Why did the prop go slower into wind?

[e-geezer]

I was holding my (electric) model and had the motor going full speed with an 11 x 8 inch folding prop. I rotated the model and noticed the prop went round slower (I could tell from the engine sound) when I pointed it into wind and faster when I pointed it the other way.

Why? I would have expected it to be 'helped' by the wind when facing into it. After all, if there'd been no power applied, the wind would have tended to turn the prop.

I reckon an answer to this might help me with prop selection.

[BillH]

Your probably did not hear the rpm change, rather the sound of the wind hitting the prop change.

[Ollie]

The propeller is designed to screw its way through the air. The propeller is most efficient when the prop advance through the air matches the pitch. When the wind opposes the slipstream of the prop it tends to stall the prop which loads the motor and slows the RPM.

[Sparky Paul]

The shape (airfoil) of the prop is different, front to back.

[Pook]

I was just thinking .... if you had your prop so it would free wheel with little friction & you point it at the wind then it would spin in the opposite direction to the way you spin it with the motor on, so if you have the motor spinning your prop and you point it into the wind wouldnt it be affected by the wind trying to spin the prop in the opersite direction... ??

ok i know thats a bit of an oversimplification, but couldnt it be that ?

Piers

[Salto]

Yes, what you have observed does sound counter-intuitive, but how's this for an explanation;

When not moving forward through the air your prop may be stalled and also each blade is probably just churning around in the wake of the other one. It's not operating very efficiently, and not passing full power to the air, so it "unloads" and speeds up.

Now, when you point the model into the wind, each blade sees some "new" air as the prop disc effectively moves forward into the wind. The blades also see a lower angle of attack and begin to operate more efficiently, imparting more power to the air, and drawing more current from the batteries. So the prop slows down.

When you point the model away from the wind the prop blades are forced more into their own wake, don't bite into clean air, don't impart full power to the air, and the motor unloads and speed up.

Graham.

[HarryC]

My goodness, there is some real junk in this thread!

If you point a prop into the wind and let it freewheel it turns the same way that the engine would turn it to create a wind in that direction. Pointing it into wind helps it spin faster, just as it would when the plane is in motion, the faster you go e.g. in a dive the faster the prop will turn.

"When not moving forward through the air your prop may be stalled and also each blade is probably just churning around in the wake of the other one. It's not operating very efficiently, and not passing full power to the air, so it "unloads" and speeds up."

A stalled blade is "loaded", an unstalled blade is "unloaded" and speeds up.

"Now, when you point the model into the wind, each blade sees some "new" air as the prop disc effectively moves forward into the wind. The blades also see a lower angle of attack and begin to operate more efficiently, imparting more power to the air, and drawing more current from the batteries. So the prop slows down."

A lower AoA is only more efficient if it drops below stalling angle, which isn't the case for the average prop which is not stalled at nil airspeed. Lowering the AoA whether from stalled or unstalled will reduce drag and increase the revs, which in turn draws less current.

"When you point the model away from the wind the prop blades are forced more into their own wake, don't bite into clean air, don't impart full power to the air, and the motor unloads and speed up."

Quite the opposite, the drag rise slows down the prop.

What e-geezer observed was a change in noise, not a change in revs. He has interpreted this as a drop in revs, when if anything it was a rise in revs.

H

[steve lewin]

I suggest repeating the experiment using a tachometer. The tachometer is a device which is far more accurate at measuring rpm than the unaided human ear. Post your results.

I haven't managed to detect a change in rpm yet using that method but it's not been very windy here lately ;).

Steve

[Salto]

HarryC,

Perhaps I didn't explain my hypothesis very clearly.

I'm not thinking in terms of the changes in drag with changes in angle of attack. I'm looking at the problem in terms of a prop running around in its own wake.

There's no disputing that the drag of a prop blade in clean air will always increase with increasing angle of attack. But the point I am trying to make is that the blades may be running around in very dirty air, and air which has a significant component of velocity in the direction of blade travel. That is, running in their own wake. One blade is "drafting" behind the other much like a bicycle rider closely following another rider in front and effectively getting a reduction in drag.

The significance of speculating that the prop may be stalled is that the wake will be much thicker from a stalled blade, and will also have higher velocity components in the direction of blade travel. This thicker wake is more likely to be giving something of a free ride to the blade following.

But when the prop disc moves forward through the air each blade starts to come out of the wake and now sees clean air with less velocity component in it's direction of travel, so it may well see an increase in drag and slow down.

Graham.

[e-geezer]

I've got a tacho and I'll try it out - see if the driven prop really does slow down into wind. You can see that this is related to my other question of how effective static measurements are. What I REALLY want to know is how to select a prop and whether static RPM and thrust measurements tell all. But maybe that should be left for another thread.

