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-   -   Do propellers really "unload" in flight? (https://www.wattflyer.com/forums/showthread.php?t=39096)

Larry3215 09-16-2008 06:49 PM

Do propellers really "unload" in flight?
 
There has been a lot of debate on this subject and it seems to me most people think that they do unload in flight.

I am one of the ones who doesnt believe there is nearly as much - if any - unloading going on as most people think.

Does anyone have any Eagle Tree or Medusa flight logger graphs they care to share that they think shows this happening?

Id like to see some graphs of hi speed setups with square or almost square props as well as some slower flying setups with more normal diameter/pitch ratios and different power and rpm ranges and flight envelopes.

This isnt really a critical or vastly important quesiton - Im just curious :)

bz1mcr 09-16-2008 09:49 PM

I would like to see some real data as well. From what I've read the fast setups will almost always release smoke (if they are going to) shortly after launch. What people say is that the props are stalled at zero plane speed and that unloads the motor some. Then while they accelerate the prop takes a bite and the motor actually slows down. That is the critical point, as the plane continues to accelerate the prop unloads and the motor spins faster and draws less current. On a plane that reaches 100 MPH or better the current at max flight speed is less than both the stall and launch current because the motor is turning faster. The difference can be significant like 10-20% less.

On planes flying less than 40 MPH the prop is not stalled at zero flight speed and the launch current spike does not occur. Also the in flight rpm and current are very close to the static values. Current at max flying speed might only be a few percent less than static.

OK, that's my belief based on reading. Now I hope some one with measured data can tell us what really happens.
Don

pd1 09-16-2008 10:51 PM

On a full sized 172 with a fixed pitch prop.
It's not uncommon for the static rpm to be about 2300.
Once the brakes are released and the plane starts to move, the rpm will jump 200 rpm.
Once in flight you can get 2700 rpm in level flight.

Judging from that I think the props unload in flight.
Just my .02
Paul

gfdengine204 09-16-2008 11:08 PM

It makes me wonder if it relates to the law of physics "A body in motion tends to remain in motion", meaning, when the plane is static on the ground, the prop is pushing (pulling) the air, but when it is airborne, the tendency to remain in motion allows the prop to turn "more easily" (for lack of a better way to explain it) and hence gets a few more hundred RPM (as in Paul's example).

Here is something I found after a quick GOOGLE search; I haven't read it completely through, but it may prove to be interesting toilet material. ;)

http://www.ozrcflying.com/2007/07/pr...r-fiction.html

Moxus 09-16-2008 11:11 PM

by unloading, do you mean stall?
well, if stall is what you mean, its dead simple to find out.
you need to know rpm, propeller dimensions and AoA that gives boundrary separation for the propeller.
as a general rule of thumb, high pitched will always stall at low speed/high power conditions. this is also the reason why hgh pitch/edf is so damn slow accelerating from stationary, even with a nuclear powerplant to spin it. propepper/impeller stalls and efficiency drops.
on the other extreme, you have 3d propellers , for example the popular dimension 12.25x3.75. it will never ever stall whatever you do. if you spin it at 10 000 rpm and drag it backwards through the air, it wont stall. because the AoA of the propeller is so small that the relative wind speed has to be negative even to produce high enough angles to get into stall behaviour.
this is also why high speed setups favorizes high diameter, high pitch, reduction gear setups, rather than small diameter and even higher pitch. the latter will save a lot of weight but it just isnt efficent at low speeds, and it cant produce enough thrust without stalling.

if you arent confuzed yet, so dont be diappointed, you will certainly be soon.
propellers has, as most of us already know, variable AoA along its diameter.
this variable AoA is only optimal (same over whole diameter) when propeller is working at design AoA. at stationary/low speeds, the AoA near centre will be higher than AoA near tip, and hence the centre airfoil section will stall while the tips works fine.
call it the washout of the propeller, if an analogy to wings helps putting the brain into gear. hence, stalled sections of the prop will vary along the diametre of the propeller as a function of rpm and windspeed.
at stationary, a square propeller will have exess of 45 degrees AoA at centre, thinking about wings again, you know thats a dead stall with humungous margins. but as speed increases, AoA will decrease and the stalled section of the propeller will decrease along its diameter.

Moxus 09-16-2008 11:13 PM


Originally Posted by pd1 (Post 476204)
On a full sized 172 with a fixed pitch prop.
It's not uncommon for the static rpm to be about 2300.
Once the brakes are released and the plane starts to move, the rpm will jump 200 rpm.
Once in flight you can get 2700 rpm in level flight.

