Hitech, flakbait, or anyone.. need some help!!

[Kweassa]

Received a lot of good help from Hitech or Flakbait, but still got some confusing issues about how RPM and throttle, props work on a plane.

 Two questions goes like this:

 If a plane wich has a safety limit of 2800rpm with a fixed prop is in a constant climb, which would yield better results?

1) If the plane goes max throttle, and controls the prop pitch to maintain 2800rpm?

2)or If the plane goes lowest prop pitch, and uses the throttle to maintain 2800rpm?

 And,

 If that same plane is in a level flight;

1) If this plane levels out max throttle, and controls the prop pitch to maintain 2800rpm

2) If this plane levels out with lowest prop pitch, and uses the throttle to maintain 2800rpm

 which will be faster?


 ...

 My take is that in both cases, with a fixed prop like that the results would be the same - approximately same climb rate and same level speed.

 Is that right?

[Kweassa]

Oh wait, now I'm confused again, I've changed my mind.

 In both cases 1) would be faster and more efficient, because while the set rpm remains 2800 constant, in both cases 1) has max throttle, which means the engine is what turns the prop and not the wind.

 In both cases, 2) would be that the props are max flattened out, which would mean max. drag, but the throttle is kept low so that the engine would not overrev, which implies that the air is what's keeping the RPM constant at 2800 - thus, max drag, low throttle, and limited same thrust(because the rpm is 2800 constant).

 Am I right?


 Then I'm confused yet again.

 Let's say there's a plane with a CSU, with RPM limit to 2800. What happens, if momentarily, during full throttle, the pilot goes to manual prop management  and pushes the RPM upto 3000 by flattening the blades?

 Will the plane slow down? Or will it be faster(if not for overrev issues)? I'm confused which might be the case sincde my limited understanding of this leads me to thinking that both might be possible..

1) If it goes faster...

 * The plane gains 200 more RPM by that way, which means the thrust increased?

*  In this case(or in usual cases) does that mean that the drag has indeed increased, but the increased thrust due to 200 more RPM overweighs the amount of drag?

* In this case, then, would it imply that if there was no overrev issues in real life, then max throttle with minimum blade angle would always be the fastest way to fly?


2) If it goes slower...

 * Okay.. the throttle is maxed out. No more increase there. But the increase in RPM was gained by not more power, but by flattening of the blade. Thus, the plane flies 200RPM faster, but again, its not the engine thats turning the props that much faster. Its the air.

* Thus, it would imply that if there was no overrev issues in real life, then the max throttle with minimum blade angle will greatly slow the plane down due to greatly increased drag of the props..

* but then again I'm confused for the third time time, in that I've always thought that "maximum speed" setting of the planes were always regulated by max RPM by prop governers. Thus, a 3000rpm limit engine actually can do more than that, and if it could, it'd make the plae faster, were it not for the safety problems.

 Oh boy my head hurts now.



 What is it with planes that create thrust? The engine itself does not create thrust, but by pistons and shafts, the rotation of the plane pushing the air back is what creates thrust.

 Then, does it create more thrust if the prop turns faster? But in order to turn the prop faster with a given engine, the blade has to come flatter, too, doesn't it?

 Does a flatter blade create more thrust(faster turn, higher drag), or does angled blade create more thrust(slower turn, less drag)?

[Straiga]

Well first, a fixed pitch prop (one solid piece of metal or wood) in a climb will decrease the RPM of the engine with full throttle. This means you can not exceed RPM limits in a climb.  In a decent you can exceed RPM limits of the engine, so throttle reductions is nesassary. Throttle sets rpm on a fixed pitch prop.

 Variable pitch or constant speed props in which you can change the pitch of the prop blade buy several different ways oil pressure or some buy counters wieghts depends on the type aircraft are controlled by the prop pitch lever in the cockpit, and throttle adjusts manifold pressure. Lets say your at a straight and level flight, and you set 2800 RPM by the prop pitch lever and increasing power from say 3100 inches of manifold pressure to 4500 in. the prop would stay at 2800 rpm changing pitch and now in a climb the prop pitch will change to maintain RPM at 2800 without a power change from 4500 in.

