prop control

[daisuke]

for those with some flight experience with fixed speed props

what is a good rule of thumb as far as prop speed settings versus manifold pressure for optimum performance? I can never seem to achieve cruise speeds without going full out throttle and having the rpms above 2500

[Rocket_Bird]

It depends on what aircraft your flying.  I think you are taking about constant speed propellers, in cruise it should be a little bit less than in climb (as to take a bigger bite of the air).  Manifold pressure should be a few inches less as well.  But it really depends on your aircraft.  Higher the RPM and manifold pressure gives you better performance, however its all about economy (and minimal stress to your engine).  I don't fly constant speed propellers personally, except in FS2004  ;D, so im sure theres other people who can make a better explanation than I.

[daisuke]

yeah I mean constant speed props

I don't have a pilot's license (yet) and therefore don't fly aircraft, but I have been using the simulator to learn what I can about all this buisness... this was the one thing I didn't get. I guess I was under the impression that there was quite a bit of play in the prop speed department, that you could move it within about 30 or 40% of the movement range of the prop controls, but apparently moving within only about the top 10% really makes any sense for cruise speed flying. am I correct in assuming this? I also assume that lower manifold pressures and lower prop speeds would allow better efficiency, but who wants to fly slow when you're not worried about running out of gas (I mean... I'm not going to sit in front of a screen for 3 hours while I fly from san fransisco to LA at economy speed)

thanks for the help so far!

[OTTOL]

yeah I mean constant speed props

I don't have a pilot's license (yet) and therefore don't fly aircraft, but I have been using the simulator to learn what I can about all this buisness... this was the one thing I didn't get. I guess I was under the impression that there was quite a bit of play in the prop speed department, that you could move it within about 30 or 40% of the movement range of the prop controls, but apparently moving within only about the top 10% really makes any sense for cruise speed flying. am I correct in assuming this? I also assume that lower manifold pressures and lower prop speeds would allow better efficiency, but who wants to fly slow when you're not worried about running out of gas (I mean... I'm not going to sit in front of a screen for 3 hours while I fly from san fransisco to LA at economy speed)

thanks for the help so far!

That's a good point(sim world vs real world). Who wants to fly slower when they don't have to?
Another factor is the fact the fuel burn dynamics programmed into the sim are marginal at best. The Baron is a good one to experiment with. On a real aircraft with fuel flow meters, you will notice a change in fuel flow, not only with change  in MP but with changes in prop speed, mixture, altitude and angle of attack(none of which are reproduced accurately in the sim). I used to fly a Chieftain(the real one not the sim)and there are specified numbers in the operating manual for high speed OR long range cruise and everything in between.
In the sim I fly at max power most of the time. Why not?
In the real aircraft one of the first lessons that you'll learn is that pushing the limits of engines and airframe will usually yield minimal results.  I'll give you a good example:
When you are really "pushing it" in the Chieftain, you'll maintain 170-180kts. That's three miles per minute. Long range cruise for the airplane is more in the neighborhood of 150kts or two and half miles a minute.
On a 300 mile trip that equates to one hour and forty minutes vs. two hours even. Of course that's if you maintain those speeds from takeoff to landing and you don't do that!(unless maybe you ARE in the sim). From many years of experience I can tell you that when you factor in the climb, descent and entry into the arrival environment, that 20 minute difference is reduced to a ten minute difference or maybe even less at really busy airports.
Now here's the real big element. Long range cruise yields about 19 gallons per hour, per engine. Max power burn can exceed 26 per side.
1.6hrs = 83.2gal@high cruise   2.0hrs=76gal@long range cruise.......or about a $21 difference when using the current price average in the US.
If you think 10minutes is worth an extra twenty bucks then firewall those suckers! If you're flying for a penny pinching, charter outfit, you may want to pull it back to save some gas, the engines, the airframe and perhaps have the fuel necessary to shoot an extra approach or two if the destination airport is weathered in.  ;)

[daisuke]

Thanks for the input, I have read a lot to the effect of high speed flight simply not being worth it, the situation that sparked this whole question was that I downloaded the lockheed connie and tried flying it at close to cruise speeds, then I read some real life accounts of flight in the connie and seeing that I basically had to keep the engines almost at the redline to maintain cruise, I could see why it was called the best 3-engined airliner.

I have found the baron in the flight sim to be one of the most realistic as far as engine controls go, but I hate flying the thing because of the porpoising it does when overcontrolling changes in pitch

But on to one more doubt I have about the control of the props. In the case that you do want to cruise at a slower speed, then it's obvious that you simply reduce the manifold pressure, however is a reduction of prop rpms normally done as well? By this I mean... seeing that at full power you can only reduce the prop rpms slightly before losing performance, it doesn't seem to make sense to reduce them even more if you're going to be flying slower than that, and yet from what I have been able to gather, the prop speed is reduced significantly when cruising slower.

I suppose that much of this could have to do with the fact that it's only a sim. I'd just like to learn and apply as much as possible before ever getting my feet wet in a real airplane.

[Rocket_Bird]

it helps save on fuel when your reduce your prop rpm

[OTTOL]

it helps save on fuel when your reduce your prop rpm

Correct.....and daisuke's assumption regarding prop speed reduction in the sim vs real world is correct. Prop speed in the sim does not have the same affect that it does in real life. The fuel saving techniques used in real life(lower RPM, leaning the mixture...)don't seem to make a difference, so you might as well fly with everything forward.
It's been awhile but if memory serves me correctly max MP/RPM for the Chieftain were in the neighborhood of 39"/2700RPM/23-28gph. Long range cruise numbers were  29-30"/2400RPM/19gph. So you can see that there is a benefit in RPM reduction but the most drastic change is in MP.
You do reach a point were the drag produced by the AOA of the wing negates any attempts at power and rpm reduction for economy. Plus, the idea of going any slower than 150kts in a Navajo seems a bit sacreligious! Honestly, I used 150kts in my example but my speed was usually in the 160-170kt range. It's purely psychological but it felt slow going below 160kts!  :-[  ;D

[Rocket_Bird]

Actually, it seems to work in the sim too... from my observations... Like the Beech King Air 350, you pull your props back your fuel flow goes down

[Skligmund]

There are other considerations also:
Noise and vibration.
Engine life.

[beaky]

Having a constant-speed prop is like having a transmission. Basically the idea is to get more thrust with less manifold pressure. In real life, this saves gas and wear and tear without having to go slower. So the limits apply to avoiding too much manifold pressure for a given rpm. All such equipped aircraft have specific settings of throttle and prop pitch for specific flight modes.
 As a general rule of thumb, and mind you this is second-hand, in RL constant-speed (variable pitch) prop-equipped airplanes, keep things "square" or under... eh, on second thought, forget it; this guy'll explain it better...
http://www.avweb.com/news/airman/184483-1.html

[Sytse]

err... I don't know if this was brought up yet, cuz I'm too lazy to read all the posts, but did you make sure you were at cruising altitude? Your altitude sometimes can make a big difference in speed. Or did you already knew that?

[Ivan]

Pitch controls engine load (resulting in temperature)... Anyone that has the Yak-12 knows that.

Running 600-700mbar pressure with full pitch gives almost the same speed as running at the same power setting at 50% pitch