Author Topic: Propeller questions  (Read 4871 times)

Offline Frank N. O.

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Propeller questions
« on: April 09, 2007, 04:30:46 AM »
I was wondering about this and after a forum-search didn't quite seem to show it as already asked I thought I'd jump in head-first and ask in search of knowledge.

Now I know that a propeller, like a rotor, is basically a wing and therefore I was wondering if it could stall so you'd loose propulsion? Maybe due to a hard manvouver?

How big a selection is there for replacing propellers on a certified aircraft (like there are various suspension-parts and wheels etc. for cars). Maybe a carbon-fibre propeller that's lighter and could therefore perhaps improve performance, reliability and decrease torque?

How much does the nose-cone help aerodynamics btw? Some old planes didn't have one, some have ones that are cone-shaped while others are rounded. Does it depend on the shape of what's behind the propeller which cone-shape is the best? For instance if there are intakes near the propeller or if it's a wide body or a narrow engine-cowling that's behind the prop?

Frank
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Offline Africaspotter

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Re: Propeller questions
« Reply #1 on: April 09, 2007, 06:01:45 AM »
One way of stalling a prop would be to stop it.

Felix
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Offline Thunder Eagle

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Re: Propeller questions
« Reply #2 on: April 09, 2007, 04:16:26 PM »
Jet compressors can stall: http://en.wikipedia.org/wiki/Compressor_stall .  I believe this is what ultimately led to Maverick losing control of his F-14 in Top Gun.   ;)  As for normal propellers, the only case I can think of where they would stall is with variable-pitch props, especially when feathering.
« Last Edit: April 09, 2007, 05:35:29 PM by Thunder Eagle »

airtac

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Re: Propeller questions
« Reply #3 on: April 09, 2007, 04:19:33 PM »
Interesting questions Frank ::thinking::
I have never thought much about whether a prop can stall, it's true that they are an airfoil and there is a point of angle of attack when any airfoil stalls but a fixed pitch prop, for instance, does not change it's angle of attack and a full feathering constant speed prop would only reach that critical angle in the process of being feathered.  A propeller can be made to lose efficiency due to overspeed when the tips go supersonic.   Undoubtedly, extreme maneuvers like tumbling change the relative wind direction momentarily which I suppose could be considered as stalling the prop---I've never really thought about it, wasn't something I considered important. :-\

There are a number of composite props on the market and there are more being designed, they do claim to increase engine response and power due to lighter weight and more efficient shapes.

Prop spinners serve the purpose of smoothly directing the airflow over the prop hub and yes the exact shape is determined by where the designer wants the air to go.   A lot of older aircraft didn't use them because there was not enough aerodynamic advantage at slow airspeeds to warrant the expense or effort to design and install one.

I'm sure the experts here can correct or add to that ;D

Cheers, Jim



Offline TheSoccerMom

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Re: Propeller questions
« Reply #4 on: April 09, 2007, 05:47:58 PM »
Frank,

I have been mulling over your questions, but was too dumb to figure out how to put my incredible level of ignorance into words.  Jim summed it up perfectly (big surprise there huh).   ;)   I was thinking that yes, hypothetically, the prop could stall but I don't know exactly how and when and where on the structure that might happen....  not being an engineer.  When you look at the prop blades, the angle is constantly changing from the hub out to the tips, to compensate for the difference in speeds from the hub out to the tips.  Near the hub, great angle, further out, finer angle, because the distance traveled by the tips is so much greater.  So...  an airfoil, yes, but one where the centers of lift are constantly moving, I assume.  Again -- Jim summed it up best, I think.  I wish I was an expert aerodynamicist so I could sound all brainy.

I, too, thought back to when the engine fails, you decrease the drag of the windmilling prop by slowing the airplane enough to prevent the airflow from keeping it windmilling -- you "stop the prop".  Then you resume your best glide speed.... (and lean over to your buddy with a grin and say "YOUR AIRPLANE"  ha ha just kidding).

And spinners on slow airplanes are probably like fancy mods on your Model T...  I mean, if you're chugging along in your 65 HP machine, you probably don't give a hoot about whether you even have a spinner.  I can tell you, though, a spinner that's screwed on just a hair off "center" will drive you crazy when you watch it -- it looks like it is wobbling like crazy.  At least that is what my psychiatrist said happened to me.    :D

 ;)   
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Offline TheSoccerMom

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Re: Propeller questions
« Reply #5 on: April 09, 2007, 05:55:19 PM »
And I KNOW this isn't strictly in tune with the thread here (there's that darned ADD again Stef!!), but there was an air attack pilot a few years back whom I met while "standing by".  He was a retired airline captain, and had started in the airlines when it was strictly a prop world..  DC-4s, -6s, -7s, Connies...  you get the drift.

