how manifold pressure and pitch relate to power, don’t you? First of all it is important to understand that "manifold pressure" has nothing to do with the variable pitch prop – you have that with any piston engine. It is the pressure whith which air/fuel mixture enters the cylinders. Higher the pressure means more air entering the cylinder means more power. With a fixed pitch prop, higher power always results in higher RPM. Hence the RPM gauge is all you need to know how much power you are generating. A MP gauge is not needed. Now enter the VP prop. The "black lever" does exactly the same as it does on a fixed prop – it opens the throttle, hence increases manifold pressure and power. With the "blue lever" full forward there is no difference between a VP and a fixed prop. More power, higher RPM. You just have the additional MP gauge to look at and impress passengers with. When you reduce power, let’s say to a 22/23 cruise, this is what happens:  - you pull back the black lever — manifold decreases — less power    If this is all you do, you essentially fly it like a fixed pitch    prop at very high RPM.  - you pull back the blue lever — this tells the prop governor to    reduce RPM — prop governor increases pitch of the prop —    prop has higher angle of attack — higher resistance —    RPM decreases — even less power Why bother? The decreased prop pitch will result in a more efficient operation at cruise speed. Even better – by setting a specific RPM rather than having a lever to directly control the pitch the prop adjusts itself to higher cruise speed. You accelerate – prop moves through air more quickly – prop angle of attack decreases – prop governor will reduce pitch to keep prop speed constant — pitch more appropriate for the higher speed. This is similar to an automatic gearbox changing gears as you speed up, just smoother. When you want to fly at 65% power there are normally several settings to achieve this – e.g. 21/24 or 22/22. Remember, higher RPM – higher power, higher manifold – higher power – you can reduce one and increase the other and still have the same power output. Now back to your deceleration. You reduce MP to 18". The prop governor will decrease pitch to maintain RPM, but you still have reduced the engine power. Its just the governor making it impossible to notice this on engine RPM so you need that MP gauge. You could reduce RPM aggressively to the same effect. But if you do that you will probably blow the Engine up – this is equivalent to trying to accelerate from 10 to 20 mph in fifth gear with full throttle – the power output is reduced by brutally squeezing the RPM down on an engine that "wants" to produce more power. Phew, this has gotten rather long. I hope that this is understandable. Maybe somebody can explain to me why maneouvering speed (Va) increases with higher weight – this is something I *never* understood. Keep curious! Christof PS. I assumed that you know the relationship between power, attitude, speed and rate of climb/descent – if not I have just made a fool of myself for replying to an obvious troll… ;-)

Response:

I know how it works (sort of!), and I know the settings to use in the PA23 Apache, but how do you really use the constant speed/ variable pitch prop? What does it do (apart from the obvious) that the fixed pitch one doesn’t? Say you want to decrease speed whilst remaining straight and level. I know that in the PA23 I select 18"/2300 (down from 22"/2300) and maintain altitude by pitching up. I know that because I’ve been taught it by the venerable George Orive, CFI. But why do I do that? What’s the theory behind the reduction in power? What I’m really trying to ask is for a comprehensive guide of what the blue lever and the black lever really do to your aircraft.

Response:

– Hide quoted text — Show quoted text – I know how it works (sort of!), and I know the settings to use in the PA23 Apache, but how do you really use the constant speed/ variable pitch prop? What does it do (apart from the obvious) that the fixed pitch one doesn’t? Say you want to decrease speed whilst remaining straight and level. I know that in the PA23 I select 18"/2300 (down from 22"/2300) and maintain altitude by pitching up. I know that because I’ve been taught it by the venerable George Orive, CFI. But why do I do that? What’s the theory behind the reduction in power? What I’m really trying to ask is for a comprehensive guide of what the blue lever and the black lever really do to your aircraft.

             I wanted to respond but I really don’t get what you’re asking — sorry.  You appear to be asking why you reduce power in order to reduce speed, but I can’t imagine that’s what you mean!?  Bruce Before you buy.

Response:

If you put a Spitfire engine in a car then you probably won’t need much of a gearbox… but you will pay a heavy price in fuel consumption.

Not necessarily. I recall a blurb in Mechanix Illustrated back in the mid 60s about a fellow that did this. He found a Merlin in a salvage yard and had it installed in his Silver Cloud. He said he got the same mileage (about 16 mpg) before and after. George Patterson,  N3162Q.

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