Author Topic: Mah draw at higher altitudes? (Read 2178 times)

Russell Bond · « **on:** December 12, 2010, 08:02:27 PM »

Does anybody know if you take more or less out of the battery as you increase altitude?

Also, if it is a hotter day, does the battery usage alter? I'm assuming on this last question that you would have to increase rpms slightly to compensate for the lower density, thereby using more power from the battery.

PJ Rowland · « **Reply #1 on:** December 12, 2010, 08:15:14 PM »

Russle, when its hotter, the air is thinner, so you would require more power.

I see no difference between electric and IC in this instance. Same with Alti..
try a bigger venturi

Russell Bond · « **Reply #2 on:** December 12, 2010, 08:17:37 PM »

Howard Rush · « **Reply #3 on:** December 13, 2010, 12:31:41 AM »

Maybe in the Southern Hemisphere.

For increasing altitude or increasing temperature, the air density goes down. It's PV=nRT again. If you are flying at the same speed in less dense air, you'll use less power. I made a spreadsheet to calculate battery energy required for a given amount of drag. You can have a copy if you like.

Internal combustion engines produce less power when air density goes down. This is probably a bigger effect than the reduction of power required to overcome drag. Hence you do things like bigger intakes and more nitro when density decreases.

Rudy Taube · « **Reply #4 on:** December 13, 2010, 12:53:45 AM »

Hi Russel,

I 2nd what Howard said.

It's not by accident that the fuel burned for us to drag several hundred thousand pounds of aircraft and passengers six vertical miles is offset by the fuel saved by flying the rest of the flight in real "thin" low drag air.

When moving up to E power we will have to set aside some of what applied to our wet power systems. Our wet systems are sensitive to thin air unless you turbo/super charge them. Just like humans in thin air, they find it hard to breathe and their power decreases. They will lose about 25% of their power at a density alt. of 7,500'. We would see this on many elevated (>3,000') places on a hot summer day. Less so at lower elevations, but still a factor.

Just one more advantage of E power.

..... But like Howard pointed out, larger venturi, more nitro etc. works too.

This is just addressing the two different power systems. There are differences in aircraft/wing performance and feel during maneuvers in thin air, but that is for another thread somewhere beyond this E section of the forum.

Russell Bond · « **Reply #5 on:** December 13, 2010, 12:54:58 AM »

Thanks Howard & Rudy,
I thought I read somewhere that you use less power as the altitude increases. That's great as the Nationals in 2 weeks are at 1100' above sea level.

I'd certainly like the spread sheet please, do you need my email?

Rudy Taube · « **Reply #6 on:** December 13, 2010, 02:23:46 AM »

Russell,

This chart may give you some idea of what you are looking at for your Nats.

If your field is at 1100' and your air temp is 35C then you and your plane will think it is at around 1169 meters (3834'). This means your friends wet systems will be down about 17% on power. While your E power will remain the same as if it were at sea level. The loss is not linear, just like our atmosphere distribution. So if they are GOOD friends then tell them to increase their power. .... if they are NOT good friends, tell them nothing.

BTW: The rate of climb under these conditions will be decreased by around 28% with a wet system. And a little decrease even with our E systems, due to prop and wing lift issues. Not to worry though, you will have an advantage over most of the wet guys there. You may want to increase your RPM by a few hundred to give you the same feel you have at a lower alt. .... Good luck, I hope you get another trophy for E power! :-)

    Density Altitude "Rule of Thumb" Chart


    STD TEMP     ELEV/TEMP 80 deg F     90 deg F     100 deg F     110 deg F     120 deg F     130 deg F



    59 deg F     Sea Level     1,200     1,900     2,500     3,200     3,800     4,400



    52 deg F     2,000     3,800     4,400     5,000     5,600     6,200     6,800



    45 deg F     4,000     6,300     6,900     7,500     8,100     8,700     9,400



    38 deg F     6,000     8,600     9,200     9,800     10,400     11,000     11,600



    31 deg F     8,000     11,100     11,700     12,300     12,800     13,300     13,800

Russell Bond · « **Reply #7 on:** December 13, 2010, 04:15:07 AM »

NAH!!!....Don't think I'll tell them...hahaha!

