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RE: Maximum altitude?? - 4/4/2012 8:45 PM   
opjose



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ORIGINAL: IflyPATTERN

I thought the 400ft rule was never passed and/or only applied when within 3 miles of an airport? Regardless, this means I break the rule flying just f3a, and that glider/soaring flyers break them even more so. This just doesnt make sense to me...

Brandon


Things are still fuzzy about this given the FAA's proposed wording, but note Bill Hanson's reply in the following link particularly regarding UAV's.

400'ft rule

You'll be operating well outside the RC/AMA scope with your project.

Good luck it sounds interesting.

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RE: Maximum altitude?? - 4/4/2012 9:08 PM   
UStik


 

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I always though that an electric motor was most efficient at it upper limit of rpms...

That's right, but as you see in the diagram the drive (including prop) doesn't "unload" much with more speed, or the other way around, even static rpm is quite high. So the motor's efficiency is not far from maximum and the propeller's efficiency is paramount.

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RE: Maximum altitude?? - 4/4/2012 9:24 PM   
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Status: offline 		Thanks, I just wanted to be sure I understood it properly. A geared drive would be a good option from what I see...

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RE: Maximum altitude?? - 4/4/2012 9:32 PM   
lasers200


 

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Status: offline 		This has been done before at LSU. Go to the Burbank flying club and ask for a older gentleman named George. He can help.

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RE: Maximum altitude?? - 4/5/2012 12:22 AM   
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Status: offline 		I don't know enough yet to really add scientific/engineering input, but I do know that some of the highest flying full-scale airplanes out there have very long, very skinny wings.

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RE: Maximum altitude?? - 4/5/2012 12:42 AM   
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I did some geometric triangulation calculations... based off my scaled 2D sketches. With how I currently have things layed out, the biggest prop I can swing is about 19.5 inches. Is this, in you guys' opinions, plenty big enough, or still too small? Keep the good ideas coming. Ill try to keep you guys updated as best as possible with Easter coming up this weekend.

Brandon

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RE: Maximum altitude?? - 4/5/2012 4:24 AM   
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Status: online 		To throw a wrench in the plan here, but if the balloon idea is out, have you considered an internal combustion engine? You could load the plane with fuel and get up to altitude, then enjoy a lighter weight at cruise until it runs dry and have the lightest weight when gliding back down.

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RE: Maximum altitude?? - 4/5/2012 6:53 AM   
UStik


 

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Status: offline 		There is not much choice of motor kv and prop diameter/pitch, anyway, so you don't have to think that hard about optimum drive.

There are only a few big folding props, those I know of are the Graupner CFRP 20x12 (and maybe 17x10, here) and the aeronaut CAMcarbon 18.5x8/10/12/14/15 (here).

I found one really low-kv motor (AXI) which is even made for a 10s LiPo. They recommend their special 30x20E propeller but it's not a folding prop. It might still be good for this project though. I didn't check what matches.

But may I remind of further problems that already have been mentioned. The long runtime of the drive means there will be some heat to be dissipated from the battery. Maybe the 10 cells should be installed separately and not as a pack to get rid of the heat. Maybe it's not a problem because half an hour runtime means only a 2C load, but it has to be checked. As well has to be checked if the battery has to be warmed or at least insulated during motor-off flight (descent in the cold air at 15k ft).

Due to these problems I would use a radio with telemetry (besides the UAV control devices) because an ESC with integrated telemetry would save weight and hassle. Add temperature sensors for motor and battery and you have the drive under full control. Unfortunately, Multiplex announced those ESCs more than one year ago but still doesn't ship them, so there seems to be a problem with them. Maybe it's a problem with the switching BEC, though.

And an idea regarding flying the thing: The ESCs have a break that stops windmilling so the prop blades can fold by the airflow and drag is reduced. You may want to have drag, though, for landing approach control. If the throttle stick is slightly advanced from the off/break position, the motor turns slowly but the blades unfold and windmilling makes for an airbrake effect. Saves separate spoilers but requires some charge left in the battery for landing (which is good for stretching the approach as well.) But it has to be checked if the drive would give enough braking effect at all.

