Hints for high altitude flying?

On Tue, 13 Mar 2007 14:57:00 -0600, I said, "Pick a card, any card" and Jennifer Smith instead replied:

A few critical questions here.

Fossil fuel or electric?

What's your altitude above sea level from where you launch? Is that altitude figured into the 9000ft you cite above or are you already at 5000 feet before you launch?

-- Ray

Oh, sorry... electric. Launch altitude is around 9000ft ASL.

On Tue, 13 Mar 2007 15:26:08 -0600, I said, "Pick a card, any card" and Jennifer Smith instead replied:

At 9000ft, you are at about 75% air density which affects both your prop efficiency and your wing lift. This, in turn, affects your overall loading. You'll have to take all that into account when designing a high altitude craft. Also, ambient air is cooling your motor and you may have to sink some of that heat from the motor or risk causing it to overtemp. Same goes for your other electronics. The air density is calculated into the operation of those units.

I worked on some quotes for an autonomous NASA high altitude glider a few (well, many, actually) years ago. The wings had to be very strong and light. They also had to be made with the upper surface able to gather solar energy.

NASA gave up on the project after receiving no bids. Wonder why? The estimated cost by all bidders was around 5 times the allowed cost by NASA. Perhaps it can be done today. That's for the young engineers to work out. Bless 'em.

-- Ray

On Wed, 14 Mar 2007 06:41:57 +0900, I said, "Pick a card, any card" and Ray Haddad instead replied:

Here's a useful link for you:

Ray

If the prop you are using provides good performance at lower altitudes I would not increase the pitch. Planes tend to have the most satisfying performance when speed set by rpm times the pitch is roughly equal to realized air speeds. Under these conditions the prop is not 'slipping' in the air very much. It sounds like you are not building much air speed so you have all kinds of slip in the prop. More pitch will just make the slip worse. At high altitude you will need a larger diameter prop. Depending on how marginal the whole system is it might even need less pitch and even more diameter. I would only go to three blades if forced to by lack of ground clearance and even then doubt if it will solve the problem.

More rpm may well help. But if the prop is a poor match to air density and drag from the rest of the system it would be better to first improve the match before dumping more rpm into it.

With any changes make sure your heat dissipation is still adequate or the motor will cook.

You really are not all that high so it should be fixable.

You say if floats ok so total lift does not sound like it is a problem.

If you are not getting much air speed and still are able to control the plane the control surface areas are ok.

Holy cow Jenny. Thats HIGH. I go dizzy up there.

The plane will need to fly faster, as the air will generate less lift, so in general you add some pitch, but also cos the air is thinner you need to move more of it, so you add a bit of prop diameter as well.

So if you flew an 8x6 at sea level you might be on a 9x6, 9x7 or 10x7 at altitude.

I just ran a Motocalc simulation on one of my planes that I fly on an

11x7, and going to a 12x8 or possibly an 11x10 seemed to give the right answers for 9000 feet altitude.

It looks like more pitch is a shade more important than more diamter.

I would. Definitely.

Sadly to stay up and climb,in the thin air, the plane needs to fly faster.

Right starting point, wrong conclusions.

Wont need bigger control surfaces - the plane will fly faster - has to - so these have similar effects.

| If the prop you are using provides good performance at lower altitudes | I would not increase the pitch. Planes tend to have the most | satisfying performance when speed set by rpm times the pitch is | roughly equal to realized air speeds.

I think the (rpm * pitch) needs to be slightly higher, actually. (But you may be looking at ground rpm rather than the faster unwound rpm in flight.)

| At high altitude you will need a larger diameter prop. Depending on | how marginal the whole system is it might even need less pitch and | even more diameter.

If so, then it would have benefited from that down lower too.

| I would only go to three blades if forced to by lack of ground | clearance and even then doubt if it will solve the problem.

Agreed. As a rule of thumb, the more blades your prop has, the less efficient it is. Some FF guys even use one-blade props. | More rpm may well help.

