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Which has the least drag ? - 1/14/2013 4:27 PM   
cyclops2



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Status: offline 		Phase 3 U2 powered by a pusher prop.

Inlet openings closed with flat plates. Each opnening is 4.7 sq. inches of flat area.
Tail opening closed by the pusher motor mounting plate.
Tapered plastic water bottle cut to a streamlined inlet covers.

I have enough battery to set prop speed to some minimum speed to possibly reduce air duct resistance if that is the most efficient way to reduce drag. It will run WOT for 26 minutes.

Thanks


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RE: Which has the least drag ? - 1/14/2013 6:59 PM   
da Rock



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Status: offline 		You're going to need to cool the pusher motor.

route the air entering the existing intakes over your ESC, motor, battery (as much as possible) and use what scale exhaust opening you can.

If the air entering the existing intakes has a way to get out, there should be less drag at those intakes. How much in any case would be anybody's guess. The reason wind tunnels are so popular is pretty much because Aeronautical Engineering formulas so often can't give reliable answers.

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RE: Which has the least drag ? - 1/14/2013 8:20 PM   
cyclops2



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Quickie.

Does a stopped prop or a slow spinning prop have more drag ? Read it but can not remember .

Thanks

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RE: Which has the least drag ? - 1/14/2013 9:10 PM   
vertical grimmace



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RE: Which has the least drag ? - 1/15/2013 12:58 AM   
HighPlains


 

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Status: online 		A stopped prop has lower drag than one that is slowly turning.

You want as little air as possible to go through the inside of the model.

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RE: Which has the least drag ? - 1/15/2013 2:39 AM   
countilaw



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quote:

ORIGINAL: HighPlains

A stopped prop has lower drag than one that is slowly turning.

You want as little air as possible to go through the inside of the model.



This really doesn't make much sense.       I  turning prop is, at least,  slicing through the air,  a stopped prop is  just blocking the air. 

Common sense says that you want  air to flow through the model, if just to supply cool air to the  electronics.    That makes sense !!!


Frank



< Message edited by BMatthews -- 1/17/2013 11:21 PM >


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RE: Which has the least drag ? - 1/15/2013 3:17 AM  1 votes
HighPlains


 

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Status: online 		Frank,

Sense, or even common sense has little to do with aerodynamics If you just took the time to do a basic search on the RCUniverse site, with the following phrase "drag of stopped prop" you would find almost 180 posts on the subject, most of which will tell you that a stopped prop has less drag than a freewheeling one. And with a very few exceptions, they would be correct. Now perhaps you have drilled down a little further, because there are some very special limited cases where they would be incorrect, but these are very special combinations of factors.


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< Message edited by BMatthews -- 1/16/2013 9:02 PM >

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RE: Which has the least drag ? - 1/15/2013 1:12 PM   
sensei



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Status: offline 		A stopped prop has lower drag then a turning prop, it's just that simple. Have you ever noticed that you can slow an airplane down much better with your engine at a low idle then dead stick? That's because that fan is acting like a full disk brake.

Bob

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RE: Which has the least drag ? - 1/15/2013 1:55 PM   
cyclops2



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Thanks guys. I knew it had a odd ball logic to it. Old age is really getting to me.


Many thanks to all of you.

Rich

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RE: Which has the least drag ? - 1/16/2013 9:12 PM   
BMatthews



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Status: offline 		A folder on a pusher motor of a U2 makes a lot of sense. Especially if you'd be belly landing the model.

If it were me doing a U2 pusher setup I'd likely close one duct with a nice streamlined front and leave one open as a source for cooling air for the motor and ESC. And if the opening around the motor is less than sapcious I might put some dump vents just ahead of the motor to try to aid in keeping the air flowing as well as practical.

The other option would be to close both scale intakes and install NACA like recessed vents in the belly or sides just ahead of the motor area in the tail to cool the motor and ESC.

Frank, the braking effect of a windmilling prop have been understood for a lot of years. It's why props on failed engines are feathered instead of left to spin in the airflow. Granted in that case they angle them for minimum drag but even with fixed props there's less drag from sitting still rather than freewheeling. As mentioned already a freewheeling prop generates lift in the wrong direction and it acts similarly to a drag disc the same size as the prop diameter.

This "feature" is put to good use in the autogyro where the freewheeling rotor is the lifting source.

Now as mentioned there ARE exceptions. A case in point is found on rubber models. The difference is that with really high pitch angles at some point there is less drag from a freewheeling prop than by stopping it. But that only occurs where the pitch to diameter ratio is more than 1:1. And we simply don't see many props with a higher pitch than diameter used in electric or glow/gas situations.

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RE: Which has the least drag ? - 1/16/2013 9:58 PM   
cyclops2



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Well I guessed right Bill ?

I put the 12 MM motor & 7 : 1 gearbox in it with a APC 7 x 6" pusher prop. The motor is well past the the rigid bulkhead. At 14 watts of motor heat at WOT I think it can stay cool enough. Will check it with increasingly longer bench runs to be sure.

I agree with your eliminating my stalled inlet airflows. They equal a flat plate area of 9.5 square inches. I really do not have a lot of power to give away with the tiny inrunner motor.

Back to the recycling for another water bottle.
I will do a hand toss in the 2' grass to make sure she is a good COG glider with everything in neutral.