Thanks for the chart, that's a keeper!

BTW, the days are expected to be about 35c....yuk!

Howard Rush · « **Reply #8 on:** December 13, 2010, 08:16:09 AM »

Here are some Web sites with air properties calculators:

http://www.digitaldutch.com/atmoscalc/
http://aero.stanford.edu/StdAtm.html
http://www.aerospaceweb.org/design/scripts/atmosphere/

Keith Renecle · « **Reply #9 on:** December 13, 2010, 09:52:37 AM »

Hi Russel,

My 1820 gram Electron uses around 1800 to 1900 mAh in Johannesburg South Africa (6000 ft ASL) and 2200 to 2400 at sea level depending on the temperature. As the guys say, it is indeed similar to a wet setup and will change with air density, but the effects are negligible on the electric setup. You will just notice the decrease in battery useage at the higher altitude. The neat thing is that it's dead easy for you to set your rpm up a little without the need for head shim, nitro, or prop changes. I would guess that your rpm change will be very little, but just set it to suit the speed that you're comfortable with. With the heat that you mention (35 degrees C) just make sure that all the components are well cooled and that none of them are on the edge of their spec. Good luck with your Nats, and please post the results.

Keith R

Howard Rush · « **Reply #10 on:** December 13, 2010, 10:29:06 AM »

Quote from: Rudy Taube on December 13, 2010, 12:53:45 AM

It's not by accident that the fuel burned for us to drag several hundred thousand pounds of aircraft and passengers six vertical miles is offset by the fuel saved by flying the rest of the flight in real "thin" low drag air.

It's not by accident, but maybe not for that reason. That's mostly induced drag. For a given lift/drag ratio, drag will be the same, because lift = weight is the same. Energy expended is force x distance = drag x distance, so you'd expect it to take the same amount of fuel to get there, regardless of air density. With the lower density at altitude, though, an airplane has to fly faster to get the requisite lift, so it gets the passengers there quicker and makes more money per day than it would flying lower. Other stuff is involved, like jet engines working better in cold air, but I've either forgotten or never understood most of it.

Rudy Taube · « **Reply #11 on:** December 13, 2010, 04:36:58 PM »

Hi Keith,

Thank you for the real life data, this is exactly the info. we need. A 20% mAh savings sounds good.

Because your ground speed increases in relation to the planes actual airspeed do you adjust your lap times by much?

Regards,

WhittleN · « **Reply #12 on:** December 13, 2010, 05:25:28 PM »

I recently went from 4,650 MSL to 200 MSL. Power consumption up by 150-200 ma no adjustment to RPM still 5.35 sec lap times.

Norm Whittle

Rudy Taube · « **Reply #13 on:** December 13, 2010, 11:37:16 PM »

Thanks Norm,

Do you remember what the temperatures were at both elevations?

WhittleN · « **Reply #14 on:** December 14, 2010, 06:32:29 AM »

Rudy
Temperature in Utah low 80's at Chehalis WA high 70's
Norm

Rudy Taube · « **Reply #15 on:** December 14, 2010, 10:23:47 PM »

Thanks Norm,

This means that you were going down from a DA of about 6,500' to about 600'. I assume you are using governor mode so that would mean that your RPM should remain the same at both Alt. using E power. With no other variables it is amazing that your lap times did not vary with the variation in ground speed? It could be that the increase in prop efficiency in the lower dense air, exactly made up for the increase in all three drag factors including the lines. I don't think we have any accurate drag data for our CL planes at our low Reynolds number and low speeds, so it would be difficult to see how this could happen.