Judging from that I think the props unload in flight.
Just my .02
Paul

that happends because AoA decreases as speed increases, and hence the propeller spins lighter, and motor can rev more.
by unloading, i think he means that propeller stalls, and is lighter to spin for that reason, but im not sure if thats exactly waht he means though.
a full sized 172 has such low pitch that i dont think it will ever stall even if dragged backwards through the air.

pd1 09-16-2008 11:29 PM

MOXUS, I think you maybe right, we might differ in the interpretation of the question.

The 172 with a 150 hp Lycoming engine uses a 74 inch diameter prop.
The pitch can vary depending on the prop.
Allowable pitch range is 56 inch climb prop to 62 inch for a cruise prop.

Just rambling now, airframe drag is zero when the plane is stopped, airframe drag is much higher when the plane is actually moving.
Yet the engine power can be reduced 25% while in cruise and maintain the same rpm as static.
So all that said, my interpretation was the prop will spin faster or produce less load on the motor while in flight.

paul

Flash1940 09-16-2008 11:46 PM

My 2 cents here....couldn't resist.. It has always been my understanding about propellers that when you look out there at about 75% from the hub at the angle the blade makes one can rationalize that the incidence angle is near stall while the aircraft is at rest. This is even more dramatic when using a high pitch prop.....over 6 or 8.
As the aircraft picks up velocity....the air coming in to the leading edge is seen to be at a lesser angle....or in other words....the angle of incidence of the blade becomes less and true prop blade lift begins to increase....the aircraft picks up even more speed and all is wonderful with the world....the prop has unloaded....so-to-speak. The aircraft is trying to catch up to the speed of the prop blast. This is why in real aircraft (if we can) we use lower pitch at takeoff....then at altitude we watch the manifold pressure and adjust the mixture and prop pitch for optimum cruise speed and fuel efficiency. If any have other input....speak up.....even if you differ from me....it's perfectly fine....

Flash

quorneng 09-17-2008 12:04 AM

A fixed pitch propeller WILL speed up as the air speed increases. This becomes a significant problem with aircraft speeds over 250 mph. If you set the pitch to achieve maximum airspeed at maximum engine revs then the static revs are so low that take off became a problem. The solution is of course variable pitch props!
Models tend to fly below 250 mph(!) and electric motors have better power characteristics at reduced motor speeds than IC engines so whilst prop unloading is there it is not that significant.

Moxus 09-17-2008 12:17 AM


Originally Posted by pd1 (Post 476219)
MOXUS, I think you maybe right, we might differ in the interpretation of the question.

The 172 with a 150 hp Lycoming engine uses a 74 inch diameter prop.
The pitch can vary depending on the prop.
Allowable pitch range is 56 inch climb prop to 62 inch for a cruise prop.

Just rambling now, airframe drag is zero when the plane is stopped, airframe drag is much higher when the plane is actually moving.
Yet the engine power can be reduced 25% while in cruise and maintain the same rpm as static.
So all that said, my interpretation was the prop will spin faster or produce less load on the motor while in flight.

paul

yep, thats true. that is because if RPM is kept constant, the AoA to the propeller will decrease as speed increases, hence lightening the load on the motor by lowering required momentum to spin it at constant speed.
so i think we are thinking the same, just trying to explain it in different ways.
and as a n.b., some cessnas has constant rpm propellers, wich means that they can change motor rpm independent of propeller rpm (variator), but i dont know if this applies to the 172.
anyway, high pitched propellers will be stalled at static spinning, and therefore turn lighter than at AoA < stall angle.
but for all propellers, regardless of pitching, it is true that motor load (momentum) will decrease as speed increases if rpm is cept at a constant level.

trebel-t91230l 09-17-2008 12:34 AM

trebel
 
Tha apparent "unloading" is due to the angle of attack lowering as the a/c moves forward. Consider operating full throttle with the brakes on; the blade angle and angle of attack are the same (same as blade angle}. As you move forward the angle of attack lessons so the blade appears to unload. It actually wold be the same as running full throttle with the brakes on and decreasing the blade angle. a 10 deg blade angle would have a 10 deg AOA with brakes on but only approx 2 deg at cruise speed.
In a dive the AOA could reverse, example Minus 5 deg. You can really hear it wind up in a dive.
Wish I had some of my old books (Retired from Spartan, one of the courses I taught was props.) Hope my explanation isn't confusing. If you have any questions I will try to find a reference to explain it better.
Dick

fr4nk1yn 09-17-2008 01:07 AM

:ack: You guys couldn't make it simple eh?

