KAUSpilot
Topic Author
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

Variable Pitch Propellors:

Thu Apr 18, 2002 4:47 am

Hello, I'm a low time private pilot with all my 90+ hours in cessna 172's and 150's. I've been thinking about getting checked out in a 182.

Can anyone explain thoroughly the whole concept of variable pitch propellors and manifold pressure? What is manifold pressure? When do you adjsust the RPM in a variable pitch prop aircraft? What is "feathering" the prop, and when do you do it? What determines the amount of engine power being produced, manifold pressure or rpm's? Are there any good online resources for learning about this topic?

Thanks!
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 5:02 am

Everything you want to know about propellErs and more:

The Pelican's Perch, by John R Deakin.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
ThirtyEcho
Posts: 1409
Joined: Tue Jan 01, 2002 1:21 am

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 5:47 am

I recommend the 182 highly for transitioning to Complex aircraft. I have a lot of time in them and they are very easy to handle. Great instrument airplane, too. Generally, unlike the 172, you can put 4 adults in the seats, some baggage, carry full fuel and still be inside the envelope and legal. The only vices that they have are a weak nosewheel and that built-in headwind. The stick forces are a bit heavier but you'll get used to that in about 15 minutes. It will probably make the 172 less of an airplane in your mind, however.
As for that prop thing, some real tech heads on here to handle that one. Be sure to check the POH for the particular airplane that you are flying to be sure but you should only need to exercise the prop on runup once.
 
KAUSpilot
Topic Author
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 5:57 am

Well I plan to get the complex endorsment in either my schools 182RG or Mooney M20C. I'm really torn as to which airplane I should choose to do training in. On one hand I grew up in the town in which mooney's are made, the mooney cruises faster, and the mooney is 30 bucks cheaper per hour. On the other I already have experience in comparable types to the 182RG, it's about 15 years newer, and it has more horspower so I could get the high performance endorsment. Can you carry four adults with baggage and full fuel in a mooney M20C and remain within CG limits? Anyone have any comments in favor/against these two types?



VS.

 
KAUSpilot
Topic Author
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 6:46 am

PS: thanks for that page FredT, I am in the process of reading through all that information. So far from what I gather the manifold pressure gauge simply reads the pressure in the manifold of the engine (duh). With the engine off this is about the same as atmospheric pressure.

By adjusting the throttle, you allow more fuel/air to flow through the manifold and see an increase in pressure. By closing the throttle, less air is being fed through the manifold and the pressure drops. By increasing the RPM's, you also get a slight drop in manifold pressure because the faster the cylinders "pump" the stronger the vaccum they create. The latter is simply a side-effect of RPM adjustment from what I understand.

All in all, I suppose you could summarize that manifold pressure is used to monitor the amount of fuel air mixture being fed to the cylinders. The angle of attack of the propellor must be lowered if the manifold pressure is reduced (the engine is working with less fuel to produce the same RPM) and the AOT must be increased if the manifold pressure is increased

Please read over this and tell me if I got anything wrong.
 
ThirtyEcho
Posts: 1409
Joined: Tue Jan 01, 2002 1:21 am

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 8:15 am

I'd bag the Mooney. There is no step up like there is to the C210, which is a real travellin' machine. I don't know about the CG thing on a Mooney but my bet is that it doesn't haul what a C182 can; plus, when you are in the Bahamas and want to rent an airplane for some sightseeing, betcha can't find a Mooney.
As for the manifold pressure gauge, did you realize that it is also a standby altimeter? Ask your instructor.
 
Ralgha
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 8:51 am

Plain 182s are not complex, though they have a constat speed prop. As to all those questions, I've said it before, and I'll say it againj, and I'm going to keep saying it until people learn.

Ask your instructor. Bad information kills, and you get a lot of it here.
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
 
bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 10:15 am

That is a very informative website. I suggest the reader to take a look! Worth your time.
-bio
 
sccutler
Posts: 5567
Joined: Thu Jan 27, 2000 12:16 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 12:13 pm

Go for the Mooney; only wimps need electricity to retract the gear...

