Propeller acts as brake?  
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Propeller acts as brake? - 3/22/2008 5:30:22 AM   
gaRCfield


 

Posts: 2107
Joined: 5/20/2008
From: Burlington, VT, USA
Status: offline 		Hello,
I'm assembling my first plane, Hobbico Nexstar, and I noticed in the instruction manual that it uses the 11x5 prop for several reasons, one being that it 'acts as a brake when the nose is pointed down."

How does that work?

Thanks.

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RE: Propeller acts as brake? - 3/22/2008 6:05:21 AM   
wellss


 

Posts: 73
Joined: 6/14/2002
From: St. Catharines, ON, CANADA
Status: offline 		The pitch ( 5" ) is the theoretical forward advance for each revolution. If the airplanes airspeed is higher than the theoretical advance, the prop blade will be operating at a negative angle of attack, effectively making reverse thrust. So, in your Nexstar example (nice plane btw, I love em as trainers), if your engine is idling at a reliable 3000, the advance is 15000 inches / min, or 14 MPH. Your plane probably makes a landing approach at 25-30 mph, so you have a braking effect, allowing for a steeper angle without picking up speed. I know these airplanes can maintain altitude on 1 click of throttle, so they're hard to get down sometimes. A stiff headwind helps too...

(in reply to gaRCfield)
       Post #: 2

RE: Propeller acts as brake? - 3/22/2008 6:24:06 AM   
Campgems


 

Posts: 2181
Joined: 2/18/2006
From: Arroyo Grande, CA, USA
Status: offline 		First, it assumes low or idle speed. If the engine it full bore opened, it's just going to go faster.

A rough example is the comparison between the spiral shaft in a childs top. You push down on it and it turns the top. That shaft is a screw with a very course pitch. Enough of a pitch that it will impart a truning movement when forced straight through the threads. Now compare that to a screw and nut from the hardware store. In comparison, you have a very fine pitch and no amount of pushing will cause the nut to turn.

If we compair that to the props, the finer the pitch the less the wind trying to push through it will be able to turn it. So, using the abouve example, a courser pitch will tend to windmill more than the finer pitch. It' all mechanical advantage. The fine pitch will allow the motor to transfer more enegery to move the plane, but not a fast. The course prop takes more enegery to get to the same rpm but will move the plane faster. The fine prop will not transfer the rush of air past it to rotation of the motor so it acts as an air brake. The courser pitch will feed rotation into the motor and make it dificult to get to the low rpm to act as an air brake.

Comparing a 11x 5 to an 11x6 would not result in any noticable difference in braking power. Comparing a 11x3 to an 11x9 though would be noticable.

I tend to fly the low pitch props on slow planes. I need to watch my landing speed as I can easilly go to slow to quickly and stall them. The higher pitched props will result in less braking, but also be faster, so which is better. Untill you get to the point that you can tell the difference, go with what was recomended for your setup. It will be the best compromise. After you get a few hours under your belt, you can start expermenting. If you are anything like I was first starting out, I got the chance to buy a lot of props, so expermenting with size and pitch wasn't very costly, well additionaly cost anyway. In my first couple months, a good day was one that I still had a spare prop left when I went home.

Don.


(in reply to gaRCfield)
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RE: Propeller acts as brake? - 3/22/2008 3:37:28 PM   
Rodney



Posts: 4998
Joined: 12/8/2001
From: FL
Status: offline 		This effect can be quite noticable. Land your plane with the engine at idle and compare how it floats compared to landing the plane with the engine off(deadstick). You will see that the plane has a flatter glide and requires a longer approach with the engine off than with it idleing.

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RE: Propeller acts as brake? - 3/22/2008 5:16:06 PM   
da Rock



Posts: 7007
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		Keep in mind that you might not see this braking depending on your engine and it's idle speed setting.

I got a buddy who really struggles with landings. He also isn't very confident of his engine management skills, and almost always has faster idle settings. He is afraid of deadstick landings. So he winds up with airplanes that don't slow down for landing worth spit. And his downlines are all on the fast side. Last new airplane, I got to land the sucker because it wouldn't slow down. It wouldn't. Turned out an engine that ought to be able to tick over on idle was doing about 2,000. Think it had 8" pitch.

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RE: Propeller acts as brake? - 3/22/2008 5:35:57 PM   
Jim Thomerson



Posts: 2889
Joined: 10/9/2002
From: Austin, TX, USA
Status: offline 		As a side note, for a while (haven't checked lately, the records are published in MA from time to time) The maximum speed record for a model glider was faster than the speed record for a powered airplane. Way this is done is that you climb the airplane way up then dive it and level out through a speed trap. Part of the reason was that the prop, even a high pitch prop on a racing engine, was acting as a brake in the dive. Also there were some problems with how the Russians were timeing the speed runs.

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       Post #: 6

RE: Propeller acts as brake? - 3/22/2008 6:03:54 PM   
vertical grimmace


 

Posts: 1286
Joined: 3/13/2004
From: ft collins , CO, USA
Status: offline 		I think the speed record has been set by Dynamic soaring akin to slope soaring. I have seen these going around 300mph.
One note, I am instructing a friend with this Nextstar aircraft and it did not fly well until I removed the wing droops and I never installed the brakes in the first place. Also, IMO the prop that comes with the plane is junk. I do like running low pitch but I think a standard APC 11-5 would work better. I put a 10-6 on his plane and it works perfectly.
Also speaking tuning engines, the needle restrictor on this engine, needs to be removed. I turned the needle in 1 full turn beyond where the restrictor would have it stop. It is impossible to have a well flying plane if the engine is not tuned properly. It is just as important not to be too rich as it is too lean. An engine will never idle and advance properly if too rich.