He claimed that during a ground delay somewhere, he had to answer the stewardess' question as to why the delay, and he told her basically "Well, we flew quite a ways to get here, so we need to wait while they wind the propellers back up.  It'll be a while yet."  He SWORE that she fell for it, and that everafter, she always checked that the "Props were wound up enough" to depart. 

Needless to say, every day after that, he always asked me if my props were "really ready", if we got a fire call.  He was a pretty funny guy and I admit, I believe the story..........    ;D       ;D       ;D
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Offline Baradium

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Re: Propeller questions
« Reply #6 on: April 09, 2007, 06:09:25 PM »
Speaking of prop airliners...   Is the only diference in a DC-6 and a 7 larger engines and a tail?  (which I would assume larger engines might imply a larger payload as well).

One of the few remaining DC-7s is sitting here on the ramp in Fairbanks (it's also owned by Brooks Fuel) and they are supposedly going to put it back in service at some point (they are very slowly working on it).  They still are regularly flying the twin sister to the downed DC-4.  The Carvair hasn't been flying a little while and has a "hut" around the #1 engine, reckon they are doing something with it (wow, gee, that might explain the reason for putting the thing around that engine huh?).

Also speaking of compressor stalls, I've had 3 of those since I've been here.


The T-41 I did my high performance in didn't have a spinner on it... made it look meaner.  That was a fun airplane to fly.
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Offline spacer

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Re: Propeller questions
« Reply #7 on: April 09, 2007, 11:08:59 PM »
Spinners are generally aerodynamic in nature, and also help smooth airflow into the cooling ports in the cowling, where provided. They help somewhat even in smaller aircraft, though not quite as much. Variable pitched props may enjoy some added protection of their hub mechanisms as well.
One thing, though... those things are freakin' expensive for a simple aluminum cone. I have two sitting in my garage: one from a DA20, the replacement of which cost $900. This plane is equipped with a fixed wooden prop, nothing special up front. The other is from a Beechcraft Duchess, the replacement of which cost (from what I was told) $8000.
The latter cost more than the value of my pickup truck!!!


Offline tundra_flier

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Re: Propeller questions
« Reply #8 on: April 10, 2007, 07:50:53 PM »
As for props, some of the new composite props used by homebuilts and other experimentals would certainly help the performance of a lot of certified planes.  The problem is the certification.  For example, I think my Cessna 150 is only approved for 4 props, all solid aluminum, all 69 inches.  Just 2 different pitches each of McAully and Sensenich.  The cost of getting a composite prop certified or STC'ed for the plane is just too expensive to be worth it.

Phil

Offline Frank N. O.

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Re: Propeller questions
« Reply #9 on: April 13, 2007, 12:33:07 AM »
Thank you all for your replies, very informative, and soccermom that was one great story  ::rofl:: I also do seriously believe that some people might believe that, at least earlier in time if they haven't had any real exposure to engines etc. (note I didn't specify a gender since I'm sure it's not a factor).

Baradium: Remember to post the pictures and data on your T-41 like we talked about in the chat yesterday so this hot-rod can be shown to the world :)

Frank
« Last Edit: April 13, 2007, 12:35:20 AM by Frank N. O. »
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Offline happylanding

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Re: Propeller questions
« Reply #10 on: April 20, 2007, 09:00:05 PM »
Really interesting question Frank, indeed!  |:)\

http://www.diracdelta.co.uk/science/source/p/r/propeller/source.html

I would never be unable to say it in other words, and I'd probably create new physics laws. I presume that article can answer your reply!  :)
I give that landing a 9 . . . on the Richter scale.

Offline chuckar101

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Re: Propeller questions
« Reply #11 on: April 20, 2007, 10:54:22 PM »
I'd have to agree with airtac.  Since the relative wind for the propellar never really changes thoughout normal maneuvers.  And on the aerobatic aircraft that tumble backwards there has to be some law or test showing if you spin the prop fast enough it will make it's relative win and counteract the rw from falling backwards.  I'm going to stop now before a start sounding really wrong.
WOW I did that!

airtac

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Re: Propeller questions
« Reply #12 on: April 21, 2007, 02:10:47 AM »
Speaking of prop airliners...   Is the only diference in a DC-6 and a 7 larger engines and a tail?  (which I would assume larger engines might imply a larger payload as well).

Longer fuselage too............