...is it that old? Or retrofitted?

Mooneys are cool.
...three miles from BRONS, clear for the ILS one five approach...
 
ThirtyEcho
Posts: 1409
Joined: Tue Jan 01, 2002 1:21 am

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 12:35 pm

About the gear thing, we could take that all the way and say that only wimps need electricity to start the engine. I do love mechanical flaps, though; you can just hold the detent button in and use them up and down like infinitely variable mechanical spoilers. That is great going into short fields with tall trees at the end of the runway.
You are going to hate the laminar flow wing on the Mooney. If you fly Mooneys you will never make a good landing again.
 
cosync
Posts: 635
Joined: Sat Nov 17, 2001 7:53 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 12:42 pm

ok firstly im just a knowledgable flight simmer who reads alot of aeronautics book. and im only 15 aswell Smile
I thought that a variable pitch proppeler is just the same thing as a constant speed proppeler.
one that uses a governer to change the pitch of the blades? or is there a type of prop called variable pitch where u directly change the pith mechanically and then put the throttle up to maintian RPM?
 
KAUSpilot
Topic Author
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Joined: Mon Jan 28, 2002 2:15 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 12:59 pm

Variable pitch and constant speed mean the same thing when you're talking about propellors as far as I know.

The propellor maintains a constant speed (RPM) regardless of power settings by varying the angle of attack of the blades (pitch). This is all only to a point, however...

For instance if you set the RPM's relatively low, use a fairly high power setting, and then begin a descent, the angle of attack of the propellor blades can only increase so much before before the effects of windmilling cause an uncommanded increase in RPM.

One Thing that bothers me about Manifold pressure:

"The rule: Manifold pressure depends on ambient pressure, the position of the throttle plate, and the speed at which the pistons are moving up and down. Manifold pressure does not indicate "power," unless other things are taken into account. "

Really, how useful is MP if it isn't an accurate reading of engine power? Isn't that the whole point of having the MP gauge in the first place? The author of the article implies that the suction action of the pistons has a very significant effect on the mp reading, stating that when the pistons start pumping slowly at low rpm, the suction effect dissipates and the MP begins to creep towrad ambient atmospheric pressure.

I would guess that the operating procedures for most airplanes take this "piston suction" into effect when specifying reccomended manifold pressure setting for various flight configurations, but I suppose different manifold pressure readings needed to obtain a given power setting will vary day to day depending on the weather. I definitely need to discuss this with my CFI.
 
cosync
Posts: 635
Joined: Sat Nov 17, 2001 7:53 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 2:28 pm

jeesus kauspilot.
u know ur shit dont yah.
id just forget about it and trust it. who cares. just keep it in the green Smile take it for granted Smile
 
flyf15
Posts: 6633
Joined: Tue May 18, 1999 11:10 am

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 3:30 pm

Cosync, It is very good and important to learn about how these types of things work. "Just keep it in the green" is a mentality that will get you killed, if you ever fly a real airplane. I also fly a plane with a constant speed prop (Cessna R172K...aka Hawk XP). The whole idea of manifold pressure, prop setting, etc can be quite confusing when you're just beginning to fly constant speed prop aircraft or have not yet flown them and it is really good that KAUS is trying to find out what they're all about.

-----

KAUS, you'll find there are quite a few "rules of thumb" for operating constant speed props. Such as the square rule and multiple cycling of the prop during runup to flush oil in and out of the prop hub. You'll also find that only some of these rules of thumb are applicable to the aircraft you fly even though they are all often taught as fact. Quite a bit of it has been passed down from the days of flying B-17s and P-51s and just doesn't apply anymore. Be sure to look in your POH for whatever aircraft you fly and/or talk with a qualified and knowledgable instructor about how to properly operate the type.

