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More drag - prop spinning at idle or dead stick?

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More drag - prop spinning at idle or dead stick?

Old 07-18-2008, 10:55 AM
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insalacosm
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Default More drag - prop spinning at idle or dead stick?

My club acquired a new field that has a very small runway which requires any plane larger than 40 size to be slowed down to a crawl to land to prevent rolling over the berm and into the weeds. Large trees prevent low approach angles which further compounds the problem. Since I'm not yet comfortable side slipping my plane into a landing, I've been killing my engine on approach in an attempt to slow the plane as much as possible and not overshoot the runway. One of the guys at the field yesterday thought that by allowing the prop to spin at idle, I could slow the plane more than by killing the engine.

I did a few calculations and a prop with a 6" pitch (I'm using an APC 13x6) spinning at 2k RPM moves forward at a speed of 11.4 mph. Since this is less than my landing speed which is probably 15 mph, I'm thinking he may be onto something...

Any thoughts on this?

Thanks,
Steve



Old 07-18-2008, 11:08 AM
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Nathan King
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Default RE: More drag - prop spinning at idle or dead stick?

A prop spinning at a nice low idle will actually create more drag than a stopped prop.
Old 07-18-2008, 11:25 AM
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Default RE: More drag - prop spinning at idle or dead stick?

A spinning prop is less drag. The props of rubber band powerd models are made to let the prop spin. If you ever have one where the prop gets stuck, you will know it doesn't glide very well at all. If you land dead stick with a large prop you might notice that it's easy to land short because the prop has more drag than at idle. If the idle is too fast you might have trouble getting the plane slow enough to land.
Old 07-18-2008, 11:48 AM
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Default RE: More drag - prop spinning at idle or dead stick?

Insalacosm, you answered your own question with the number you calculated. When landing at 15mph, if the prop is making a slipstream speed calculated at 15 mph its like its not even there. As the prop slows down to less than 15mph of prop wash the plane will slow as the prop acts as an airbrake on the plane. If the prop speeds up past the point where it moves air at more than 15mph then the plane will accelerate.

I've heard the best way to make a plane land in a short distance is flaps or spoilers though I have no experience with either. There's an active thread on the subject in this forum that you may find interesting.
Old 07-18-2008, 12:47 PM
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Default RE: More drag - prop spinning at idle or dead stick?

A Spinning prop causes more drag than a stationary prop.

If you really want to slow it down fit a 3 or 4 blade prop, when back off it's like hitting the brakes.

Combine that with flaps and you can land most pretty short.
Old 07-19-2008, 12:39 PM
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Campgems
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Default RE: More drag - prop spinning at idle or dead stick?

I like the APC wide sports series, IE the 12 1/4 x 3 3/4 is a great prop for both hauling you off the runway and hitting the brakes when needed. There are a few others, the 13x4W ia another good one if you engine can swing it.

If your radio will allow it, setup up air brakes. Full up ailerons and slow up elevatro to full travel. This requires a flaperon setup. A couple of the guys at our field fly 33% birds and use the air brake setup for stopping. It lets them keep the air speed up until they touch down then hit the skids

We got a couple guys flying jets also and they have real brakes, It is interesting how fast they land and how quick they stop.

Just don't try to slow down to much on landing, I've crunched more than my share making a nice slow approach and the wind suddenly dies and I stall. Flying to close to the stall speed isn't good until the wheels are on the ground.

Don
Old 07-19-2008, 01:17 PM
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Default RE: More drag - prop spinning at idle or dead stick?

A spinning prop is less drag.
If you really think so, I suggest you put it to a test. Lift a model to altitude with another plane and do drop tests. Remove the glow plug so the prop freewheels. Compare glide distance to same airplane with stopped prop.
Old 07-19-2008, 02:03 PM
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Default RE: More drag - prop spinning at idle or dead stick?

Just go to the local WalMart and buy one of those little spinning-propeller-on-a-stick things for kids. Then push it around. Now stop the prop by shoving a pin thru the prop and into the stick and push it around again.
Wow, mystery solved!
Old 07-19-2008, 02:52 PM
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Default RE: More drag - prop spinning at idle or dead stick?

Good examples.
There has been lengthy discussions on this, in forums, the model press, etc.
And the mathematical formula is quite complex, but does indicate just how much drag a spinning prop produces above a stationary prop.
Old 07-21-2008, 11:38 AM
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Default RE: More drag - prop spinning at idle or dead stick?

