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Propeller acts as brake?

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Propeller acts as brake?

Old 03-21-2008, 11:30 PM
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Jetdesign
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Default Propeller acts as brake?

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.
Old 03-22-2008, 12:05 AM
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wellss
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Default RE: Propeller acts as brake?

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...
Old 03-22-2008, 12:24 AM
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Campgems
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Default RE: Propeller acts as brake?

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.

Old 03-22-2008, 09:37 AM
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Rodney
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Default RE: Propeller acts as brake?

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.
Old 03-22-2008, 11:16 AM
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da Rock
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Default RE: Propeller acts as brake?

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.
Old 03-22-2008, 11:35 AM
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Jim Thomerson
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Default RE: Propeller acts as brake?

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.
Old 03-22-2008, 12:03 PM
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Default RE: Propeller acts as brake?

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.
Old 03-22-2008, 12:26 PM
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Campgems
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Default RE: Propeller acts as brake?

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
Old 03-22-2008, 02:56 PM
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Default RE: Propeller acts as brake?

I got the Nexstar ARF, so I wasn't planning on putting the restrictor on.
Old 03-22-2008, 04:40 PM
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Default RE: Propeller acts as brake?

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.
Old 04-05-2008, 10:45 PM
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Default RE: Propeller acts as brake?


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.
Old 04-06-2008, 01:31 AM
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Default RE: Propeller acts as brake?

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.
Old 05-28-2008, 04:20 AM
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Default RE: Propeller acts as brake?

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
Old 05-28-2008, 05:51 AM
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da Rock
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Default RE: Propeller acts as brake?

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.
Old 05-28-2008, 07:38 AM
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da Rock
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Default RE: Propeller acts as brake?

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
Old 05-28-2008, 08:29 AM
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Default RE: Propeller acts as brake?

Freewheeling props have much more drag than a stopped prop. In the past, I had several glow plugs blow out of crystalized heads in Formula One airplanes, so the engine freewheeled on glide. Lost about half the glide distance with the props turning.

I'm not surprised that the mixture on a model engine would be way off in the rockies vs. an engine setup for sea level.
Old 05-28-2008, 12:21 PM
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Default RE: Propeller acts as brake?

I am afraid that is the usual anecdotal evidence without any science or measurement to back it up

If it really is try - WHY?
Old 05-28-2008, 12:23 PM
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Default RE: Propeller acts as brake?

Sorry I meant true not try
Old 05-28-2008, 01:25 PM
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wellss
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Default RE: Propeller acts as brake?

It's true because the propeller is at negative AoA. It's geometry. When the propeller is turning and the airplane is moving forward, the propeller scribes a helix through the air.

Take a point on the propeller blade, say the tip. Using the original example of 11 x 5 propeller...

With the RPM at 3000, the tip speed is 98 mph, right?

We'll say the airplane is moving 25 mph forward, so now the helix angle is

tan(helix) = forward speed / tip speed
= 14.3 deg

The geometric twist angle of the propeller blade ( relative to the chord line ) can be figured by knowing the distance that the tip travels in one revolution ( pi x diameter ), compared to the pitch ( the advance for one revolution ).

tan( twist ) = pitch / circumference
= 8.23 deg

Since the twist angle is less than the helix angle, the angle of attack is negative and the propeller is making reverse thrust, or acting like a brake. If you were to reduce the helix angle to less than the twist angle, say by sitting static on the ground. Then, the propeller makes forward thrust.

Likewise, the helix angle is reduced when the RPM is increased, so by going to full throttle at 25 mph, then the propeller makes forward thrust again. It probably starts pulling at 5000 RPM or so at that speed.
Old 05-28-2008, 02:14 PM
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Default RE: Propeller acts as brake?

