Mr Spaceman From Canada, joined Mar 2001, 2776 posts, RR: 18 Posted (6 years 10 months 1 week 2 days 2 hours ago) and read 22918 times:
Hi guys.
I have a mental block about aircraft with constant-speed props.
For example:
As explained in the September, 1996 issue of FLYING magazine, as well as in the operating procedures for a Mooney Bravo in Microsoft's Flight Simulator 2004, during cruise flight the manifold pressure is to be set to 34 inches (via the throttle) and the prop rpm set to 2,400 rpm.
What I don't understand is ......... if you pull back the throttle to reduce the manifold pressure to say 25 inches (to reduce fuel flow), and keep the propeller set at 2,400 rpm's ......... why does the aircraft lose airspeed? (about 10+ knots at 14,500 ft)
Why does the airspeed start dropping when the engine power is reduced even though the prop is still set at 2,400 rpm?
Why doesn't a prop spinning at 2,400 rpm's continue to pull an aircraft through the air at the same speed when the engine power is reduced?
CFIcraigAPA From United States of America, joined Jan 2004, 223 posts, RR: 5 Reply 1, posted (6 years 10 months 1 week 2 days 2 hours ago) and read 22933 times:
Your throttle setting governs how much power the engine is producing. With more power output at 34'', there is more power to turn the prop. This allows the prop to operate at a higher pitch while staying at the selected 240 RPM, thus "biting" more air and producing more thrust to pull the aircraft forward. In essence, the throttle is actually controlling the pitch of the prop at higher power settings.
CM
Mr Spaceman From Canada, joined Mar 2001, 2776 posts, RR: 18 Reply 2, posted (6 years 10 months 1 week 2 days 2 hours ago) and read 22928 times:
Hello CFIcraig APA.
Thank You, for your explanation. I really appreciate it!
These words really help me to finally understand whats going on.
"In essence, the throttle is actually controlling the pitch of the prop at higher power settings."
OK, so it's the reduction in the propeller's pitch (the prop's flyweights are moving), during the same rpm's that is reducing the airspeed. Great!!! Now I know.
Jetlagged From United Kingdom, joined Jan 2005, 2327 posts, RR: 15 Reply 3, posted (6 years 10 months 5 days 1 hour ago) and read 22653 times:
Quoting CFIcraigAPA (Reply 1): In essence, the throttle is actually controlling the pitch of the prop at higher power settings.
Not really, but it's one way to think about it. What actually happens (at any power setting, from a stabilised condition) is that reducing power means the propeller pitch has to go finer to maintain RPM, this reducing the thrust developed. Conversely increasing power will make the propeller go to coarser pitch to maintain RPM, thus increasing thrust.
The power lever doesn't control pitch unless your talking about single lever control designs, or beta mode (reverse thrust).
The glass isn't half empty, or half full, it's twice as big as it needs to be.
LeanOfPeak From United States of America, joined Oct 2004, 509 posts, RR: 1 Reply 4, posted (6 years 10 months 5 days ago) and read 22647 times:
It was not meant that the throttle literally controls the propeller pitch. It was meant that a change in the throttle results (More or less directly) in a change in the propeller pitch.
It is something akin to the advice sometimes given to beginning students learning to land..."Pitch to control airspeed, throttle to control sink rate." Certainly, neither of those is actually what the change to that control commands, literally (Though the result does indirectly occur). But it does get the student thinking about the interrelatedness of the throttle and the pitch and gets them one step closer to the paired adjustments to throttle and pitch that are necessary to get a particular desired result.
MrChips From Canada, joined Mar 2005, 818 posts, RR: 0 Reply 5, posted (6 years 10 months 4 days 18 hours ago) and read 22616 times:
It makes a lot more sense if you compare the prop control to a gearshift lever in car with a manual transmission.
The position of the prop control (and the corresponding RPM setting) can be thought of as the gear you have selected, and the throttle is exactly the same as in a car.
For example: If you keep your car in gear but take your foot off the throttle, you will lose speed. If you add a bit of throttle, you will maintain your speed, and if you put your foot to the floor, you will accelerate.
If you had a manifold pressure gauge in your car, you would see the exact same thing in the airplane as you move the throttle.
Jetlagged From United Kingdom, joined Jan 2005, 2327 posts, RR: 15 Reply 6, posted (6 years 10 months 3 days 16 hours ago) and read 22585 times:
Quoting LeanOfPeak (Reply 4): It is something akin to the advice sometimes given to beginning students learning to land..."Pitch to control airspeed, throttle to control sink rate."