Someone mentioned that their CL plane feels different at different altitudes. Did you find this to be true? Since you were flying at a slower "airspeed" at alt. I assume you could tell the difference in lift (or lack of?) coming out of the corners and at the top of the HG?

Thanks again for your data.

Bill Little · « **Reply #16 on:** December 14, 2010, 11:52:31 PM »

Well, Mah used to go down a bit in the holla to do her drawin' We had to draw it from a branch.. She usually sent we'uns so the girls wouldn't fall and get hurted up.

WhittleN · « **Reply #17 on:** December 15, 2010, 06:42:52 AM »

Rudy
I had to move the handle line spacing in 2 holes per line when I went to the lower altitude. Otherwise the thing turned way to tight.

Bill
You do what yo mama says you hear.

Norm

Rudy Taube · « **Reply #18 on:** December 15, 2010, 07:33:46 PM »

Norm,

Neat solution, I'll bet your wing and control surfaces were very happy with all that thick air to play with!

I'm still puzzled that your lap times did not change. My old text books tell me that there should have been a change. Obviously I'm missing something. Maybe Howard will have some ideas on this. I just drive airplanes, he, Brett, Paul, and others on our forum design stuff that really fly out of this world.

Howard Rush · « **Reply #19 on:** December 15, 2010, 07:50:14 PM »

I'd check those textbooks again.

Howard Rush · « **Reply #20 on:** December 15, 2010, 09:11:36 PM »

To elaborate: The electric stunt system keeps RPM constant. In level flight, what the prop does and airplane drag other than induced drag are both pretty much proportional to air density. According to my ciphering, induced drag would decrease as density increases. I figure induced drag is lift^2 x 2 / (span^2 * pi * Oswald's efficiency factor * density * true airspeed^2) . Does that look right? For a 64-oz. airplane of 60" span with elliptical lift distribution, flying 55 MPH, I get .029886 lb. of induced drag at 4,000 ft. and .02633 lb. at sea level, a difference of .003556 lb., which is 0.389 watt, a drop in the bucket.

I left off the conclusion. I'd guess the level-flight speed would be pretty much the same.

Keith Renecle · « **Reply #21 on:** December 15, 2010, 11:06:33 PM »

Quote from: Rudy Taube on December 13, 2010, 04:36:58 PM

Hi Keith,

Thank you for the real life data, this is exactly the info. we need. A 20% mAh savings sounds good.

Because your ground speed increases in relation to the planes actual airspeed do you adjust your lap times by much?

Regards,

From 6000 ft down to sea level or close to it, my lap times are faster. A comfortable lap time for me with a constant rpm is 5,2 secs per lap on full length lines, and this goes down to around 4,9 to 5,0 secs. I just lower the rpm around 200 or so until I'm comfortable. I normally set my timer for a flight of 2 minutes so that I can get a few quick flights out of one battery. It's so great to fly at low altitude coming from the high altitude in Johannesburg with thin air. Usually my first few wingover pull-outs end up at around 20 feet above the deck! When the coastal guys fly up here, the situation is reversed so things can get quite exciting!

Keith R

Howard Rush · « **Reply #22 on:** December 15, 2010, 11:51:23 PM »

I live near sea level. The first time I went to Nils Norling's contest in central Oregon, I think I got a little grass stain on the tail on the first wingover.

Rudy Taube · « **Reply #23 on:** December 16, 2010, 02:01:35 AM »

Hi Howard,

Thanks for your input. At our low speeds with lightly loaded, high lift wings I would not expect the induced drag to be very much at all. But I would think that the total of all four drags (Plate, Parasitic, Induced, and Form) on the plane and the lines would add up to enough to make a small difference in TAS between the two altitudes thick and thin air. Even a little Cessna 172 at a constant power setting gains about 6 knots TAS between these two altitudes. We travel at about 50% of the Cessna's 120K speed so this would mean an approximate 3 K gain. On 64' E to E lines this would be about 4.9 sec at 6,000' and 5.1 sec at SL. I know these are two very different Reynolds #s, and there are many other variables between the two different sized A/C, and the relationship may not be linear, but I would expect the speed #s, whatever they are, would both still go in the same direction.