[steve lewin]

Not sure you need a new thread, most of your question was covered in your Static thrust thread. Static thrust and rpm do not tell you all about flight conditions and no-one imagines that they do. There's some debate about whether they tell you anything useful at all but they certainly don't tell you everything.

Most of us select a prop by flying with several and choosing the best. You can select a reasonable range to try using static thrust, pitch speed and/or other numbers from Motocalc/Electricalc, static tests etc. (after current I look first at max. climb rate) but to optimise for YOUR particular model and YOUR particular way of flying you need to fly. All IMO of course.

Steve

[Martyn McKinney]

If you hold your running motor and prop outside the window of a moving car, you will discover that at some speed the motor will become a generator. At this point the angle of attack of the blades is zero and I would have expected the motor be be turning faster than its zero speed condition.

Also as an experiment you might want to try holding the fuselage of your plane out the window of a moving car with the motor battery removed.

You will discover that at some speed (probably about 30 mph) you will be able to use your transmitter to drive the servos,

The circuitry of most speed controllers allows the voltage generated by the motor to power the receiver and servos. If the motor battery were connected it would recharge the battery.

[e-geezer]

Yes there is some debate Steve. This is one. What I would like to know is this:

I measure the static thrust and RPM (and current if you like) over time for a given prop, battery, motor.

How should I adjust the figures for actual flight? Up? Down? And by how much?

Anybody know?

[Martyn McKinney]

If the pitch/diameter ratio of the prop is less than about 0.7 the static thrust measurement will be approximately twice the thrust at the prop's optimum flying speed.

Props with pitch/diameter ratios greater than about 0.7 are stalled when static thrust measurements are made. The measurements therefore are not a good indication of the thrust available while flying.

The prop's optimum flying speed may be estimated by calculating the maximum speed the prop would go in theory.

MAX SPEED(mph)=RPM/60 X PITCH(in)/12/88*60

Optimum flying speed = MAX SPEED X (1-5/(18.25 X P/D)

I have a small wing with an RPM of 19,000 and a 3/2 prop

MAX SPEED(mph)= 19,000/60 X 2/12/88*60 = 36 mph

OPTIMUM FLYING SPEED = 36 X (1-5/(18.25 X 2/3)

=36 X (1-5/12.16) = 36 X (1- .41) = 36 X .59 =21 mph

The reason that this is the optimum flying speed is that this is the speed that the prop would have to be travelling so that its angle of attack is approximately 5 degrees where the lift/drag ratio is max.

If the speed is greater than this speed, the angle of attack would decrease from 5 degrees. The lift/drag ratios of airfoils with angles of attack less than 5 degrees decrease very rapidly.

At slower speeds, which would increase the angle of attack, the lift/drag ratio also decreases but not as rapidly.

The rpm of the motor will also increase slightly and the current will decrease as the speed of the aircraft increases. This also may be estimated from the motor constants.

[Sparky Paul]

Anyone... ANYONE!!!!
Put a prop on a stick.
Hold the prop so the wind makes it rotate.
Measure the rpm.
Mount the prop on the stick backwards.
Hold the prop so the wind makes it rotate.
Measure the rpm.
Then, and only then, pontificate and prognosticate on the wonders revealed!

[raptor22]

Originally posted by HarryC
My goodness, there is some real junk in this thread!

If you point a prop into the wind and let it freewheel it turns the same way that the engine would turn it to create a wind in that direction. Pointing it into wind helps it spin faster, just as it would when the plane is in motion, the faster you go e.g. in a dive the faster the prop will turn.

"When not moving forward through the air your prop may be stalled and also each blade is probably just churning around in the wake of the other one. It's not operating very efficiently, and not passing full power to the air, so it "unloads" and speeds up."

A stalled blade is "loaded", an unstalled blade is "unloaded" and speeds up.

"Now, when you point the model into the wind, each blade sees some "new" air as the prop disc effectively moves forward into the wind. The blades also see a lower angle of attack and begin to operate more efficiently, imparting more power to the air, and drawing more current from the batteries. So the prop slows down."

A lower AoA is only more efficient if it drops below stalling angle, which isn't the case for the average prop which is not stalled at nil airspeed. Lowering the AoA whether from stalled or unstalled will reduce drag and increase the revs, which in turn draws less current.

"When you point the model away from the wind the prop blades are forced more into their own wake, don't bite into clean air, don't impart full power to the air, and the motor unloads and speed up."

Quite the opposite, the drag rise slows down the prop.

What e-geezer observed was a change in noise, not a change in revs. He has interpreted this as a drop in revs, when if anything it was a rise in revs.

H

just read the thread... was wondring if someone would say something before i got to the end! Guess thats why the AMA president keeps attacking sites like this!