And yet another flying thing: A R/E plane may be tricky to land in gusty wind because there's no separate yaw and roll control, even less crossed controls. And worse, a high-aspect-ratio wing makes for big yaw and roll inertia. So I would just practise in a simulator, even if the plane had ailerons, or are you used to gliders?

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RE: Maximum altitude?? - 4/5/2012 12:38 PM   
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Status: offline 		IC motors is bad for high altitude because the air is thinner, and the carburettor of model IC motors isn't sophisticated enough to compensate.
So your motor will stop due to thin air even before the model is out of visible range (been there done that).
Notice that full scale planes with piston engines and carburettor has a mixture knob just as easily acessible as the throttle knob.

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RE: Maximum altitude?? - 4/5/2012 4:15 PM   
IflyPATTERN


 

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Status: offline 		Ustick,

Thanks for the great info and links you have provided. You have been very helpful! The only problmem I have with the Axi is it weighs over 3lbs! I am looking at this motor: http://www.f3aunlimited.com/webstore/index.php?main_page=product_info&cPath=9_22&products_id=44 It only weighs 18oz and will have propper wattage. Now, there is a geared version of this motor- so I might look into that.

I plan to have a "braking" system set up on the motor so that when it comes time to land I can slow the plane down. I don think that will be all too difficult with the folding prop in the equation(?)

Jester, Its not that I'm against the balloon idea- I think it could work! I feel most comfortable working with airplanes, and since the Research group has sorta already done the balloon thing, I dont think its as good of an idea. I wish an IC engine would be applicable in this case! (coming from a YS sponsored pilot )

Also, just curious... What exactly will be advantageous to having a motor (120kv) that will spin a 18-19" prop more slowly than a motor (1300kv) that would spin the same prop at a higher RPM?


Brandon

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RE: Maximum altitude?? - 4/5/2012 5:32 PM   
UStik


 

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RE: Maximum altitude?? - 4/5/2012 5:36 PM   
UStik


 

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Status: offline 		Yeah, those low-kv motors are heavy, that's just what I wanted to show. The Neu drive with gear seems to be well suited, maybe even a bit too powerful. You see, 1300 motor kv divided by 6.7 gear ratio gives 194 drive kv. That's a bit more than the AXI but the recommended props (about 20x14) would even fit. And I think you would find a matching centerpiece for the 10 mm prop shaft.

An efficient propeller needs a low power loading, that is Watts power per square meter prop disk area. The slower the plane flies the lower the power loading has to be. You may read about the theory here, especially the second picture (diagram) about optimum efficiency. Compare a prop drive to a jet engine: the jet has to accelerate a small amount of air to high speed to get the same thrust as a propeller which accelerates much more air to much less speed. The latter is more efficient, same airspeed provided.

According to the theory, you have to choose a big enough prop disc for the power the airplane needs for climb and for the intended airspeed. If you choose a too small prop disk you need a stronger motor because more power is wasted, and that is done by turning faster. So you can't compare to different kv motors on the same prop. The high-kv motor would have to be much more powerful two turn the same prop with more rpm. The bigger the prop disk is for a given thrust needed the slower the prop has to turn. So for the same plane: big prop - slow rpm or small prop - much rpm (and losses). Sorry, this explanation isn't really thought-out.

Anyway, for cases as this one a motor-gear combination is virtually always much lighter than a low-kv motor. Don't even think about doing without a gear.

By the way, airbrake effect is well possible with a folding prop, no problem. It's just that the effect lessens with speed so it's more for glide angle control. And you have to test the effect before relying on it, maybe on another model. And you have to check that the ESC doesn't prevent it.

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RE: Maximum altitude?? - 4/5/2012 6:18 PM   
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Status: offline 		Motors don't work the same so there's no real need to run at higher RPM. What DOES happen is that there's going to be some point where the internal DC resistance becomes significant at higher current levels and it drops the overall efficiency. What you want to do is pick a motor that you can run at the best peak efficiency for current and voltage. That'll give you the most output per lb of motor and battery weight. The RPM where this will occur is set by the Kv value. I've not seen any real advantage of high Kv motors over lower Kv motors other than when wound on the same frame the higher Kv value options use less turns of heavier wire than the low Kv option. So the DC resistance is lower for the high Kv version which means there's less heat generated at higher currents.

