*OT* Variable Pitch Propellers *OT*
- Thread starter PRB
- Start date
Throttle means MP, torque applied to the shaft and thrust. You could leave the engine theoretically at max RPM the whole time and the system would work OK. The reason we don't do this IRL? High RPM causes high engine stress and wear and reduces the specific fuel consumption. Not so important in FS.....
Usual problem with FS modeled constant speed systems, insufficent range of prop blade angle change and the prop is pitch locking against a stop, at which point it becomes a fixed pitch. Another area? Minimum Goverened RPM set too low, or way to high. Another area with geared engines? Boloxing up the gear reduction ratio.
T
Usual problem with FS modeled constant speed systems, insufficent range of prop blade angle change and the prop is pitch locking against a stop, at which point it becomes a fixed pitch. Another area? Minimum Goverened RPM set too low, or way to high. Another area with geared engines? Boloxing up the gear reduction ratio.
T
warchild
Charter Member
Not sure what it could be. Its a standard setup.. reducing MP will leave the rpms at 2700 with 100% on the cps, and the plane will eventually ( over a few miles ) slow down frm top speed. but decreasing rpms while leaving the MP alone will slow it down a whole lot faster.. ninety percent of the time, on landing, i find myslf reducing the manifold pressure to below 30 inches, and using the cps to control the speed i'm flying at. I'm basically flying the props..
the throttles were set up several times, always the same way. plug them in and set the axis for each pair to thrust, pitch and mixture.. pretty standard..
FS does constant speed props and engines quite well except for multi stage superchargers. The one thing it does not seem to model well is a MP phenomenon where MP will increase if the prop RPM is reduced first. Proper operation of prop/engine controls require the MP to be reduced first and then RPM to avoid things like blowing cylinders off the case.... For power increase, RPM up first the MP. Same reason.
Neither wings nor propellers create lift by upper section low pressure, high pressure on the bottom side is equally important, especially at higher AOA. Props create much of their thrust by creating a reaward mass flow of air, much like turbojets or turbofans, but tend to be more efficent at low speeds than the later two.
The prop tables as well as the calculations for engine HP using displacement, compression ratio and RPM do darn well when run through the various curves and tables in the .air file.
T
Neither wings nor propellers create lift by upper section low pressure, high pressure on the bottom side is equally important, especially at higher AOA. Props create much of their thrust by creating a reaward mass flow of air, much like turbojets or turbofans, but tend to be more efficent at low speeds than the later two.
The prop tables as well as the calculations for engine HP using displacement, compression ratio and RPM do darn well when run through the various curves and tables in the .air file.
T
The proper way to fly any large radial piston engine with a constant speed prop, is to use max RPM for takeoff and usually METO MP. As soon as practical reduce the MP to a climb value (say about 45") and the RPM to a climb value of (say 2550 RPM). Airspeed is varied to allow for proper cooling. Cruise, reduce MP slowly to cruise table value, say 34" or so and RPM as necessary for required speed or range. For descent, MP is usually reduced (slowly) to achieve the desired descent rate with RPM left at cruise value. On approach RPM is increased, sometimes to max, sometimes to the cruise value. MP adjusted as necessary to maintain approach speed. For Naval aircraft the Prop speed may be at the climb value as rapid throttle advancement for waveoff can cause an overspeed, which for the R2800 is quite hard on the engine.
For taxi prop levers are left full forward. Do not think of these as 100 % or 0%. Fiddling around with the Prop levers was a great way to ensure very short engine life.
T
For taxi prop levers are left full forward. Do not think of these as 100 % or 0%. Fiddling around with the Prop levers was a great way to ensure very short engine life.
T
warchild
Charter Member
yeah, well, i still cant figure out why if i set the gear ratio to its true 2.2:1 ratio the plane wont fly under 500 mph.. Right now, with it set to 1.44:1 i'm getting the correct top speed, manifold pressure, temps and everything else, but as soon as i change it to 2.0 Mp shoots up to over 50 at idle, and all hell breaks loose.. However, that minimum governed rpm is certainly worth re-investigating..
Addendum. 16:9 as a gear ratio works ( 1.77:1). This matches specifications on th R 2800-AM11 with 2250 hp..
Brett_Henderson
Charter Member
When you get into big radials, with gear-reduction, and super/turbo charging; it gets complicated in a hurry... and the MSFS engine/prop modeling falls apart.