--Alex

[Dave Hederich]

>> There's some debate about whether they tell you anything useful at all but they certainly don't tell you everything.

Steve, is there really a legitimate debate about this? I can't recall anyone really producing any evidence that static thrust does not precisely replicate hovering and extreme high alpha flight performance. I've actually measured instances of motor combinations that produce slightly more static thrust than the all-up weight of the aircraft. When I hold the aircraft in a vertical orientation, apply full throttle and release the aircraft, it slowly climbs vertically, proving that it has slightly better than 1:1 thrust-to-weight as predicted by the static testing.

The only debate here would be whether or not hovering and high-alpha flight are "useful." There's no debate about the accuracy of the prediction.

[steve lewin]

You're right of course Dave, I was too imprecise. Static measurements tell you something hovering and also something about take off capabilities, in particular the ability to accelerate from rest.

I guess I'm just biased because I don't (can't) fly 3D so I find that they don't tell me much any flight modes that I ever find my planes in. I generally try quite hard to avoid stalled or near-stalled conditions. I'm afraid I'm a bit old fashioned and I don't really think of a thing that looks like a plane but is pretending to be a helicopter as "normal flight" ;).

So will you settle for "Static measurements tell you something about a very small portion of a plane's performance envelope, typically at zero to very low speeds, but there's some debate as to how well they predict behaviour in any other parts of the flight envelope" ?

Static thrust is often measured with the prop in a stalled condition. If you manage to closely replicate that condition by having the plane not moving forward then it's a pretty good predictor.

Steve

[Mikey-flies]

As far as I can tell all the props (and ducted fans for that matter) unload (spin faster) in a dive or when heading into gusting wind. My T3D (ips A drive, 11x7 HD prop) with pixie7p will freewheel when I shot off the throttle if it is moving more than a crawl.

All my free flight planes (After the rubber winds down the props act very much light a prop on a stick) props will rotate in the same direction while gliding as they do while under rubber power. The runtime of the rubber will often be LESS in the air then on the ground as well due to less resistance to the prop as the aircraft speeds up (the rubber applies the same force to the prop shaft whether or not the prop has any resistance).

[Dave Hederich]

Steve, your settlement might be too generous. I sure wouldn't take the side of the debate that static thrust measurements predict behavior in parts of the flight envelope other than the slow ones. ;)

Actually, as I get more interested in higher speeds, I find myself anticipating the withdrawal pains of no longer being able to rely on the simple static thrust measurement to predict low-speed thrust performance. It's going to take a little more sophistication to predict the right prop for best performance at higher speeds.

[steve lewin]

It would be nice to find some theoretical basis for choosing props for any part of the flight envelope but I don't really mind having to fall back to the old-fashioned "Fly it and try it" method. The *calc programs get you into the right area and the cost of buying a few props to test is not too worrying.

With the things I fly I'm rarely worried about dragging out the last bit of performance so I can nearly always use all the props for something even if they're not optimal. After all it's another excuse for the crash "What can you expect with a non-optimal propeller fitted". Makes a change from "Radio interference" ;).

Steve

[raptor22]

Originally posted by steve lewin
After all it's another excuse for the crash "What can you expect with a non-optimal propeller fitted". Makes a change from "Radio interference" ;).

Steve

thats not a problem where i fly.....theres both strong downdrafts and radio interference on my channel.:p

--Alex

[e-geezer]

The runtime of the rubber will often be LESS in the air than on the ground as well due to less resistance to the prop as the aircraft speeds up (the rubber applies the same force to the prop shaft whether or not the prop has any resistance).

Interesting. Have to think about that.

[e-geezer]

But with a rubber-powered model, the number of 'powered' prop rotations is a 'given', whereas with an electric or petrol motored model, it isn't - if the model has an easier time in the air (e.g. level flight in still air) you'd get more prop rotations. So I don't know what to think about this.

Maybe we should think in terms of energy and power..

At least from the graph above I learnt that the plane speeds up until thrust equals drag.

So I suppose that if you streamline your model it will fly faster. Will it stay up longer and/or travel further?

[HELModels]

Nobody has got more half baked, backwards, unproven, almost right theories about rotating wings than me. I think I will take a small motor with prop and mount it in a vice and then use compressed air from 2hp compressor as wind. I will report results. Hmmm. The vice has influence. I'll figure out something and report back.

;)

[raptor22]

electrostorch,

i think that the stream of air is too thin for that to be reliable, but the prop shoud pseed up, anyway.


So I suppose that if you streamline your model it will fly faster. Will it stay up longer and/or travel further?


of course; it has less holding it back to either go faster or use less power for the same speed (for the same size wing, airfoil, etc). it like have better bearings, oil, tires and so on for your car; less resistance.

