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Maximum altitude??

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Old 04-04-2012, 10:11 AM
  #76
IflyPATTERN
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Default RE: Maximum altitude??

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...

Im trying to keep everything as simple as possible, as Von had already suggested. Im liking the idea of doing away with ailerons and using a sort-of V-tail instead. Also, I am gathering I should look into using alower Kv electric motor in conjuntion with the biggest prop possible. This all sounds reasonable and I shall try to work around that. If anyone has any motor, prop, battery, and esc reccomendations- I would be more than happy to listen. Id rather not have a gearbox. Its just one more thing to have to maintanance and possibly fail.

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Old 04-04-2012, 10:39 AM
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Default RE: Maximum altitude??

Gear should be no problem, there are maintenance-free gears which virtually never fail. Maybe you end up with a gear-less solution but you shouldn't exclude a gear in the first place. Maybe you just can't find a low enough kv motor. The mentioned AXI F3A has only 235 kv but is fed by a 10s LiPo battery, which is a good idea. But then it turns just too fast for your project. A gear practically reduces kv, just divide the motor kv by the gear ratio.

The problem of lower air density mentioned by BMatthews is another thing to watch out for but I thought we get the basic points clear first. Aerodynamically, all gets faster with altitude, that is airspeed increases and the prop turns faster. The motor is unaffected by air density but turning faster means giving less torque. So there is just a maximum altitude (ceiling) for the airplane and it needs a stronger drive than the average model to get that ceiling above 15 k ft.

Temperature and moisture effects are yet other (smaller) influences and have been mentioned as well.

I forgot: The "paddles" have some aileron effect but are no full replacement, especially not with a high-aspect-ratio wing. What you need is dihedral for stability and basically a R/E plane. Polyhedral wings handle nicely and effective dihedral should be 9 degrees, that's a good rule of thumb.

To illustrate what I mean: The blue/yellow parkflyer has 3 / additional 12 degrees dihedral on the inner/outer wing panels, giving 8.8 degrees effective. The Motorglider has 3 on the wings and additional 30 degrees on the tiplets, giving 9.0 equivalent. Both are typical shoulder-wing R/E models.

Edit: Naah, I was wrong. I forgot that the average model has a strong drive. Just tried with a calculation for my Sr. Telemaster with a weak drive. It's not exactly overpowered but would still climb at 18k ft (half the ground air density), just with only 58% climb rate and 18% more air speed. It would need about 40 minutes to get at 18k ft, so that's the reason for a stronger drive (which I have).
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Old 04-04-2012, 11:35 AM
  #78
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Default RE: Maximum altitude??

Gear drives are not all that bad today, and using one would lower your current draw, this may actually give you a bit more air time as a side effect.
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Old 04-04-2012, 11:35 AM
  #79
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Default RE: Maximum altitude??

Notice the 40 minute estimate to reach 15k ?

That is probably going to be a problem is you expect battery power to get you there. There are a couple of apps around that work out all the electric parameters based on available components. They might or might not be able to accept battery sizes required for such long motor runs. There aren't too many model applications today that require such long, strong runs. Those apps would be worth playing around with once you've got some idea of the motor you might be using.

Efficiency might also be worthwhile. I think inline motors (often with gearing) have a significant edge over outrunners.
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Old 04-04-2012, 11:42 AM
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Default RE: Maximum altitude??

I always though that an electric motor was most efficient at it upper limit of rpms...
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Old 04-04-2012, 11:45 AM
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Default RE: Maximum altitude??


Quote:
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.

[link=http://amablog.modelaircraft.org/amagov/2012/02/15/president-signs-faa-bill-includes-protection-for-model-aviation/]400'ft rule[/link]

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

Good luck it sounds interesting.
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Old 04-04-2012, 12:08 PM
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Default RE: Maximum altitude??


Quote:
ORIGINAL: iron eagel

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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Old 04-04-2012, 12:24 PM
  #83
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Default RE: Maximum altitude??

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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Old 04-04-2012, 12:32 PM
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Default RE: Maximum altitude??

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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Old 04-04-2012, 03:22 PM
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Default RE: Maximum altitude??

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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Old 04-04-2012, 03:42 PM
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Default RE: Maximum altitude??

Ok,

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.

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Old 04-04-2012, 07:24 PM
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Default RE: Maximum altitude??

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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Old 04-04-2012, 09:53 PM
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Default RE: Maximum altitude??

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, [link=http://www.graupner.de/en/products/a19e1365-97ab-4565-bb27-a7b06a1c1fea/productcategory.aspx]here[/link]) and the aeronaut CAMcarbon 18.5x8/10/12/14/15 ([link=http://www.aero-naut.de/en/products/aircraft/airplane-accessories/propellers/camcarbon.html]here[/link]).

I found one really low-kv motor ([link=http://www.modelmotors.cz/index.php?page=61&product=5360&serie=20&line=GOLD]AXI[/link]) 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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Old 04-05-2012, 03:38 AM
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Default RE: Maximum altitude??

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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Old 04-05-2012, 07:15 AM
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Default RE: Maximum altitude??

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...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?


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Old 04-05-2012, 08:32 AM
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Old 04-05-2012, 08:36 AM
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Default RE: Maximum altitude??

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 [link=http://www.mh-aerotools.de/airfoils/propuls4.htm]here[/link], 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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Old 04-05-2012, 09:18 AM
  #93
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Default RE: Maximum altitude??

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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Old 04-05-2012, 10:41 AM
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Default RE: Maximum altitude??

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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Old 04-05-2012, 01:52 PM
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Default RE: Maximum altitude??

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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Old 04-05-2012, 01:57 PM
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Default RE: Maximum altitude??


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.
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Old 04-05-2012, 02:08 PM
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Default RE: Maximum altitude??

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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Old 04-05-2012, 03:11 PM
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Quote:
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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Old 04-05-2012, 04:43 PM
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Default RE: Maximum altitude??

Quote:
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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Old 04-05-2012, 04:53 PM
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Default RE: Maximum altitude??

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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