I totally agree that a slowly turning propellor generates drag and your explanation for it BUT and I am sure you could see this coming


if the prop is stationary then the negative AoA is greater- so why isn't the drag greater? If the drag does decrease when the prop is stationary, then you must theorise that at some point there is a change of aerodynamic regime over the prop. At what RPM does that happen 1000 RPM?- 100RPM? - 10 RPM? -1RPM?


I have never had an answer to this and think that is is one of those assertions that everybody believes is true because everyone says it is.

Frankly, I don't
Old 05-28-2008, 02:46 PM
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Default RE: Propeller acts as brake?

taken from http://www.djaerotech.com/dj_askjd/d...mill_prop.html

The blades of the stopped (but NOT feathered) prop will have the chords almost perpendicular to the airflow. Their drag will approximate that of a flat plat that size, perpendicular to the airflow. This drag could be very high. If the prop is spinning freely, the angles of attack along the blades are very low and so is the drag, as long as airspeed and rpm are kept as low as possible.
There is quite a long bit of discussion in that link. But what applies to our situations can be summed up in the above quote.
Old 05-28-2008, 02:53 PM
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da Rock
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Default RE: Propeller acts as brake?

It appears that it very much matters what the pitch of the prop is, how fast it freewheels, and what it's chord is.

The drag from the stationary prop is primarily a function of it's frontal area. There will be a slight additional induced drag from the shape generating negative lift. After all, even "backwards" it's an airfoil encountering airflow at an AOA.

The drag from the same prop freewheeling would be induced drag created by the lift generated from a greatly less AOA. The amount of drag should be abundantly apparent from observation of the rpm.

BTW, in the old days, rubber powered freeflight models almost always had freewheel devices that came in when the rubber played out. hhhhmmmmmmmmmmmmm
Old 05-28-2008, 03:19 PM
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Default RE: Propeller acts as brake?

But I am not talking about windmilling propellors which I think will have the least drag.

We are arguing whether a slow turning prop on a power plane at idle has more or less drag than a stationary prop with a stopped engine.I have yet to see any convincing evidence or adequate explanation as to why it should have more drag.

Most people would think the drag increases with difference between the actual airspeed and the speed implied by the helix of the prop at the RPM which it is turning. This difference is greatest if the prop is stopped. If the drag is actually less with a stationary prop, then at what speed of rotation does it suddenly become less and why?
Old 05-28-2008, 03:43 PM
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wellss
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Default RE: Propeller acts as brake?

Ah, I see what you're saying.

The AoA is greater with the prop stationary, but the airspeed isn't. The drag ( or reverse thrust, however you wanna look at it ) is going to vary with speed squared ( in this case RPM^2 ). At some point, RPM is going to dominate. We know that airfoils make their greatest lift at some angle ( 10-20 degrees or so ) and with the greatest airspeed. Beyond that angle, lift is lost as the wing is stalled. Then, if you further reduce airspeed, you lose even more lift. This is the same thing that is happening to the propeller blade as it slows down and the angle of attack goes beyond 10-20 degrees negative, on its way to 90 degrees, where it is stationary. That's the best way I can explain it. If you want to work through the calculations yourself, try this page...

http://www-mdp.eng.cam.ac.uk/library...ler/prop1.html
Old 05-28-2008, 05:45 PM
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da Rock
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Default RE: Propeller acts as brake?


ORIGINAL: gaRCfield

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.

When the throttle is reduced quickly enough, the prop will change from producing thrust to creating drag. It is basically a propeller that creates lift when rotated. As the airplane accelerates, the speed of rotation must increase to continue to accelerate the airplane. When the speed of rotation ceases to produce acceleration, the speed stabilizes. If that speed of rotation is then decreased, the prop causes deceleration.

http://www.djaerotech.com/dj_askjd/d...mill_prop.html explains in detail the causes of that deceleration. Which are linked to the pitch of the prop, it's blade area, it's speed of rotation, the initial speed of the airplane, the drag of the airplane, the percentage of cloud cover, the density altitude, the number of spectators and a billion other things.

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