So a prop DOES unload(?). Now how much?

I made a thread awhile ago over at RCG on this.
The thing that got me interested was a plane that claimed to fly, IIRC, 20mph over it's pitch speed.

pd1 09-17-2008 01:32 AM

Full sized unloads between 20 and 25 per cent.
I think the models should be in that ball park, or less.

I have not seen any plane that would go above it's theoretical pitch speed.
Unless the motor went much higher RPM than it was supposed to.
If a motor has a max RPM determined by KV and Voltage, it's not going to go faster than that in level flight.
Down hill is a different animal.

There is still loss or slipping as the prop pulls.
The prop is not 100 per cent efficient.

Paul

fr4nk1yn 09-17-2008 02:42 AM

Thanks, That's what I figured even back then being a n00b.
If I remember right he was claiming a speed of over 114mph although it only had a pitch speed of 88mph.
Even now I see him claim crazy speeds with pitch speeds well below his "actual" speed.

Flash1940 09-17-2008 02:55 AM

(Pitch X RPM)/1056 = Speed (Mi/hr)......with NO slip. Usually multiply the results by 0.85 (15% slip) and you'll be pretty close to reality.

Flash

Larry3215 09-17-2008 03:28 AM

Excellent - lots of discussion.

Don nailed it on the head in the second post. Most everyone else repeated the same basic premis in various levels of detail, so we are all more or less on the same page - as far as the theory goes.

In addition I have seen a number of Eagle Tree flight data graphs claiming to show this power unloading in flight. Graphs that clearly show a drop in power as time and/or speed increases during a flight.

However, Im not so sure these graphs PROVE the point because there is one additional factor that no one has mentioned so far, that I have always suspected plays a very large roll in this power drop over time.

Our batteries are NOT constant power sources the same way an internal combustion engine is (more or less).

We all know that battery voltage is not constant. It is dropping all the time during the flight and in some cases very dramatically during full power runs.

I suspect that the majority of the apparent power drop as speed increases is actually due to the drop in battery voltage as power is consumed and pack voltage drops.

The thing that most people forget is that the power consumed by a motor varies as the CUBE of the voltage. The amps vary as the square of the voltage.

If the load is constant and you double the voltage the power consumed goes up by a factor of 8 times! The amps go up by a factor of 4 times as much. Cut the voltage in half and the power consumed goes down to 1/8. The amps drop to 1/4.

So if you have a pack thats holding say 10.5 volts under a 30 amp load at the start of a speed run and the voltage drops by .2 volts during that run, the power to the prop will drop by almost 6%. A .5 volt drop will result in almost a 15% drop in power!!!

You have to remember that any drop in voltage also means a drop in RPM and therefore a drop in the amps as well. Its not a simple linear thing at all.

The trick is in gathering ALL the data that would be needed to verify what exactly is causing the power drop - voltage sag or prop unloading. I suspect that its both and that voltage drop is a large majority of the cause. Much more than "unloading".

I have yet to see a single Eagle Tree data file that contained all the info you would need to make a reasonably accurate assessment - either way.

Thats why I was asking for Eagle Tree or Medusa data files - we need voltage and amps and rpm at a minimum. Speed would be nice and altitude and throttle position would be nice. Most likely we will have to take the pilots word as to how the plane was flown at any given moment along the plot.

I plan to make some flights later this weekend specifically to test this so we will see.

In the mean time if anyone has any plots Id love to see them so we can take the data apart and see whats what :)

Larry3215 09-17-2008 03:40 AM

Oh - just to keep it interesting there is also pack heating under hi loads that can lower the Ir of a pack and actually increase its voltage - or at least reduce the amount of voltage drop :)

Tram 09-18-2008 04:31 PM

According to what I've seen via the EagleTree, things tend to unload quite a bit in the air..

I have had setup that draw 45 amps on the ground pull 35 amps in the air..

If I get some time tonight, I'll do some runs on the ground and read amps/watts and post a graph compared to the inflight data I sent you Larry..

I have to put myself firmly into the "things unload in the air" group of myth believers.. :)


Jeff
www.CommonSenseRC.com

Larry3215 09-18-2008 05:31 PM

I've been looking at the data you sent over and Ive decided I was 100% wrong - then I changed my mind back to my original stance, then back again, then back again!