Alas, you can't just buy RPM at the hobby store. RPM comes with a smaller prop, or more volts, a different motor or a different gear ratio. It's easier to just put on a prop with a higher pitch.

| With any changes make sure your heat dissipation is still adequate or | the motor will cook.

I suspect that the effective cooling the motor will get will actually remain the same at altitude (assuming the same prop), as the plane will have to fly faster, which will move the less dense air across the motor faster.

However, you need more power in less dense air, so the motor will generate more heat. So yes, you've got to be wary of that.

| You really are not all that high so it should be fixable.

Depends on the plane and power system. If it was marginal down lower, it might not work at all up here without changes.

| You say if floats ok so total lift does not sound like it is a | problem. | | If you are not getting much air speed and still are able to control | the plane the control surface areas are ok.

It usually boils down to power system. As an extreme example, if you've got half the air density (which corresponds to roughly 18,000 feet), then you'll need to fly 41% faster to get the same lift and I think you'll need twice as much power to maintain altitude (but you're going twice as fast, so your `mpg' doesn't change.) However, your plane will still be just as maneuverable as it was down lower -- you can still do a 180 degree turn in X seconds -- but you'll need more space to do it, because you're going faster. The added speed cancels out the lowered air density.

As for the `power needed', I'm ignoring effects like engines and jets being more efficient at certain ranges and pressures and such. It really gets to be a whole lot more complicated.

and talk about the additional factors somewhat if you want to read it, but they don't really matter much to us.)

Fortunately, the original poster doesn't live at 18k feet, so things aren't quite so bad.

| I suspect that the effective cooling the motor will get will actually | remain the same at altitude (assuming the same prop), as the plane | will have to fly faster, which will move the less dense air across the | motor faster.

In retrospect, I think that's wrong. Using my example from earlier, if the air density is 50% sea level, then the speed is 41% higher, so we're moving 1.41 * 0.5 or about 70% of the air across the motor as we'd do at sea level. So it's a double whammy -- less cooling, but your motor is working harder.

| However, your plane will still be just as maneuverable as it was | down lower -- you can still do a 180 degree turn in X seconds -- but | you'll need more space to do it, because you're going faster.

I goofed there too. Yes, if you pull back on the stick, it'll put the same force on the plane (since you're going faster in the less dense air) but the plane has a lot more inertia than before (because it's going faster) so it'll be less maneuverable.

Oh well, can't get everything right the first time around :)

Take a look at

for a description of how altitude affects electric models. MotoCalc can take altitude into account in its predictions as well.

Stefan Vorkoetter

Jennifer,

Why not measure the air speed at altitude? Just time how long it takes to go 200 meters in level flight and no wind. It only takes two people and one stop watch. Turn this into miles per hour or Km/hr whichever units are comfortable for you.

Then measure the prop rpm on the ground and from rpm and pitch calulate what the air speed should be in theory. This calculated air speed will be low as the prop unloads in the air and turns faster then on the ground.

If your meaured air speed is 95% or better of what the rpms and prop pitch say it should be TNP is correct, you need more pitch to get the plane to fly faster.

I suspect you will find measured air speed is well below 95% of theory. In that case you need more diameter at the same pitch as I suggested. If the air speed is well below the prop speed your present prop is wasting a lot of energy blowing air instead of generating thrust. The thing is not a ducted fan after all. Putting more pitch on it will simply make the situation worse.

The reason I suspect you are short on diameter rather then short on pitch is because you say the plane floats very well. To me a floater is a plane that still has enough lift to be controlable at something like 15 mph. So it does not sound like lift is the problem. It sounds like air speed is the problem. Air speed is a matter of matching the prop to the conditions and drag of the plane to get the most useful thrust from a given power input. There are lots of combinations where less pitch and more diameter give you more thrust and therefore more airspeed. There are also lots of combinations where the opposite is also true. Depends on where you start.

Thanks for all the hints and pages! I think my course of action is clear now: I'll try a larger prop with maybe a slightly higher pitch and add additional cooling to the motor and ESC. We'll see how it goes :)

Thanks again Jenni