With the above said: We have Keith's example showing the opposite from what we normally would think would happen. I even contacted Keith to verify that I did not misunderstand his #s. He verified that he had to REDUCE his RPM by a few hundred at SL to keep his lap time from being too FAST.

Keeping his RPM the same his plane traveled FASTER in the thicker, more drag, SL air than it did in the thinner, lower drag, 6,000' air. He speculated that it may be the prop being more efficient at SL. That seems to be the only variable we have that would explain this. That could be the difference, with their increase in thrust (thicker air, more lift on the props wing) overcoming the increase in A/C drag? Since lift is so sensitive to any increase in speed, and our props are turning so much faster than FS props I would think that any increase in fluid density would cause a nonlinear increase in thrust (compared to the FS example I used). Keith suggested that Howard may be able to explain this anomaly. I agree.

Howard: ........

PS: I apologize to Russell for taking his thread off subject. His original topic question was answered long ago, we use LESS mAh at altitude for a constant RPM. We are now talking about any change in LAP TIMES (TAS) at the different altitudes, and the different WING performance in different air densities at the same speeds. Dean may want to split this thread to keep the two issues separated?

Rudy Taube · « **Reply #24 on:** December 16, 2010, 11:10:39 AM »

Russell,

Since I have already muddled up your thread by introducing TAS and lap time changes, I will try to relate them to the second part of your original post.

In CL we have a unique feature that puts a constraint on our ability to have the plane "feel" the same on the controls at different density altitudes. That constraint is the lap times that we are comfortable with in CL. You were correct when you speculated that we should increase power (RPM or prop pitch in our ECL case) if the goal is to keep the planes wing and control surfaces happy and "feel" like we want them to, like they do in thick air at SL.

It is easy to see that the wing only cares about how many molecules of fluid (in our case air) go over it in a given time period to produce the amount of lift (or force if we are talking about the control surfaces) we need to do our maneuvers. This is really IAS (Indicated Air Speed), this is how fast the plane THINKS it's going. If you could talk to your plane and ask it how fast it was going your plane would count the # of molecules of air going over it in a given time and tell you its speed. This is the ONLY speed your plane cares about. It will feel the same at the speed IT tells you it is at no matter what the DA is. If these molecules are spaced farther apart, as in your thin high altitude air, then we have to drive the plane at a higher TAS (true airspeed) to keep the # of molecules of air the same going over our plane per time period. We care about TAS because this will mean that at a higher DA we would have to increase the TAS which will mean FASTER Lap times in CL. These lap times may be uncomfortable for many, but the plane would like them. if we want to keep the plane happy, because it wants to always remain at the same IAS it had at SL to perform (feel) the same, then we need to increase the power at high DAs.

In RC when we go to a higher DA contest we just do what you said and increase our power settings and use MORE throttle for our maneuvers so the plane is at the same IAS (Indicated Air Speed) as it was at SL, and the plane is very happy. We pilots are not quite as happy because we are now flying a plane that is flying faster in both time and space to our reference point, feet planted on the ground. We have to speed up our thinking to keep up, BUT the plane "feels" the same, and performs the same as it did down at lower DA, (SL in mine and Howard's case).

So in CL it is a trade off between feel and lap speed. If when flying at high DAs and you want to avoid the grass stains on the tail that Howard experienced and you want that nice solid "feel" you had at SL, then you will have to add power (ECL, increase RPM or higher pitch prop) and suffer with FASTER lap times. .... OR Keep your lap times the same and tell your plane it will have to learn to fly at a slower IAS with fewer molecules of air to work with per time frame. The plane won't feel good, but your lap times will remain the same and the plane will "look" to you like it is flying at the same speed as you are used to at SL.