 Now in a desent RPM would stay at 2800 by changing pitch without a power change.  Large increases of airspeed in a desent, some would say rpm would increase above what rpm was set for, because the prop would not be able change pitch enough to prevent over speed. Well why have a constant speed prop system to begin with. The P-51D manual has a standard max rpm 3300, high speed dives are talked about and nothing notes of any prop overspeed problems for any given manifold settings or prop pitch. I have taken a P-51D from high altitudes dives into compressability and have not noticed any over speed problems. I have an LOA and over 500hrs in the P-51D.

 High altitudes were there is thin air, prop pitch angels would be different than at lower altitudes were there is more air, but would show the same rpm settings.

 Turbo props have prop levers to adjust prop pitch (rpm) and throttles adjust torque (power). The higher altitude torque decreases while rpm stays the same, decrease alititude torque increases prop rpm remains the same, for a given prop setting.
Same for manifold pressure as in torque.

Lets talk prop drag, I used props to slow down the airplane like the King Air 300 I fly, I enter the pattern at 1500 agl, pattern altitude for turbo props, torque at 400 anything less you get the gear horn. Props set at cruise 1950 rpm, at 174kts I bring in approach flaps this pitches up the airplane so I trim 3 down at the same time, but the 300 is a clean airplane and takes awhile to slow down, so I go to full props, 2200 rpm now what this does is it sets up a pretty good sink rate, so I pitch up and trim and down goes the airspeed, at 156 gear down no pitch change at all just drag, at 142 in the white arc on the airspeed indicator I bring in 10% flaps and torque to 800, slowing to the target 110 airspeed for the approach-blue line(best single engine rate of climb), now Im turning base flaps to 60% with a 700 fps rate of decent, then the turn to final approach, flaps to 80% adjusting torque for rate of decent and targeting the touch down point, if Iam hot, reduce torque and pitch up, if Iam slow pitch down bring in the torque. At this point If you brought back the toque all the way to idel you get so much drag and set up a large sink rate, you would stall by to much pitch up to kill the sink rate this would be unrecoverable. Thats how much drag some props can produce. When over the numbers slow to 87Kts-red line (minimum controllable airspeed single engine). After touch down pull throttles to idle then pull up the throttles into beta and pull back for reverse prop thrust.

 For max power in any airplane, fixed pitch or constant speed prop, is full throttle for fixed pitch props or full manifold pressure and high rpm low pitch for constant speed.

 For better cruise, reduced rpm for fixed pitch or reduced manifold pressure and low rpm high pitch for for constant speed.

 You should have more manifold pressure for a given prop rpm
like 2900in. 2800 rpm.
 Some manufactures have resenace ranges for power and rpm settings to stay away from, which means that the prop blades can flex and bow one way or another from the opposite blades causing vibration that cause damage to the engine and props.

Hope this helps
Straiga

[flakbait]

Fixed pitch means the blades don't move, hoss. Also, you can't intentionally over-speed the engine by simply throwing a Curtiss into manual and going to full increase RPM. It has limits, the same as any other prop, which prevent an operator from using full manual increase RPM to over-speed the engine. Now if you're using WEP, that's a different story because additional power is being produced. I'll explain that in a sec. You can't over-speed a Hamilton by using manual pitch control because of the design; it is physically impossible. Aeroproducts props are the same way; inside each prop butt is a piston with fixed travel limits which controls blade pitch, so you can't grab the lever or hit a switch to gain any RPM. The engine, and the prop, balance each other. When one is pulling or putting out more, the other reacts.

Think of it this way: you want to drill a hole through a wood block, so you pull the trigger on your drill to get it up to speed. The drill running at full RPM is no different than a prop at full low pitch. There's a torque load on the prop, and the bit, but not much of one so you've got full RPM. Now ram that bit into the wood block and what happens? It slows down to a balance point where the torque of the bit is countered by the torque of the drill. You pushing against the drill is the same as aerodynamic drag and dead weight in any aircraft. Props maintain RPM the same way: as long as the torque load from the engine matches the torque load from the prop meeting the air, you have constant RPM. Should the prop's drag and torque load increase from dialing back pitch, RPM will decrease because the engine is putting out a fixed amount of power. Its a lot like trying to move a 500 pound weight with a Freightliner, only to have the weight suddenly jump from 500 to 800 pounds. The engine power is a fixed constant in level flight, so changing the load on the engine will either cause it to bog down or speed up. Just like a drill.