As for gears they will always soak up SOME power both for the friction of the teeth meshing as well as the additional bearings. So if they can be avoided then it is the better option. What determines if you go geared or not is how the propeller efficiency climbs with an increase in size vs the amount of power used up by any reduction gearing. To keep things simple I would strongly suggest sticking with direct drive low Kv motors at first.

The other good news is that since you'll be wanting the prop and motor to work best at a relatively low climb speed you can perform ground based static thrust testing for the props and actually trust the data to a much higher degree than for normal model flying. The options you'll find work the best will be the ones which provide the most thrust and those which are a tad higher in pitch and diameter without running the current consumption up higher than you want. The reason I say you can use the ones that are a LITTLE higher in pitch and diameter is because the drop off with pitches higher than around 8 inches occurs due to the blades stalling during static thrust testing. But once in the air for prop testing you may well find that those "nearly" options work better than the best ground performers.

Some of the older F3J class gliders were fine with polyhedral and rudder plus elevator. But in looking up images of those from today they seem to have downsized a little and gone to ailerons. But there were a lot of 4'ish meter span models that flew back 10 to 15 years ago with rudder, elevator and flaps or spoilers. And it's not like you'll be doing aerobatics with it... To get the most out of dihedral in this way it's best to go with a 5 to 7 panel style wing with each panel towards the tips becomeing smaller and at a higher angle to simulate the shape of an elliptical curve as much as practical. It puts the strongest responding part of the dihedral out at the tips where it has the most leverage to roll the model from the yaw to roll coupling.

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RE: Maximum altitude?? - 4/5/2012 7:41 PM   
UStik


 

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Status: offline 		The AXI and the Neu motors are not comparable power-wise, the AXI is more powerful and weighs 840 g more. But even if a direct drive and a gear drive are comparable and the weight difference is smaller then this weight put into more battery usually outweighs the 5% gear losses. It's often like in this case: The low-kv motor having the required kv is just one power class higher than the higher-kv gear combination and thus has considerably more weight. AXI offers geared outrunners for a reason: "The possibility of spinning extremely big propeller compared to the size of the unit".

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RE: Maximum altitude?? - 4/5/2012 10:52 PM   
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Status: offline 		Thrust determined by newtons law (throw air in one direction, and you get pushed by the same force in the other direction), is determined by M * V.
That is, the mass of the air the propeller throws, multiplied by the velocity it accelerates the air with.

This means, the same amount of thrust can be produced by take a lot of air and give it a small acceleration (big prop, spinning slowly).
Or by taking a small amount of air and give it a big acceleration (small prop, spinning quickly).

However, the energy needed to produce this thrust is determined by 0.5*M*V².
That is, doubling the mass accelerated will double the energy input needed.
But doubling the velocity of the same air, will quadruple the energy needed.

Thats why big props spinning slowly is more efficient.
But big props spinning slowly is also heavier, so size has to be kept within the limits of reason.

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RE: Maximum altitude?? - 4/5/2012 10:57 PM   
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ORIGINAL: Von Ohain

IC motors is bad for high altitude because the air is thinner, and the carburettor of model IC motors isn't sophisticated enough to compensate.
So your motor will stop due to thin air even before the model is out of visible range (been there done that).
Notice that full scale planes with piston engines and carburettor has a mixture knob just as easily acessible as the throttle knob.


Pretty sure Maynard Hill cracked the problem on this. I know he set an official altitude record with an IC engine that was around 8k feet and made flights over 10k ft.

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RE: Maximum altitude?? - 4/5/2012 11:08 PM   
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Status: offline 		You can workaround it by having a servo operating the mixture needle.
However, this is "blacksmithing" at best and definetly not keeping it simple, if that is an aim.
Servo operated mixture needle requires you to adjust the mixture in-flight, and you have to guess what needle position is required for which altitude.
Even worse still, you also need to guess the altitude of the aircraft unless you got telemetry with real time feedback.

And no model IC motor manufacturer will provide you any chart for altitude vs mixture, hence you need to work out on your own how to adjust the motor for which altitude.
In my opinion, that is a university project of its own.
And since propulsion is not the main study of this particular project, I'd say its just not worth it to do it more complicated than necessary.