--Alex

[Sparky Paul]

Originally posted by ElectroStorch
Nobody has got more half baked, backwards, unproven, almost right theories about rotating wings than me. I think I will take a small motor with prop and mount it in a vice and then use compressed air from 2hp compressor as wind. I will report results. Hmmm. The vice has influence. I'll figure out something and report back.

;)
.
Use several different pitches, both ways...
Find the relationship between pitch and rpm...

[Tim Green]

When incoming windspeed increases, an electric driven prop will increase RPMs. Mainly as a result of the increase in voltage.

RPM is directly proportional to volts.

As the prop/motor unloads as the plane increases in speed, the current draw on the motor decreases (the motor's working less hard, as the windspeed nears pitch speed).

As the current draw on the motor decreases, the volts to the motor increase. Thus the RPMs increase.

I believe that it's the battery losses which come into play, in making the voltage increase as the prop unloads. With less current, the battery is more efficient, providing more volts to the motor - hence more RPMs. Putting it another way - the battery has internal resistance, which causes voltage loss in the cells - and this loss decreases as current decreases.

[Martyn McKinney]

The internal resistance of batteries is usually significantly lower than the armature resistance of most brushed motors.

Although it is true that the battery output will rise slightly, the major contributor to the increase in the motor's speed is the reduction in voltage drop in the armature windings.

The RPM of any DC motor is equal to Eg X Kv, where Eg is the generated voltage. Under no load conditions Eg is almost equal to the applied voltage.

As the load on a motor increases, Eg decreases because of the increased voltage drop in the armature resistance and the motor speed drops.

Placing the motor-prop combination facing the wind reduces the load on the motor, decreases the armature current and therefore decreases the voltage drop not only in the battery, but more significantly in the armature, resulting in a higher RPM.

[Tim Green]

Martyn - I left out wire losses, because the only place I can measure battery voltage is at the battery leads. And as changes in voltage measured at the leads is caused by current changes across the internal resistance of the battery, this is the only drop I felt comfortable discussing - I can measure it, and I can see it changing with changing load, and I know what causes it.

So thanks for sharing your knowledge of the winding losses. Something I'm at a loss (pun intended) to measure, and don't have enough background in it to "know" how much those losses might be. I suppose I could use the published mohms for each motor though, to determine what these losses would be.

At any rate, just so we don't veer too much from the origin of this thread - It seems we're in agreement that the RPMs increase when the motor unloads as the prop faces into increasing wind speeds (when the plane dives for instance) - and that the RPMs increase because the voltage increases. Which is what the original poster was asking about.

[Martyn McKinney]

Tim:

For your interest I have attached a drawing of a simple model of a DC motor.

It includes the 3 motor parameters

Armature resistance Ra (Ohms)

Motor Constant Kv (RPM/Volt)

No Load Current Io (Amperes)

From these, the motor characteristics may be determined under most any load conditions.

Eg (the generator voltage) is the back voltage generated by the turning armature.

RPM is equal to Eg X Kv

Io (no load current) is approximately constant over a range of input voltages. It arises mainly from brush loading, but a small portion is due to bearing friction and windage.

As the load increases the armature current increases.

Eg=Ein-(Ia X Ra)

Power in = Ein X Ia

Power Out = Eg X (Ia-Io)

[Tim Green]

Martyn,

Thanks for the info. It helps me to understand what you're talking about.

As to the losses within the battery itself. We can quantify the battery losses ...The resistance of the CP1700 cells I use regularly is .0055 ohms per cell. In a 10 cell pack, that's 10 * .0055 ohms = .055 ohms.

Now I've measured a CP 1700 10 cell pack under load, at 36A, and it shows 10.5v using my whatt meter. And this same pack's no-load voltage is about 13v.

So, that's 36A * .055R = 1.98v expected loss due to battery impedance, and probably some additional losses due to solder joints, etc. which would explain the 2.5v loss I'm actually seeing.

At any rate, a couple of volts is a lot of RPMs, before we even take into account the wire losses you've described.

My brushless motor in this same setup (Kontronik Fun 500-27) has an internal impedance of .0158ohms. At 36A this would be dropping .56 volts. But I don't know whether I can just multiply it like this, or have to take something else into accout too - you probably know whether I can just do it like this or not.

I see you're Canadian. Cool. I was born in Ottawa, some time ago. My folks are from Moose Jaw, Sask.

[Martyn McKinney]

Brushless motors are much more efficient than brushed motors because the resistance of their armature windings is much lower than that of brushed motors and their Io is much smaller because there is no brush friction.

Typical brushed motor efficiencies 60 to 80%

Brushless motors 80 to 90 %

You can plug your numbers into the previous equations, but you will have to know Io and Kv.

You're right, it is cool in Canada.