Steve:

I believe that stopping the propeller is not the best solution to your problem.
Besides, landing at dead stick in a regular basis doesn’t leave any margin for aborting landings with less than ideal approaches.

Drag, high or low, is not significant at the low speed of landing.
Increasing only the drag of a model, without increasing lift, just makes it drop from the sky more like a stone, at a more vertical angle of gliding, especially against a strong wind, instead of landing in a shorter run.

The key for a short run landing is low stall speed, since your speed at landing should be around 1.2 of that value.
But low stall speed depends on the weight of your model (try to make it as light as possible) and the lifting capacity of the wing (the higher the better at landing).
But a model like that cannot fly fast and be very maneuverable; hence, you need models able to change their aerodynamics characteristics for slow landing-take-off and for fast flight.

To enjoy your new short landing strip better, you need models with STOL capacity (short take-off and landing).
Just research about STOL, and you will find a lot of ideas and solutions to your problem.

An excellent article for full scale airplanes, but fully applicable to models, can be found at:

http://www.zenithair.com/stolch801/design/design.html

There is also a lot of good and practical information in Andy Lennon’s book “Basics of R/C Model Aircraft Design”.

Regards!!
Old 07-21-2008, 08:03 PM
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Default RE: More drag - prop spinning at idle or dead stick?

Depends on the pitch, and diamerter of the prop in relation to the airplane.
Old 07-21-2008, 08:41 PM
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B.L.E.
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Default RE: More drag - prop spinning at idle or dead stick?

If you want a really dramatic comparison, compare the sink rate of a deadstick helicopter doing an autorotation landing verses the sink rate of the same helicoptor falling with the rotor speed at zero.
Old 07-21-2008, 09:33 PM
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Campgems
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Default RE: More drag - prop spinning at idle or dead stick?


ORIGINAL: B.L.E.

If you want a really dramatic comparison, compare the sink rate of a deadstick helicopter doing an autorotation landing verses the sink rate of the same helicoptor falling with the rotor speed at zero.
Can your pocket book stand a couple of these demos?

Don
Old 07-21-2008, 10:20 PM
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Default RE: More drag - prop spinning at idle or dead stick?

ORIGINAL: Sport_Pilot

A spinning prop is less drag. The props of rubber band powerd models are made to let the prop spin. If you ever have one where the prop gets stuck, you will know it doesn't glide very well at all. If you land dead stick with a large prop you might notice that it's easy to land short because the prop has more drag than at idle. If the idle is too fast you might have trouble getting the plane slow enough to land.
Rubber powered free flight planes have props with a very high pitch. The higher the pitch of a prop, the more that freewheeling is favored. Also, rubber powered props tend to have lots of blade area meaning lots of drag when stopped. Props with a low pitch, such as the ones used by 3-D fliers, tend to brake the plane if allowed to freewheel. Less drag if the prop is stopped.
A helicopter in autorotation is the most extreme example of a very low pitched prop.

High performance sailboats that zig-zag downwind sweeping back and forth across the wind will quite often beat similar sailboats that just head dead downwind, the extra speed gained by heading up a little more than makes up for the extra distance traveled. It's the same principle.
Old 07-28-2008, 10:28 PM
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Default RE: More drag - prop spinning at idle or dead stick?


ORIGINAL: nmking09

Depends on the pitch, and diamerter of the prop in relation to the airplane.
It would also depend on aircraft speed. If the plane is going faster than the calculation above it will act as a brake, however if it is going slower it will provide some thrust.
Old 07-29-2008, 07:24 AM
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Default RE: More drag - prop spinning at idle or dead stick?

Rubber powered free flight planes have props with a very high pitch. The higher the pitch of a prop, the more that freewheeling is favored. Also, rubber powered props tend to have lots of blade area meaning lots of drag when stopped. Props with a low pitch, such as the ones used by 3-D fliers, tend to brake the plane if allowed to freewheel. Less drag if the prop is stopped.
A helicopter in autorotation is the most extreme example of a very low pitched prop.
The pitch and size of the prop only makes the differance larger or smaller. The fact is the spinning prop will always have less drag. The reason is fairlyobvious. If spinning the leading edge is not hitting the air at a perpendicular angle, but is slicing through the air near the pitch angle. Also it is now facing the air backwards and is acting like an upside down wing. Another way to look at it is if a prop spinning at a speed corresponding to 15 MPH has no drag, and one spinning at 10 MPH has some drag, then one not spinning at all has even more drag.
Old 07-29-2008, 07:28 AM
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Default RE: More drag - prop spinning at idle or dead stick?