Rich

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RE: Which has the least drag ? - 1/16/2013 11:57 PM   
ARUP


 

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quote:

ORIGINAL: cyclops2


Thanks guys. I knew it had a odd ball logic to it. Old age is really getting to me.


Many thanks to all of you.

Rich

Just think how autogyros work!
For the U-2 model NACA type ducts very near the motor and batteries would be the best option but it wouldn't look scale. Having the open ducts at the front to cool the motor and batteries is needed but there is a lot of 'wetted' area drag because of this. My $0.02

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RE: Which has the least drag ? - 1/17/2013 12:43 AM   
cyclops2



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Oh boy.

I can do some test glides without the inlet cones first & the sealed duct.

Then tape on the inlet cones & check the glide distance.

My concern is if I can not climb fast enough even with almost a 1/2 hour of WOT power.

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RE: Which has the least drag ? - 1/19/2013 4:08 AM   
Lownverted



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quote:

ORIGINAL: cyclops2


Well I guessed right Bill ?

I put the 12 MM motor & 7 : 1 gearbox in it with a APC 7 x 6'' pusher prop. The motor is well past the the rigid bulkhead. At 14 watts of motor heat at WOT I think it can stay cool enough. Will check it with increasingly longer bench runs to be sure.

I agree with your eliminating my stalled inlet airflows. They equal a flat plate area of 9.5 square inches. I really do not have a lot of power to give away with the tiny inrunner motor.

Back to the recycling for another water bottle.
I will do a hand toss in the 2' grass to make sure she is a good COG glider with everything in neutral.


Rich


Rich,

Neat project! A couple of minor things, one your prop is on backwards, the numbers should face the front of the plane. Second with that 7:1 gearbox, that "E" prop is not going to produce a lot of thrust. At your lower rpm you'll likely be much better off with a "SF" style prop. They work much better in for higher thrust and lower speeds.

Good luck!
Red

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RE: Which has the least drag ? - 1/19/2013 4:56 AM   
vertical grimmace



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quote:

ORIGINAL: sensei

A stopped prop has lower drag then a turning prop, it's just that simple. Have you ever noticed that you can slow an airplane down much better with your engine at a low idle then dead stick? That's because that fan is acting like a full disk brake.

Bob

If it has a low pitch. High pitch racing props , or high pitch period make slowing a model down difficult. I have always done better landing my racing planes deadstick.

Again, why not use a folding prop? There are many available.

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RE: Which has the least drag ? - 1/19/2013 5:37 AM   
HighPlains


 

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Status: online 		There is a big difference between an engine at idle, dead stick with the prop stopped, and one where it is "windmilling" (turning over the engine, being driven by airspeed). I once had the glow plug blow out of the head on a ST X40 Formula One racing motor and the airplane came out of the sky with about half the gliding distance it normally had when deadstick. Idle? Surely you jest. Those engines had 22K+ and zero as your two options. Those props were nearly square, with 8 1/4 diameter and 7 3/4 pitch.

Anyway, it sure was a surprise to see the drag a windmilling prop added. It helped me to understood why on some fixed pitch light planes they tell you to nearly stall the plane to stop the prop if there is no chance of a restart.

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RE: Which has the least drag ? - 1/19/2013 4:45 PM   
Lownverted



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Status: offline 		Totally agree with the windmilling prop providing more drag. Another eye opener is flying a powered glider with a folding prop and no brake. It is nearly impossible to get it to fold without a brake. You need to stall it nearly straight up to get it to stop and fold.

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RE: Which has the least drag ? - 1/20/2013 2:21 PM   
cyclops2



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Eagle Eyes saw the borrowed prop & motor From the GWS P-51.

I ran both types of 7" & the only real difference was 4 amps versus 5 amps & 40 mph versus 49 mph on a tach. Thrust seems the same with my scale.

Does anyone make a folder this small ? 7 x 5" or 6" ? Will I need to attach a small spring between the blade hubs to get & keep the blades folded ? I did the spring on a piper cub because it seemed to not have enough speed to close. I will probably run with the inlets sealed with the cones.
Do not want to do vertical stalls to close the folder.

There is a lot more to these gliders than........Find the COG & toss.

I appreciate all the training you guys are giving me.

Thank you
Rich

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RE: Which has the least drag ? - 1/20/2013 3:38 PM   
da Rock



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quote:

Another eye opener is flying a powered glider with a folding prop and no brake. It is nearly impossible to get it to fold without a brake. You need to stall it nearly straight up to get it to stop and fold.


When that happens it suggests the prop blades are bound up. There is no value having any friction holding them open or closed. They are safely held by the size of the bolt and structure. Stiff folding does nothing of any value. Their rotation holds them open. Overtightening the bolt doesn't help hold it either. Locktite helps if you're worried, but quality manufacturing goes just as far.

All of my blades have flopped around forever. Never lost one either. Now, I have always purchased quality folder spinners forever too. The retailers that sell the blades worth buying sell spinners worth buying as well. Not all my gliders have the brake turned on and every one of them fold reliably. They fold reliably at any airspeed but remember that airspeed is right after shutting off the motor and seldom is the glider's airspeed right then slow. Pulling the throttle off firmly works great.

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RE: Which has the least drag ? - 1/20/2013 6:05 PM   
cyclops2



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Rock

I agree. I had to sand a few thousandths off of 1 blade untill it would flop open or closed.

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