If IC is even a realistic option, i'd recommend turboprop.
Gas turbines is by nature much less sensitive to both mixture and thin air.

My first choice would still be electric, because its simplest to operate, most reliable and most versatile.
Possibly also cheapest, given the trouble with making a piston IC work with altitude.

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RE: Maximum altitude?? - 4/6/2012 12:11 AM   
jessiej



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ORIGINAL: gaRCfield

I don't know enough yet to really add scientific/engineering input, but I do know that some of the highest flying full-scale airplanes out there have very long, very skinny wings.



True,, but also true that there are reynolds number problems when going from large to small.

That being said, how many reading this thread have seen a current generation F3C freeflight model using a folding wing to go from low aspect ratio for a 450-500 foot climb in 5 seconds to a high aspect ratio for a sailplane-like glide?

Not sure how/if that approach could be applied to the project under discussion, but those models are sure impressive to watch, and the restrictions on power/engine run vs duration provide food or thought.

jess

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RE: Maximum altitude?? - 4/6/2012 1:43 AM   
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Pretty sure Maynard Hill cracked the problem on this. I know he set an official altitude record with an IC engine that was around 8k feet and made flights over 10k ft.


Actually Maynard Hill set a record altitude of 26,919 feet 42 years ago. It was that project that he invented the electrostatic wing leveler that he also used to maintain a fixed climb angle through the flight. That system measured the voltage of both wingtips, and used the difference in voltage to level the wings. Another sensor mounted to the tail was used to control pitch. The technology was going to be sold by World Engines, was advertised once and never mentioned again. Turns out that the US government thought it would be too easy to weaponize into cheap drones.

The sensor exploited the fact that there is a voltage gradient of about 100 Volts per meter of altitude. However to detect it you must first ionize the air and use a very high input impedance amplifier to sense it. This is easily done with a small amount of radioactive isotopes, in fact you most likely have 3 or 4 in your home right now. They look like smoke detectors and sell for ten bucks at Walmart.



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RE: Maximum altitude?? - 4/6/2012 1:53 AM   
da Rock



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quote:

ORIGINAL: j.duncker


quote:

ORIGINAL: Von Ohain

IC motors is bad for high altitude because the air is thinner, and the carburettor of model IC motors isn't sophisticated enough to compensate.
So your motor will stop due to thin air even before the model is out of visible range (been there done that).
Notice that full scale planes with piston engines and carburettor has a mixture knob just as easily acessible as the throttle knob.


Pretty sure Maynard Hill cracked the problem on this. I know he set an official altitude record with an IC engine that was around 8k feet and made flights over 10k ft.



Maynard's infernal combustion powered altitude record for a model was something like 26,000 iirc.


ooops.......... post overlap, sorry.

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RE: Maximum altitude?? - 4/6/2012 5:39 AM   
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Status: online 		On the balloon topic, I didn't mean just using a balloon to get the samples. I meant using a balloon to haul your plan up to altitude then take over powered flight from there. Since the plane wouldn't have to climb under motor power, your electrical system would only need to make enough power to make the plane cruise. That would let you use a much less powerful motor system saving significant weight, or would let you keep the weight the same and have a lot more loiter time up high if that's what you would want. If you don't need any substantial loiter time at altitude, a pure glider could make the flight simplifying the project substantially.

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RE: Maximum altitude?? - 4/6/2012 8:32 AM   
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Status: offline 		Forgot to illustrate BMatthews' advice to use a multi-panel wing with an odd number of panels. The picture shows a 5-panel wing were the center segment is a piece of its own. The biggest bending stress of the whole wing is in the middle of this segment where no wing joiner is. The main spar is C/F reinforced, otherwise the wing is a traditional wood construction. It's a three-piece wing with two joiners quite far outboard which can be simple and lightweight. And the pieces are shorter than the fuselage while two pieces would be longer.

You see the upright V-tail and may imagine that it produces an adverse roll when used as rudder. The inverted V-tail produces proverse roll. The effect is small, though.