I hate to disagree, but your analogy isn't really close. Pitch does control airspeed, much more directly than power setting. Excess thrust directly relates to vertical speed. So it's more than just a saying to get student pilots thinking, it's what actually happens in flight.
I was aware that CFIcraigAPA was deliberately over-simplifying, but he made a point that this happened only at high power settings. In fact the throttle affects pitch at any power setting, but only indirectly. The propeller lever controls pitch directly.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
Kay From France, joined Mar 2002, 1873 posts, RR: 4 Reply 7, posted (6 years 7 months 1 week 4 days 6 hours ago) and read 22328 times:
I have been thinking about this too lately. I can't believe that I know how to operate it, yet still haven't got to the bottom of its functioning. What is still unclear is the following:
1- Propeller pitch is automatically adjusted continuously by the plane. Therefore, if you have two levers, one for throttle and the other for rpm, playing with either will have an effect on pitch. True or false?
2- The throttle lever will increase power by opening the "butterfly" of air entrance to the engine. But what about the rpm lever? How does this one increase power?
3- To make things more complex, I have read that there are "automatically-adjusting" systems and old non-automatic ones. My interest is for two planes, the Diamond Katana Rotax and the Cessna 182. What matters is that these two planes be in the same category, because I learned how to operate the first one well, and sooner or later will have a go at the second.
SlamClick From United States of America, joined Nov 2003, 10019 posts, RR: 76 Reply 8, posted (6 years 7 months 1 week 4 days 4 hours ago) and read 22323 times:
Quoting Kay (Reply 7): But what about the rpm lever? How does this one increase power?
One way of think about it: The prop lever sets the RPM limit or limit of the RPM range. When the lever is set to a position that corresponds to, say, 2400, the prop governor will change blade pitch to maintain 2400 RPM so long as there is sufficient manifold pressure to do so. It is not uncommon to see RPM fall below the set value when the throttles are brought back to idle.
The manifold pressure is the torque.
With manifold pressure around cruise value, putting the prop lever foward will permit the blades to go to a higher RPM. Pulling it back will limit them to a lower one. Without touching either lever, pulling the nose up will load the prop and cause the blades to go to a finer pitch attempting to deliver the set RPM. Pushing the nose over will unload the prop and it will go toward feather to attempt to limit the engine RPM.
All are interrelated, but the prop lever sets the range.
Happiness is not seeing another trite Ste. Maarten photo all week long.
LeanOfPeak From United States of America, joined Oct 2004, 509 posts, RR: 1 Reply 9, posted (6 years 6 months 2 weeks 3 days 15 hours ago) and read 22064 times:
Kay:
1) As I understand it, the throttle will change the pitch of the blades unless the prop is already at the stops to try to maintain a set RPM. The prop lever will change the pitch up to the point where the throttle is not set high enough for the engine to deliver adequate power to attain the set RPM.
2) Any internal combustion engine has a power curve. The maximum power output rises with RPM until it hits a peak and starts to fall off. As long as the RPM is not above maximum power (Which it would not likely be allowed to be), moving the propeller to finer pitch gives the engine better leverage and lets it, along with the prop, spin up to an RPM at which it generates more power. It is similar in concept to putting a manual-transmission car in a lower gear.
3) According to what I've seen, your Katana had a constant-speed prop. Did you not have a throttle and a prop control? Most 182's will have three levers; Throttle, Mixture, and Prop. If you have not been exposed to dealing with the prop, it's relatively straightforward. The mixture is also relatively straightforward, but you apparently haven't seen it before, so it'll likely take a little getting used to. There are some conversions out there with Full Authority Digital Engine Control (FADEC) that reduce the entire system to one lever with computer controls, but those are still relatively rare from my understanding.
FredT From United Kingdom, joined Feb 2002, 2179 posts, RR: 29 Reply 10, posted (6 years 6 months 2 weeks 2 days 3 hours ago) and read 21983 times:
Rather than to answer questions on a per question basis, I choose to give the information required to answer the questions.
The three main variables are throttle position, RPM setting and airspeed. The prop governor will adjust the prop pitch to whatever it takes to provide the set RPM at the given throttle position and airspeed. Change one of these three and the prop pitch will have to change to keep the RPM constant. The actual power output is a function of both throttle position and RPM (in constant ambient conditions).
