Quote:
Originally Posted by Shockwave
I understand how CS works, but I'm failing to understand the why.
Does it automagically find the most efficient blade angle? If I'm tooling along in cruise, wouldn't I want the prop at as coarse a pitch as possible? How would I know that by setting engine rpm?
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There are LOTS of advantages of a C/S prop. Tooling along does NOT mean as coarse of pitch as possible. Do you try to fly with your wing at max angle of attack, above stall all the time? (well may be just at landing) What RPM do you set with a fixed prop? How do you know what to set? (practical limits) Like a wing, there's an ideal AOA, based on speed & lift required. A prop needs to be at a suitable AOA for airspeed & power applied as well. Of course temp & air density affects both wing and prop. The c/s prop can handle a wider range of conditions, because of its CONSTANT SPEED or VARIABLE BLADE ANGLE.
There's more burden to figure out RPM to set, because you have a choice. A fixed pitch prop gives no choice; you take what you get. A constant speed prop is efficient over a wider range of speeds, altitudes and power conditions.
Part of the MAGIC is comes from the designers who made the prop to match HP, airspeed and airframe drag. They do this by selecting the proper airfoil, area, chord, twist and shape, just like an airplanes wing. So the prop already has the magic built in. Adjusting the RPM (indirectly affecting the AOA or pitch of the blades) is fine tuning. A Hartzell BA prop for a RV would not work on a King Air, vice verse. The prop has to be custom made for that plane/engine to be efficient.
The constant speed prop is like a transmission analogy is lame, but kind-a, sort-a fits.
A 5 speed manual car going down the freeway in 2nd gear at 65 mph is bad idea; 5th gear would be best for this condition. Now you're start up a steep hill at 65 mph, 4th gears better. That's the idea of c/s props, keep engine rpm (& torque) in ideal range. We don't have 5 gears, we have 800 gears, from 1,900 to 2,700 rpm. Unlike a car, we fly in a narrow range of engine RPMs but vary torque. A FIXED pitch prop is just that FIXED, optimal only in one condition, a compromise for all others. It's like a one speed bike verses a 21 speed bike. Both bikes get you down the boardwalk, but one will be hard to pedal up hill and too fast to pedal down hill, but the one speeder is simple and works pretty good on the flats. Another example, going 65 mph in 5th gear is efficient. 5th gear is also correct for 105 mph as well, but it's NOT efficient MPG wise. Adding a 6th gear would not help at 105 mph, because the engine does not have power to pull a 6th gear. The trans is designed for the engine and car, like a prop is for a plane and its engine. MAGIC is engineered into it!
The engine and airframe also have their sweet spot as well, which goes into selecting RPM. We fly the plane, we don't let the prop fly it. You select engine power (MAP/RPM) for a dozen other factors not just for the prop. Small variations from best RPM will not drastically affect overall efficiency. Following typical power settings (MAP/RPM) for beginners works fine.
What airspeed, power & altitude do you want to fly at? It's a choice like pepsi or coke. Go fast or save fuel? The prop gives max prop efficiency (possible) by adjusting blade angle to maintain RPM. Cool! If you change your airspeed and/or power radically, YOU the pilot needs to change RPM. Once set the prop will dutifully hold RPM, which is efficient, like a automatic transmission. Say you're transitioning from cruise back to climb, you need to increase RPM (to be efficient). Once level you need to dial RPM back again. It will not automatically change RPM, the pilot needs to do it. Aerobatics: A c/s prop varies blade angle so RPM remains constant. Awesome! With a fixed, acro takes constant throttle jockeying to keep RPM's from over revving.
The gov and prop only holds RPM constant. That's amazing. It can hold 2,500 rpm at 210 mph or 140mph or 25" or 21" of MAP. You said you know HOW it works.
The pilot decides the RPM, MAP & airspeed by airmanship. You fly faster TAS with a headwind and slower with a tailwind, right? The prop can't figure that out for you. AOM's or AFM's recommend cruise power settings.
Ask yourself first what % power you want. Than set the power (MAP/RPM) to that requirement, simple. The world does not revolve around the prop alone (no pun intended).
In general, cruise, lower RPMs give better prop efficiency, BUT the prop makes less thrust (engine makes less power) at low RPM. You need a certain amount of thrust to fly, more for takeoff and climb. Also airspeed is additive. At low air speeds you can run more efficiently at higher RPM than in higher speed cruise. Thus for takeoff and climb we use higher RPM than in cruise.
The c/s prop will not tell you what's best, but it will give you the choices that you don't have with a fixed pitch prop. A c/s prop lets you fly slow or fast as efficiently as possible. Fixed is a compromise. So for takeoff 2,700rpm, climb 2500-2700rpm, cruise (2300-2600), descent (2300-2500), approach/landing as needed.
Takeoff - Need to get up asap, so max power = max rpm. Max RPM is set by prop & engine design/limits.
Climb - high power, max / near max pwr, so 2,700 or 2,600 rpm is fine. For noise 2,500 rpm is great, gives up only 150 fpm; it's worth it.
Approach/Landing - Set power as required. At slow speed & power the prop acts like a fixed prop, about 1900 rpm based on throttle position for typical approach.
Cruise:
When you start flying your RV or a constant speed prop plane, run the numbers, figure out what YOU want. Experiment! Guys play with different power settings (RPMs). They note FF & ground speed, while tweaking RPM for max efficiency. If you find the ideal RPM for a flt cond, use it again. A C/S prop gives better overall performance & efficiency than a fixed prop, even if you're not a perfect operator.
There are other factors like smoothness. Changing RPM 50 or 100 rpm can give smoother operations.
Limitations (stay out RPM) are set by the prop manufacture and usually don't affect normal operations, however if you're going for super low RPM's, you might have to avoid some rpms. Over square, MAP higher than RPM is OK with in limits, but too much MAP w/ too low RPM is like driving your car in 5th gear at 25 mph. You're lugging it.
140 mph cruise is way more efficient than 190 mph. It takes less power to fly at 140 mph TAS; you're at a higher L/D, ut a 500 sm flt will take 1 hr longer.
The important thing, you have control over RPM, which is blade angle, at high or low speed.
Good news, there's a handful of typical/normal RPM's to choose from, from 2,300 to 2,700. WOT at or above 8,500', you take the MAP you can get. So 2,300 rpm works. If you want to go a little faster, try 2,500. It may be smoother? Experiment! May be you find flying real slow at 2000 rpm is your thing.
In summary, generally you want to fly high enough to have 75% or less power with WOT (+8,500'). Engines are most efficient @ WOT. You want to also lean for fuel efficiency, so that means setting 75% power or less. To get 75% power with WOT means flying at/above 8,500'.
Pick the lower RPM than the higher, given the choice; LEAN the heck out of it; RPM will be bracketed in a range based on practical, personal or mechanical limitations. Lower RPMs are better for efficiency but not necessarily speed. Higher RPM is NOT efficient for the prop in cruise, but if you need it to fly fast or high for a tailwind, the net gain is a plus.
WHAT EVER YOU DECIDE YOUR C/S PROP WILL AUTOMATICALLY CHANGE BLADE ANGLE FOR THOSE COND. Fixed props can't do that.
Thanks all for the correction, but its fine. For go around you want high RPM (low / fine pitch).
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