The problem is that there is critical data missing. I cant tell for sure what your doing with the throttle stick and wheather or not your climbing or diving or flying level.

At some points on the graph it looks like your doing repeated climbs/dives and changing throttle fairly often - but I cant tell exactly where the changes occur.

Voltage on the packs actually climbs rather dramatically durring some of your speed runs, then it drops in others. Current draw also does some odd things at times that I cant decide if its caused by a sharp dive/climb indicating strong unloading - or are you relaxing the throttle slightly? Or some of both?

I think what we need is a fixed flight profile where we know what the throttle is doing at least and if possible level straight runs - or a mix of climb and dive with some way to tell which is which.

Do you by chance have a phase reading RPM sensor for your Eagle Tree unit? That would help as well.

Im thinking the following flight profile would be a good place to start. The spots along the way where the throttle gets chopped would be to mark the start and end of runs or changes in how the flying is done. Those breaks would make it easy to find them on the graph.

1)Plug in the pack and do a full power static run up for 5-10 seconds then chop the throttle.
2)Take off and climb to altitude at a lower throttle setting and get lined up for a speed run.
3)allow the plane to slow down as much as possible without stalling then briefly chop the throttle then go imediately to a full power straight and level run for say 10-20 seconds. At the end chop the throttle again briefly.
4)get lined up again for a speed run then chop the throttle and go back to full. Fly level for 5 seconds, then pull to a full power vertical for as long as possible. Chop the throttle again.
5) stay up hi as and in fact climb as far as you dare - then chop throttle again and go back full. Now start a full speed level run for say 5 seconds and then go into a full power dive and hold it as long as you can then chop throttle.

That would give us:
1) a static reference point on the ground.
2) level full power run with the plane increasing speed all the way as the pack voltage drops.
3) a level run turning into a vertical climb - should show the prop loading UP - maybe
4) a level run turning into a full power dive - should show maximum UN-loading

If we have pack voltage and amp data all the way through we can calculate the percentage of unloading thats due to pack voltage drop and compare that to the total power drops/increases. If we had rpm data that would be icing on the cake.

I need to go digging and see if I can find my rpm sensor....

By the way - Trams setup is a hi speed almost square prop, so that will give us a data point on that end of things.

I'll do my runs with a relatively flat prop on a slow flyer foamie and we will see if that is any different.

If we both fly the same flight profile that should help see any differences.

Larry3215 09-18-2008 05:35 PM

By the way - full speed dives can be risky so be sure your plane isnt subject to flutter. That can get a little toooo exciting!

Tram 09-18-2008 06:28 PM


The problem is that there is critical data missing. I cant tell for sure what your doing with the throttle stick and wheather or not your climbing or diving or flying level.
Throttle stick for ~85% of the flight is left in the "WOT" position. :)



I think what we need is a fixed flight profile where we know what the throttle is doing at least and if possible level straight runs - or a mix of climb and dive with some way to tell which is which.

Most of the flight goes something like this..

Launch..

Climb to altitude.. come by for a pass.. climb.. by for a pass.. climb.. by for a pass.. lather, rinse, repeat.. ;)


Do you by chance have a phase reading RPM sensor for your Eagle Tree unit? That would help as well.
Nope.. Just the inline..


Im thinking the following flight profile would be a good place to start. The spots along the way where the throttle gets chopped would be to mark the start and end of runs or changes in how the flying is done. Those breaks would make it easy to find them on the graph.
The EagleTree is capable of reading throttle position.. I'll see what I can whip up..


1)Plug in the pack and do a full power static run up for 5-10 seconds then chop the throttle.
2)Take off and climb to altitude at a lower throttle setting and get lined up for a speed run.
3)allow the plane to slow down as much as possible without stalling then briefly chop the throttle then go imediately to a full power straight and level run for say 10-20 seconds. At the end chop the throttle again briefly.
4)get lined up again for a speed run then chop the throttle and go back to full. Fly level for 5 seconds, then pull to a full power vertical for as long as possible. Chop the throttle again.
5) stay up hi as and in fact climb as far as you dare - then chop throttle again and go back full. Now start a full speed level run for say 5 seconds and then go into a full power dive and hold it as long as you can then chop throttle.
Probably tomorrow before I could get around to that, but sure.. I'll do it..


...full speed dives can be risky so be sure your plane isnt subject to flutter.
I don't think it's possible for the XT to flutter, the way it's built.. ;)

Larry3215 09-18-2008 06:58 PM

Outstanding!

bz1mcr 09-18-2008 07:04 PM


Originally Posted by fr4nk1yn (Post 476280)
:ack: You guys couldn't make it simple eh?