We're used to car engines where the gas pedal is the go-fast lever. If the car needs more air, it just sucks in more air cause everybody knows the gas pedal gives it more gas... right? In any piston pumper you've got three knobs: throttle (air), mixture (fuel) and RPM (engine speed). Balance them correctly and you can fly for nearly 12 hours, or you can go fast enough to make your hair bleed.

Then, does it create more thrust if the prop turns faster? But in order to turn the prop faster with a given engine, the blade has to come flatter, too, doesn't it?

If you can crank over a prop faster, you will get an increase in speed to a point. You can't really increase prop tip speeds because as a prop tip goes through the sound barrier it becomes distorted from the shockwave. Eventually you'll shatter the blade. The prop blade has to go flatter to decrease torque load as well as drag so you can spend more engine power on going really fast.

Does a flatter blade create more thrust(faster turn, higher drag), or does angled blade create more thrust(slower turn, less drag)?

The flatter you go, the more oomph you get because less of the blade face is hitting the air. This comes from the drill example above. The engine has a fixed amount of power it can create, and increasing blade pitch increases both torque load on the engine and prop drag. Stick your hand out a car window and you'll instantly see what I mean. As long as you hold your hand nearly flat, you can move it around all you want. Change the angle up and suddenly the wind tries tearing your arm off. The higher the angle, the more torque required to turn the prop, so the slower your engine RPM. High pitch/low RPM is also great for making your butt uncomfortably numb, especially when you run the mixture LOP (another horse to flog).


That answer things for ya, Kweassa?


-----------------------
Flakbait [Delta6]

[Schutt]

If a plane wich has a safety limit of 2800rpm with a fixed prop is in a constant climb, which would yield better results?

Fixed prop, no change to prop pitch.
Constant speed prop, you select rpms and the prop pitch is adjusted so you get the rpms you set.
Turbo prop, you adjust pitch in a given range.

rpm of the prop can not exceed the rpm at that the prop blade tips go close to supersonic. Dont know how to calculate this but for the sake of this discussion assume you are way below that.

Now another point. The engine has a power curve and a redline. The max. power of the engine increases with rpm to a certain point (max hp) from then it falls again. With reducing mainfoldpressure or combustible (fuel,N2O,Methanol) you can get lower power output than that.

The redline is the max rpm at which you can savely operate the engine, going higher it will be destroyed.

When you have any kind of charger you can increase the power at a certain rpm, to a certain extent. Pumping to much air+fuel into the cylinder it will explode to early scrap the engine. This still leads to the above mentioned power curve where engine hp rise to max hp boosted and then fall again to redline.

max hp might be diffrent from max hp boosted.

Now to your points of rising rpms...

1. when the engine at the given altitude is at max hp boosted then increasing the rpm will give you less net hps. together with less efficient prop= slower

2. when increasing rpm pushes you above the redline the engine is dead within less then a minute, will net in less lvl speed.

3. increasing rpm from below max hp boosted and still below redline.. will net in more power aka more speed

If case 3 works then you werent at max power settings before. In ah2 terms you werent at military power.

If case 3 works but your engine heats up because its cooling is not sufficiant then you are in some kind of WEP rated power.

If case 3 works only with injection of special fuel (Water, MEthanol,N2O,MW50 usw) then you use WEP rated power.

Other than that, when you reduce the rpms it gets more effective prop but you probably come to a point where the engine delivers less power. Like a car, at idle its weak at 5000rpms you get kick bellybutton acceleration.

Ok now back to your second post.. what happens when you rev up the engine by giving less prop pitch from military power settings?
Depends on the plane and engine.
1. engine blows up
2. less speed/climb if your engine goes above max hp
3. better speed/climb but engine starts heating up (=using wep)

ciao schutt

[Golfer]

Wouldn't be this one would it?

[Straiga]

Full power, full speed without WEP for any prop airplane is full throttle(carburated, manifold pressure, or for turbines-torque).