The RPM will remain at the set RPM until the propeller hits either the fine or the coarse pitch stop. If it hits the fine pitch stop, prop RPM will drop below the set RPM. If it hits the coarse pitch stop, prop RPM will increase beyond the set prop RPM. The former is far more common and is what you typically rely on to check the mags on the ground before takeoff by checking the RPM drop as you go to either mag. The propeller is on the fine pitch stop and thus unable to compensate any power loss due to a bad mag with finer pitch, thus a power loss will show as an RPM drop.
If you, prior to the mag check, open the throttle to beyond the point where the propeller reaches the set RPM, the propeller will come off the fine pitch stop. A power loss will then be compensated for by the propeller going to a finer pitch – no RPM loss. If you had a propeller pitch indicator, you could check your mags with the propeller off the fine pitch stop by verifying that the propeller pitch change isn’t larger than a predetermined limit.
The power produced by a reciprocating engine (piston engine) is roughly proportional to the amount of air going through it. A higher manifold pressure means a higher pressure differential pushing air into the cylinders on each intake stroke, which equals a higher combustion chamber pressure, more air and more power.
A higher RPM means more cycles per time unit, more air and more power. On the other hand, as the volume of air increases, the velocity in the intake system increases and the loss of total pressure in the intake system increases. This means less pressure and less air in the combustion chambers, meaning less increase in power with increased RPM up to the point where the power consumed by friction and the propeller equals the power output and the RPM will increase no more.
The throttle is nothing more than a device which allows the operator to control this loss of total pressure and thus the power output of the engine. The manifold pressure is our indication of the loss of total pressure over the throttle valve. Close the throttle and the total pressure loss increases, meaning a decreased manifold pressure.
If the load on the engine is small compared to the enabled power output (broken pitch governor, high speed dive with high power and prop on coarse pitch stops, blades broke off…) the power demand will not reach the produced power below the design RPM of the engine. This could mean the RPM increasing until the valve springs can no longer keep the valves out of the way of the pistons*, bearings seize up, ramrods break, piston bolts break etc. Expensive and bad.
I do oppose the view taken above that the RPM setting is a limit. It is a target RPM for the propeller governor. If the actual RPM is above this target, the governor will increase prop pitch (if able) to reduce RPM. If the actual RPM is below this target, the governor will (again if able) reduce prop pitch. In a high speed dive, a propeller may very well hit the coarse pitch stop resulting in a propeller going above the set RPM, just as a propeller will go below the set RPM at low airspeeds and low power settings.
Finally, we are talking real engines in this forum.
*) Or, in the case of certain Italian motorbikes which lift the valves mechanically rather than by springs, the valve stems elongate until valves and pistons meet... happens at a higher RPM but is equally bad.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
LeanOfPeak From United States of America, joined Oct 2004, 509 posts, RR: 1 Reply 11, posted (6 years 6 months 2 weeks 2 days 2 hours ago) and read 21977 times:
Limit vs. target is a semantic argument (And those can be kind of fun at times).
The idea behind the use of "limit" is that, in practice, the governor sets the upper bound of RPM the prop will see, but the prop will easily see less RPM if the throttle setting is reduced to the point where there is inadequate fine pitch to allow the set RPM, so there is no lower bound directly linked to the position of the prop control.
The idea behind the use of "target" is that the governor is always attempting to maintain the set RPM. It may briefly overspeed (Just as a bus governed to 55 might do 60 descending the Rockies) and it may run out of fine pitch control to attain the set RPM, but the attempt is always there.
I do think I prefer "target," but not by much, and it's not a comparison I would have drawn in order to choose one or the other in the absence of this discussion.
FredT From United Kingdom, joined Feb 2002, 2179 posts, RR: 29 Reply 12, posted (6 years 6 months 2 weeks 1 day 21 hours ago) and read 21965 times:
LoP,
as I'm a sucker for semantics, here goes nothing.
Limit
The greatest or least amount, number, or extent allowed or possible: a withdrawal limit of $200; no minimum age limit.
Target
A desired goal.
We are striving to keep the propeller at the set RPM. Above is just as bad as lower. Thus, it is a target RPM. If it was a limit RPM, anything below would be fine (assuming it was an upper limit). In some engines, we do have limit RPMs as well and overspeed governors which will cut in should the propeller overspeed due to e g a faulty prop RPM governor.
Given the right (wrong) conditions, the propeller will hit the coarse pitch stop and overspeed, just as it will hit the fine pitch stop and fall below the set RPM under the right (wrong) conditions. It's the same mechanism in both cases. Most private pilots will only see one of them, but both are there.
Semantics are not just semantics. "The study or science of meaning in language forms". Two different words, two different meanings and two different modes of operation, depending on which you decide on.
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