You have to start fudging things like, prop MOI, min/max pitch, engine power-scalar, and thrust-scalar.. lotsa air-file work, and ultimately, you'll even have to customize the gauge XML, to make it all realistic from the pilot's perspective.
The core theory of the CSP itself doesn't change.. you just gotta "force" the operational range to fit the airplane.
You have to start fudging things like, prop MOI, min/max pitch, engine power-scalar, and thrust-scalar.. lotsa air-file work, and ultimately, you'll even have to customize the gauge XML, to make it all realistic from the pilot's perspective.
The core theory of the CSP itself doesn't change.. you just gotta "force" the operational range to fit the airplane.
Brett_Henderson
Charter Member
Yeah.. that's beyond the MSFS model.. it's easy to visualize though.
A normally-aspirated engine is always trying to "suck" the air into the intake manifold.. it's maximum possible MP, is whatever atmospheric pressure happens to be. At sea-level, on a "standard" day, a wide-open throttle will yield just under 29.92" of MP.
With a super/turbo charged engine, the intake manifold can be at well above atmospheric pressure. The engine "lets" air into it's combustion chamber, rather than having to "suck" it in. With a big ol' radial, with a turbine all spooled up; a reduction in RPM means that the pistons aren't "taking" the air out of the intake manifold as quickly, so the MP goes up for bit.
It would take a very complex, interactive XML gauge (hidden) to accomplish this.
Ok, I'm hijacking my own thread, sort of, which was OT to begin with... If you're supposed to increase RPM before increasing MP, what about during a landing approach, for example, when you're making small adjustments in power? Does every power adjustment require a corresponding RPM adjustment? Even little ones? If so, why do the manuals call for some landing RPM? That implies to me that you leave the RPM set there for landing. But if you have to muck with it every time you want to tweak your landing approach...
warchild
Charter Member
OK, i dont know if it's correct or not, but what i do when i'm landing ( since i cant lower the flaps all the way till 170 mph ) is to chop the throttle to idle ( i can hear the engine revving down but the rpms remain the same ) and then adjust the cps till i have 1500 rpm on the prop.. then i can apply full throttle and bring the manifold back to 20 or 30 inches, and use the cps to adjust my approach speed... sounds complex but isnt really..
Well, in the P-51, for example, I put the RPM at 2700 (or is it 3000?), like the book says, then yank on the throttle back and forth with reckless abandon to maintain speed and descent profile. In real life I would have blown up the poor Merlin, I suppose. The other cool thing the RPM control does is allow one to use the prop as an air brake, of sorts. The A2A P-51s do this nicely. If I'm coming in "too hot", advancing the RPM a tad will slow the ship down, due to the prop blades turning flatter into the wind.
Brett_Henderson
Charter Member
The 'P' in G.U.M.P.S. reminds you to set the Prop for landing RPM. This is normally max RPM, and yes, you leave it there in case of a go-around...
The concern about the MP/RPM relationship, is that you want to keep the RPMs "above" the MP.
Obviously, this gets confusing with big, turbo-charged engines. It just happens to work out nicely for small GA. You don't want a six-foot diameter prop spinning much above 2700RPM, else the blade-tips go supersonic.. and for all intents and purposes, 27" of MP is gonna be near maximum achievable.
27" / 2700rpm ... 27/27 ... That's the "squared" deal.. keep prop RPM/100 near MP in inches.. most importantly, don't let MP go much above RPM.. why 25/25 is a typical climb setting... 24/24 is a typical cruise setting. Too much MP and the engine is trying to over-power the prop, very stressful on the internal engine parts... No worry about over-powering the prop when it's at Max RPM.. so you don't need to fiddle with it during an approach, no matter how much you change MP.
**********************************
@Pam... the MSFS sounds are unrealistic. In reality, you won't hear it "rev" down, because it's not reving down.. the RPMs are constant.. What you hear in the real world, is it gets quieter as power decreases.. but does not rev down... Engine RPM and prop RPM are the same (or the same ratio, with gear reduction).. You cannot change prop RPM, relaltive to engine RPM.. they're rigidly married.
As for your landing technique ? I won't pretend to know every airplane's specific procedures.. but I'm pretty sure that will get you into trouble, in any airplane. Not only do you risk over-powering the prop (as mentioned)... you need to be at max RPM in case there's a go-around.