So a prop DOES unload(?). Now how much?

I made a thread awhile ago over at RCG on this.
The thing that got me interested was a plane that claimed to fly, IIRC, 20mph over it's pitch speed.


I know of a plane that was radar measured in level flight at a speed of 107 MPH, significantly above it's static pitch speed. This started a lot of controversy and resulted in many repeat measurments of both the static rpm and measured flight speed. The data repeated over and over. One parameter that could not be measured precisely is the pitch of the prop. Some props like the GWS 7X3.5 are known to not be what they are labled. The prop used on this plane was an Aeronaut 6X5 and as far as we could tell it was about a 5" pitch, but a difference of 10-20% would not be easy to measure.
the controversy ended when doppler sound measurements confirmed the radar speed measurements and provided an in flight RPM measurement that was in fact significantly higher than the static RPM.

Bottom line: The measured speed was well above the static pitch speed, but less than the in flight pitch speed.

Old Fart 09-18-2008 07:27 PM

2 Attachment(s)
Here's two graphs from my Brio - no "throttle monitoring, mut I can tell you the first drop is from unloading - I don't chop the throttle back that quick.

bz1mcr 09-18-2008 07:27 PM


Originally Posted by Larry3215 (Post 477097)
I've been looking at the data you sent over and Ive decided I was 100% wrong - then I changed my mind back to my original stance, then back again, then back again!

The problem is that there is critical data missing. I cant tell for sure what your doing with the throttle stick and wheather or not your climbing or diving or flying level.

At some points on the graph it looks like your doing repeated climbs/dives and changing throttle fairly often - but I cant tell exactly where the changes occur.

Voltage on the packs actually climbs rather dramatically durring some of your speed runs, then it drops in others. Current draw also does some odd things at times that I cant decide if its caused by a sharp dive/climb indicating strong unloading - or are you relaxing the throttle slightly? Or some of both?

I think what we need is a fixed flight profile where we know what the throttle is doing at least and if possible level straight runs - or a mix of climb and dive with some way to tell which is which.

Do you by chance have a phase reading RPM sensor for your Eagle Tree unit? That would help as well.

Im thinking the following flight profile would be a good place to start. The spots along the way where the throttle gets chopped would be to mark the start and end of runs or changes in how the flying is done. Those breaks would make it easy to find them on the graph.

1)Plug in the pack and do a full power static run up for 5-10 seconds then chop the throttle.
2)Take off and climb to altitude at a lower throttle setting and get lined up for a speed run.
3)allow the plane to slow down as much as possible without stalling then briefly chop the throttle then go imediately to a full power straight and level run for say 10-20 seconds. At the end chop the throttle again briefly.
4)get lined up again for a speed run then chop the throttle and go back to full. Fly level for 5 seconds, then pull to a full power vertical for as long as possible. Chop the throttle again.
5) stay up hi as and in fact climb as far as you dare - then chop throttle again and go back full. Now start a full speed level run for say 5 seconds and then go into a full power dive and hold it as long as you can then chop throttle.

That would give us:
1) a static reference point on the ground.
2) level full power run with the plane increasing speed all the way as the pack voltage drops.
3) a level run turning into a vertical climb - should show the prop loading UP - maybe
4) a level run turning into a full power dive - should show maximum UN-loading

If we have pack voltage and amp data all the way through we can calculate the percentage of unloading thats due to pack voltage drop and compare that to the total power drops/increases. If we had rpm data that would be icing on the cake.

I need to go digging and see if I can find my rpm sensor....

By the way - Trams setup is a hi speed almost square prop, so that will give us a data point on that end of things.

I'll do my runs with a relatively flat prop on a slow flyer foamie and we will see if that is any different.

If we both fly the same flight profile that should help see any differences.


Larry,
I think you are making it more complicated than necessary.

If the RPM in flight ever exceeds the static WOT RPM Then it must be because the prop unloaded. (regardless of throttle position or aircraft attitude).

You correctly point out that it is possible for in flight Voltage to be greater than static voltage because the batteries performance can improve early on as they warm up. As long as your battery is above 70 degees F during the static run, I am pretty certian that you can forget about it. But to see for sure you would need to monitor V and current. If at any current level the V is higher in flight it than it was at the same current during the static run it has to be because of the temperature effect.

Don


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