Moving to higher performance aircraft can be quite a thrill. It'll make your basic 172s seem like less of an airplane and make it so you hate flying a 152  Big thumbs up
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Variable Pitch Propellors:

Thu Apr 18, 2002 4:47 pm

KAUS,
yes, MP depends on ambient pressure and RPM. However, you set an RPM for your current phase of flight so that one is eliminated from the equation. Then there's a recommended MP setting for various combinations of RPM and altitude, giving you the power setting you want and there you are.  Smile

When the MP gauge starts going towards ambient due to low RPMs, there's that infamous deafening silence up front that's probably a bigger worry by then.  Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: Variable Pitch Propellors:

Fri Apr 19, 2002 5:14 am

The reason for Constant speed props (variable pitch is thus....

A fixed pitch propeller is a compromise in efficiency. The angle of attack of the blade which is fixed is either a climb prop, cruis prop.

fixed pitch props are a compromise of efficiency and only designed to operate in a certian range. A climb prop will not cruise well... and a cruis prop will not climb well.

Hence the invention of the constant speed prop. The angle of attack of the blade can be changed for different phases of flight.

Low pitch/high rpm for takeoff/climb.

High pitch/low rpm for cruise.

Manifild pressure (which is actually vacuum NOT pressure) measures the vacuum pressure in the manifold as a measure of power. The lower the vacuum the more power the engine is making.

The variable pitch prop is an infinetly variable transmission. You dont drive your car on the highway in first gear at 6,000 RPM do you? No you use 5th. Inversly you dont start your car in 5th gear at 1,000 rpm.

In a light twin at 500 ft the manifold pressure is lowerd to 25 inches of mercury and the props are pulled back to 2500 rpm... known as 25/25 climb.

However the proper procedure would be to go to the POH and get the optimum climb power/propr RPM.

The next time the power/prop combination is changed is at TOC...(Top of Climb) again power and pitch are per POH for ambient conditions. It is also dependant on economy or performance cruise.

Prop levers should always be ahead of the power levers as to not cause a prop overspeed condition. Power is always adjuted before prop speed when reducing power and vice vera for increasing power.

Hope this helps

JET
 
Dufo
Posts: 796
Joined: Tue May 18, 1999 2:41 am

RE: Variable Pitch Propellors:

Fri Apr 19, 2002 7:03 am

I'm about to start flying the Seneca (II) next week. Do you have any experience/hints for this twin, JETPILOT (or anyone else)? I have about 15 hours on Pa28r-200, not including everything else.

Regards,
Jernej
I seriously think I just creamed my pants without any influence from any outside variables.
 
TechRep
Posts: 1877
Joined: Sun Jan 13, 2002 6:53 am

RE: Variable Pitch Propellors:

Thu Apr 25, 2002 12:51 am

Check this out.



TechRep
 
cosync
Posts: 635
Joined: Sat Nov 17, 2001 7:53 pm

RE: Variable Pitch Propellors:

Thu Apr 25, 2002 1:01 am

too confusing tech rep.
i have no idea wot is going on in that diagram.
 
XFSUgimpLB41X
Posts: 3960
Joined: Fri Aug 25, 2000 1:18 am

RE: Variable Pitch Propellors:

Thu Apr 25, 2002 2:56 am

Cosync- that's a turboprop schematic for the prop hydraulic system- but regardless it is still a constant speed/variable pitch prop.

What airplane is that for? A Dash 8?
Chicks dig winglets.
 
bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: Variable Pitch Propellors:

Thu Apr 25, 2002 5:35 am

lol, I'm lost too with that diagram! Though pretty

What I don't really understand is why and how does RMP increase by changing the blade angle. Can someone explain? And if RPM changes, how can the propeller be 'constant speed'? Maybe it's too obvious but I simply don't get the picture.. And what are "low pitch" and "high pitch" settings referenced to? (horizontal axis, blade AOA, plane AOA...)
-bio
 
vc10
Posts: 1342
Joined: Thu Nov 15, 2001 4:13 am

RE: Variable Pitch Propellors:

Thu Apr 25, 2002 6:02 am

Bio15,
I can only suggest you look up the web site recommended by FredT at the start of this thread it is real easy to read and should make every thing clear.
Good reading little vc10

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