Sounds like you might need to plan the approach a little further out. The biggest adjustment to my technique (real world), was when I transitioned form Skyhawks and Warriors; to Bonanzas and Mooneys. While learning high-performace/complex airplanes, you're taught to be at 20" MP, 20 miles out, so you can get into landing configuration and at pattern airspeed. My first attempt at landing a Mooney became a go-around BEFORE I was even on final approach... too hot, too high.. .
Bomber_12th
SOH-CM-2025
When landing a high performance prop-driven aircraft, the common practice is to set the prop pitch the same as it is for climb - with the Mustang, it is 2700 RPM, for the Corsair, it is 2550 RPM, for the Allison P-40, it is 2600 RPM, etc. This provides a proper RPM to be at in case of go-around (you don't typically want full RPM, as it will be too much torque on go-around), and at the same time it does provide some good braking, as more prop is turned into the wind. During landing, there shouldn't be any 'jockeying' of the throttle - from my experience, just as in real life, coming off of an overhead break you should throttle back to a setting of about 28-29 inches MP in a Merlin P-51, or 25 inches in an Allison P-40. This starts bleeding the power off very nicely, as you put in the proper RPM setting, where at which time the gear can come down, and various flap settings (as indicated in the manual) are used to progressively slow the aircraft down through base and final (in case of the Mustang, you usually start by puting in 20 degrees of flaps, first, right off the break, and then put down the gear once gear speed is reached). Only smooth throttle changes should be used to make power corrections throughout landing, and properly done, the throttle should remain fairly much in one position, until over the threshold. Whether you're landing a P-51, Corsair, P-40, or anything similar, there is simply no other means to better land the aircraft, and 99% of the time, this is what is done. Otherwise, you have to start much further out, which is quite unsafe (given an engine failure), with flaps being put in earlier, to get the aircraft slowed down enough - and you don't really want to use flaps if you can help it.
Landing a high performance aircraft, an overhead break is always recommended - as is a close-in pattern at a proper altitude - very different to something like landing a Skyhawk or Warrior.
Landing a high performance aircraft, an overhead break is always recommended - as is a close-in pattern at a proper altitude - very different to something like landing a Skyhawk or Warrior.
warchild
Charter Member
Awesome. Thanks Fliger.. I've never had anyone explain those things tp me.. much appreciated..
warchild
Charter Member
Sounds like you might need to plan the approach a little further out. The biggest adjustment to my technique (real world), was when I transitioned form Skyhawks and Warriors; to Bonanzas and Mooneys. While learning high-performace/complex airplanes, you're taught to be at 20" MP, 20 miles out, so you can get into landing configuration and at pattern airspeed. My first attempt at landing a Mooney became a go-around BEFORE I was even on final approach... too hot, too high.. .
twenty miles?? Sheesh.. we were thinking that ten miles was a lot and to be honest, i'll usually drop flaps to 8 degrees a little abpve 250 knots just a couple miles out, then match the remainder of the flaps with the speed so that i dont have them fully down before 170 mph, then use a steep ( eight to twelve degree ) dive with full flaps onto the runway like one of the ways the manual suggests.. It also says you can glide it in, but I dont knowww.. thats a pretty hefty freight train to try and land without flaps..
the manual also doesnt say anything about rpms or mp while landing. it says to mainin 110 mph over the threshold.. i guess they werent counting on the plane being adapted to flight sim and having someone with very little real world experience flying it..
That's how military planes typically approach an airport to land. They come in low and fast right over the runway, then turn hard 180 degrees to enter the down wind leg, pulling lots of G and reducing power. All those Gs bleed off speed. The goal is to be at gear lowering speed as you come out of the 180 degree turn and stabilize on down wind. Then they finish off the landing in the "normal" way. If there are more than one plane in the formation, they each make the hard turn at some number of seconds of interval, in order to space them out enough...
SkippyBing
Charter Member
Who says you can't use flaps gliding?
Brett_Henderson
Charter Member
Yeah.. twenty miles is over-kill... that's for initial training.. you tighten things up as you gain experience. One time I was flying over KCMH (Columbus International) waiting for permission/vectors for final.. and the tower guy said, "got a 757 on long final.. if you can turn for the numbers now, you're cleared to land" ... this was at 2500msl and 150knots. I pulled the throttle to idle.. pitched up to bleed off speed.. dumped flaps and gear and dove for the runway..