Just for the record: Such high-performance electric gliders have usually geared drives for reasons mentioned above. Efficiency of a good gear is about 95% because nearly all friction is rolling friction, like railway wheels on rails have. The bearings are ball or needle bearings and the toothed wheels have involute toothing. That's also why there is nearly no wear. Just an annoying whine due to straight toothing...

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RE: Maximum altitude?? - 4/6/2012 3:39 PM   
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ORIGINAL: da Rock

quote:

ORIGINAL: j.duncker


quote:

ORIGINAL: Von Ohain

IC motors is bad for high altitude because the air is thinner, and the carburettor of model IC motors isn't sophisticated enough to compensate.
So your motor will stop due to thin air even before the model is out of visible range (been there done that).
Notice that full scale planes with piston engines and carburettor has a mixture knob just as easily acessible as the throttle knob.


Pretty sure Maynard Hill cracked the problem on this. I know he set an official altitude record with an IC engine that was around 8k feet and made flights over 10k ft.



Maynard's infernal combustion powered altitude record for a model was something like 26,000 iirc.




ooops.......... post overlap, sorry.


WOW Anybody know what engine setup he used?

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RE: Maximum altitude?? - 4/6/2012 4:34 PM   
da Rock



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quote:

ORIGINAL: j.duncker

quote:

ORIGINAL: da Rock

quote:

ORIGINAL: j.duncker


quote:

ORIGINAL: Von Ohain

IC motors is bad for high altitude because the air is thinner, and the carburettor of model IC motors isn't sophisticated enough to compensate.
So your motor will stop due to thin air even before the model is out of visible range (been there done that).
Notice that full scale planes with piston engines and carburettor has a mixture knob just as easily acessible as the throttle knob.


Pretty sure Maynard Hill cracked the problem on this. I know he set an official altitude record with an IC engine that was around 8k feet and made flights over 10k ft.



Maynard's infernal combustion powered altitude record for a model was something like 26,000 iirc.




ooops.......... post overlap, sorry.


WOW Anybody know what engine setup he used?



Have to look up the magazine article. A number of his different type record attempts required some out of the box solutions. At least one of them used a small percentage of gasoline in the fuel. With any luck the post with the answer will show up in about 10 seconds.

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RE: Maximum altitude?? - 4/6/2012 4:36 PM   
Von Ohain


 

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Status: offline 		If you are hellbent on doing it with piston, you can do it with fuel injection motors (I believe OS had fuel injected competition grade engines for many years now. They are priced as if they were made of gold though). Or as mentioned above, servo operated mixture (and possible automated, if you are into programming microcontrollers).
IDK how Hill did it, but said methods should make it possible. Hill was a skilled chap it seems, so I wouldn't be surprised if he had automated mixture control of some kind.
That would be a minimum requirement in my opinion to have any reliability at altitude with curburettor motors.

Its still not worth in in my opinion though, because of price and complication.
If it turns out that electric is not feasible, then go for turboprop.
Turbomachinery is in general a lot more efficient at altitude, and at extreme altitude its also the only thing that will run at all. Because it doesn't have a fixed compression ratio, it will compensate for the thin air by spooling, hence increasing its compression as needed to keep up to rated power.

Heres a price example aswell:

Fuel injected piston:
http://www.jhbol.com/store/index.php?main_page=index&cPath=64_65_70

Turboprop:
http://www.1classifieds.co.uk/-1/posts/2-For-Sale/28-general/2878822333-Wren-MW44-Gold-Edition-Turbine-Turboprop-Engine-pound-1100.html

The price is about the same, dollar for power. The turbo is twice the price, but its twice the power aswell. Plus, it has a lot of features included that the F.I. piston doesn't.

Turbo also has the added benefit of wetstart and integrated starter motor, so IF you shall have a flame-out (motor stops, in jet language) you can just restart it with a push on your radio, IN FLIGHT.
If the piston sucker stops at altitude (which it is a lot more likely to do) you will have to land the plane and restart it manually, on the ground.


That said, I still believe electric is simplest and best, but if it turns out you can't get enough battery into the plane to make electric feasible, then please consider turbo seriously before you go with piston.
Turbo has a lot to show for, for this purpose.

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