< Message edited by vertical grimmace -- 3/22/2008 6:04:41 PM >

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RE: Propeller acts as brake? - 3/22/2008 6:26:10 PM   
Campgems


 

Posts: 2181
Joined: 2/18/2006
From: Arroyo Grande, CA, USA
Status: offline 		Those needle restrictors are a royal pain. I've had a couple guys come out to the field with them on and ask for help tuning the engine. You just can't get a proper tune with them on. The engine will run, but that's about it. A couple minutes with an allen wrench and all is well with the world. If it blue and on the carb, take it off.

Don

(in reply to vertical grimmace)
       Post #: 8

RE: Propeller acts as brake? - 3/22/2008 8:56:49 PM   
gaRCfield


 

Posts: 2107
Joined: 5/20/2008
From: Burlington, VT, USA
Status: offline 		I got the Nexstar ARF, so I wasn't planning on putting the restrictor on.

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RE: Propeller acts as brake? - 3/22/2008 10:40:33 PM   
vertical grimmace


 

Posts: 1286
Joined: 3/13/2004
From: ft collins , CO, USA
Status: offline 		The restrictor comes already installed. You have to remove it if you do not want it on the engine. (which is highly recommended.) Unless you are getting the version that does not come with an engine. If you already have an engine other than an Evolution, then you do not have to worry about it. I guess I just assumed you had the complete ARF with engine and radio.

< Message edited by vertical grimmace -- 3/22/2008 10:43:04 PM >

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RE: Propeller acts as brake? - 4/6/2008 4:45:54 AM   
gaRCfield


 

Posts: 2107
Joined: 5/20/2008
From: Burlington, VT, USA
Status: offline
quote:

ORIGINAL: vertical grimmace

The restrictor comes already installed. You have to remove it if you do not want it on the engine. (which is highly recommended.) Unless you are getting the version that does not come with an engine. If you already have an engine other than an Evolution, then you do not have to worry about it. I guess I just assumed you had the complete ARF with engine and radio.


ARF is just the plane, no radio or engine. RTF comes with radio and engine (OS engine, Futaba radio). I got the OS46AX separately, so there shouldn't be any needle restrictor, although one came in the hardware kit with the plane, but it's staying there.

_____________________________

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RE: Propeller acts as brake? - 4/6/2008 7:31:41 AM   
BMatthews



Posts: 9052
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		I would not be surprised if dynamic soaring gliders are not faster than the record. But to achieve a record flight the model must fly through the properly acredited speed timing traps and that just doesn't fit in with how a dynamic soaring flight takes place. So while they may be fast it's not a record.

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RE: Propeller acts as brake? - 5/28/2008 10:20:12 AM   
davidej


 

Posts: 63
Joined: 11/6/2002
From: West Mersea, UNITED KINGDOM
Status: offline 		Been reading this thread and saw the often-stated claim that a motor at tickover provides more drag than when deadstick.

I find this counter intuitive. It would need a change of aerodynamic regime over the prop at some point to be true.

had anyone got any real evidence to support it or a sound theoretical arguement why it should be so. I find these "you will notice when ...." statements totally lacking in any real proof.

Incidentally yachtties continually argue as to whether you shoud sail with the propeller windmilling or whether you should apply the shaft brake, so they have the same problem.

Any help would be appreciated.

David

< Message edited by da Rock -- 5/28/2008 11:20:38 AM >

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RE: Propeller acts as brake? - 5/28/2008 11:51:05 AM   
da Rock



Posts: 7007
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		It's rather well documented that there was significant drag from freewheeling props on battle damaged engines in WWII. Unfortunately, that situation does not directly parallel ours. They had the capability to feather their props. A feathered prop is quite a different animal than a stopped prop oriented to an operating angle.

It's quite possible that the electric powered glider guys will be the ones most apt to have test data on the issue. They have engine braking that was created to stop freewheeling. It's for use with and without folding props. So there was reason for them to sort out the value of it in just the situation we're interested in.

I remember seeing mention made of the drag of a freewheeling prop versus stationary in modeling literature but not with supporting notation. So it's an issue that may be based predominantly on opinion in our hobby. Opinions are quite a bit more valuable than nothing at all. And are usually offered to help in the face of nothing better being available.

I fly electrics. Some have folders. Some don't. One has had a folder and not had a folder. Darned if I could detect that one or the other had less drag.

I have also read that the drag of a windmilling prop is greater because it's actually producing more lift that it would fixed. That didn't seem realistic but that's all there was, no supporting notation.

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RE: Propeller acts as brake? - 5/28/2008 1:38:26 PM   
da Rock



Posts: 7007
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		I plugged a thread into one of the Electric forums asking about this detail of fixed vs freewheeling. We'll see what the guys who might need to know, know.
http://www.rcuniverse.com/forum/m_7550340/tm.htm

(in reply to da Rock)