 Carburated airplanes with non turbo fixed pitch props can not exceed engine RPM red lines on takeoff, straight and level flight, or in a climb with full throttle. These are high load sittuations and decrease power of the engine below redline.
 In a desent with a full throttle power setting you can now exceed rpm redline limits of the engine. Increasing airspeed and with more air through the prop disk, because of the large frontal area of the prop, speeds up the fixed pitch prop increasing the RPM of the engine past redline not prop rpm, the tach is on the engine not the prop. This is why we reduce engine RPM when desending. When the engine quits the prop still turns because of the air through the prop disk. If you slow to much the prop will stop turning.  Drag is less with a spinning prop than a stopped prop for a fixed pitch prop only, not for a constant speed prop.

Engine RPM, not prop RPM, is managed buy the throttle with a fixed pitch prop.  

 Full speed is with full engine rpm for fixed pitch prop (as close to red line) and full manifold pressure and high rpm for constant speed props.

 For a given takeoff there is only a certain amount of air available. Based on altitude, temp, and moisture (density altitude), with a manifold pressure airplane, manifold pressure is the amount of air available in the manifold before entering the engine, same as a carburator (except you dont read manifold pressure). Manifold pressure decreases with altitude. Power available can be different for the same airplane on a different day (density altitude) less air on some days than other which means less power on somedays than other. Higher altitudes means less power than at sea level. This is why at high altitudes we need superchargers or turbos to boost more air into the engine.
 The fuel will always be the same, but the air density will change on a daily bases, so now we have mixture lever in the cockpit, which allows us to change the fuel air mixture in the engine. We dont change the air mixture because were getting all the air we can get for a given day but we changing the fuel to meet the air, this will give us the most power and best combustion for the engine.
 Running the engine lean can (less fuel than air mixture) can clean spark plugs, and run the engine hotter in the desents to prevent shock cooling with cowl flaps closed. Running the engine richer (more fuel than air mixture) can cool the engine in climbs with the cowl flaps open. Richer mixtures can foul spark plugs with high leaded fuels. The best fuel air mixture for a given power setting can extend the range of the airplane. Also the best fuel air mixture can give full available power for takeoff.

 Fixed pitch props come in different pitches (Climb, cruise or combonation of both) They are one solid piece of metal or wood and can not change there pitch. Cruise props (low rpm high pitch)are very bad for takeoff performance and climb, but good for long range, and climb props (high rpm low pitch) bad in cruise performance but good for stol. Thats why there a combonations of both built into one, for all around preformance.

 Single and multi-engine Turbo props have a prop range from high RPM (low pitch) redlines to low RPM (high pitch) and to full feather, like twin engine piston have.
 When a prop is in a high RPM low pitch setting the prop has a high drag in low power settings (large frontal area), when the prop is in low RPM high pitch the prop has low drag in a low power setting(low frontal area). When the props in feather the prop is like a knife to the relative wind, and very little frontal area or no drag at all.

 Piston singles have a prop range from high rpm(low pitch) to low rpm (high pitch) but not feather, but a lowest rpm setting the prop angle is very high almost a knife angle(low frontal area) to the relative wind. This is for engine out situations for best glide. If the prop was in full high rpm setting, the prop would windmill because of the large frontal area (more drag)in an engine out situation and the plane would not glide as far as the lowest pitch setting with the prop stopped with a small frontal area (less drag)at almost a knife edge to the relative wind. So if you had an engine out pull the prop to low RPM for best glide, just like feathering.

 Prop tips that go supersonic do not produce thrust.

 No Golfer its was a straight "D" model, not Tigers pony.

Straiga;
Certified Flight Instructor-instrument, Airplane Single and Multi-engine, Helicopter

ATP Airplane and Helicopter

[Golfer]

I didn't think it was Tiger's, but one never knows.

Who's was it?  I've had a chance to have a long talk (meant the world to me) with Brad H. (not posting full last name since he's not a celebrity or anything) while flying Hurry Home Honey in our airshow.  He's got LOAs in the F4U, P-40, P-51, P-47 and T-28.  If this guy told me how to tie my shoes I was going to listen to him.

I asked him loads of questions about this and that.  It's nice when guys like that take time to allow knuckleheads like me to pick and prod at their brains.  I think he's a DE with one of those rare 'any kind of piston airplane there is' authorizations.  He's gotta know something