If you want a really dramatic comparison, compare the sink rate of a deadstick helicopter doing an autorotation landing verses the sink rate of the same helicoptor falling with the rotor speed at zero.
Not the same thing. The helicopter blade is moving at a reverse angle. If you had a prop that could reverse pitch this would be the case. If you have positive pitch then there is less drag, hold the prop still it has more drag, and reverse the pitch you will have even more drag.
Old 07-29-2008, 09:06 AM
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Default RE: More drag - prop spinning at idle or dead stick?

Hi insalacosm
You have gotten plenty of advice to mull over. The description of your field could fit ours. A factory and trees on one end and another factory at the other end. I try to get my planes so they have no tendency to do a snap roll near stall. I tend to always have a high idle because I believe that is safer. However, if there is no head wind I may be forced to idle down further during the landing. With the right approach and going to idle at the right point in the landing pattern, I use up elevator to lose altitude rapidly. The whole plane can act as an air brake, but you have to watch your wings carefully. If they start waving at you you need to let up on the elevator. I work the throttle as needed to hit the landing point I have chosen, which is always near where I am standing. This gives me a much better perspective of the actual touchdown,. Landings are a big part of the enjoyment of my flying. I believe a lot of people hate the landing part; I know I did when I was learning. I even practice landing with the wind. I started this after a few hairy landings. I had not immediately notice that the wind direction had done a 180 on me. I do not panic if I am going to roll off the end of the field since I know my SPAD planes are usually not damaged. You need to also learn when to abort a landing safely, not in a panic manner. Aborting a landing, or a take off, is part of the whole experience and there are times when that is absolutely the correct thing to do. Thinking about aborts and even practicing them puts you in a better position, mentally, to safely take action when one is neccesary.
Old 07-29-2008, 09:37 AM
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Default RE: More drag - prop spinning at idle or dead stick?

From experience in many deadsticks with all sizes of models, an idleing engine give much more drag than a stationary prop (dead stick landing). Especially on my big bipes, an ideling engine will let me land much shorter and stop quicker than a dead stick.
Old 07-29-2008, 12:07 PM
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Tall Paul
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Default RE: More drag - prop spinning at idle or dead stick?

"lift" and "drag" are merely terms used to seperate the force working on an airfoil to make the force easily understood
Both a windmilling propellor and an autorotating rotor are taking energy -from- the air.
This energy slows the airplane/helicopter, as opposed to the airplane's/helicopter's response if the propellor/rotor were stopped.
Old 07-29-2008, 02:51 PM
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insalacosm
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Default RE: More drag - prop spinning at idle or dead stick?

Good discussion, thanks. I have since had three flights at the field where I landed with the engine idling and my speed was much slower than when I killed the engine. I was able to touch down and roll to a stop in about 10' compared to more than 20' when the engine was killed. I found two things helped slow the plane when keeping the engine running: 1. the propeller slowed the plane because my approach speed was faster than the thrust generated from the prop; and 2. I was also able to bleed off excess speed by achieving a higher angle of attack approach that was possible because the spinning prop helped maintain airflow over the control surfaces. Now I just need to build up confidence in my short landing and approach abilities so I can fly my larger planes at the field! One of our local FAI pilots sucessfully landed his 2m pattern plane at the field by slipping the plane in and then flairing to kill speed - it was impressive but didn't look easy.
Old 08-05-2008, 03:50 AM
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Default RE: More drag - prop spinning at idle or dead stick?

Quoting CrateCruncher "Insalacosm, you answered your own question with the number you calculated. When landing at 15mph, if the prop is making a slipstream speed calculated at 15 mph its like its not even there. As the prop slows down to less than 15mph of prop wash the plane will slow as the prop acts as an airbrake on the plane."

Excatly this is what i think. If a plane is going 15MPH and if the prop is pushing the air with 15MPH then it wil act like its not there. In this case dead stick will have more drag. Because during deadstick there is something rather than having "nothing".

But.... lets say the plane is doing 50 and the prop is still pushing 15 then i am not sure if stopped prop will have more drag then the idling one.
Old 08-07-2008, 02:57 PM
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Default RE: More drag - prop spinning at idle or dead stick?

you probably dont want to kill your engine on decent incase you have to pull out for a 2nd go for some reason. Unless you can restart your plane mid flight, i woudlnt do it, but thats just me.

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