Sorry this is so wordy. It would be shorter using math, but I think words might be better for most people. .... I like math, but I still think in words. My friend Howard is smarter than me, he likes words, but he thinks in math.

Regards,

Mark Scarborough · « **Reply #25 on:** December 16, 2010, 12:07:08 PM »

Quote from: Keith Renecle on December 15, 2010, 11:06:33 PM

From 6000 ft down to sea level or close to it, my lap times are faster. A comfortable lap time for me with a constant rpm is 5,2 secs per lap on full length lines, and this goes down to around 4,9 to 5,0 secs. I just lower the rpm around 200 or so until I'm comfortable. I normally set my timer for a flight of 2 minutes so that I can get a few quick flights out of one battery. It's so great to fly at low altitude coming from the high altitude in Johannesburg with thin air. Usually my first few wingover pull-outs end up at around 20 feet above the deck! When the coastal guys fly up here, the situation is reversed so things can get quite exciting!

Keith R

well crap, I never thoguht of this,, now I know why my pullouts are all so high at contests,, Hey RAndy Powell, now you can stop giving me grief about my pullouts, at home, 3000 foot , my pullouts are perfect!

Russell Bond · « **Reply #26 on:** December 16, 2010, 01:48:20 PM »

Hey Rudy, I don't mind as there has been a lot of useful info etc.
Thanks all for their inputs.
I'm just glad that I won't be using more battery at the Nats.

Oh shute!!!!!!!....Only 8 days till I leave and I haven't done much practice because of the windy weather!!

At least I won't have to calculate my fuel usage and fiddle with needle settings, props and venturi sizes.

Howard Rush · « **Reply #27 on:** December 16, 2010, 08:03:31 PM »

Quote from: Rudy Taube on December 16, 2010, 02:01:35 AM

Thanks for your input. At our low speeds with lightly loaded, high lift wings I would not expect the induced drag to be very much at all. But I would think that the total of all four drags (Plate, Parasitic, Induced, and Form) on the plane and the lines would add up to enough to make a small difference in TAS between the two altitudes thick and thin air. Even a little Cessna 172 at a constant power setting gains about 6 knots TAS between these two altitudes. We travel at about 50% of the Cessna's 120K speed so this would mean an approximate 3 K gain. On 64' E to E lines this would be about 4.9 sec at 6,000' and 5.1 sec at SL. I know these are two very different Reynolds #s, and there are many other variables between the two different sized A/C, and the relationship may not be linear, but I would expect the speed #s, whatever they are, would both still go in the same direction.

At the same lap time, because both propeller thrust at constant RPM and drag are pretty much proportional to density, I'd expect the airplane to go the same speed at different altitudes. Keith's results indicate that I'm missing something. The ratio of dynamic pressure (hence drag and thrust) between 15 degrees C at 6000 ft. with 5.2-sec. lap times, and 15 degrees C at sea level with 4.95-sec. lap times is 1.38. That's a bunch: more than I would expect from the prop efficiency increasing at sea level.

I don't think the Cessna analogy is useful. Constant power isn't constant RPM, and induced drag is a player with the Cessna.

Quote from: Rudy Taube on December 16, 2010, 02:01:35 AM

It is easy to see that the wing only cares about how many molecules of fluid (in our case air) go over it in a given time period to produce the amount of lift (or force if we are talking about the control surfaces) we need to do our maneuvers.

How many molecules of fluid (in our case air) go over it in a given time period would give you a number proportional to density X "true" airspeed X wing area. Close, but the kinetic energy of those molecules matters, too. I think you are trying to describe dynamic pressure, which is 1/2 X true airspeed squared x density. And it's not "only". Without viscosity, there is no lift. Then there's angle of attack.

Quote from: Rudy Taube on December 16, 2010, 02:01:35 AM

I think words might be better for most people.

The correct words might. If my airplane flies well at 5.2-second laps at 6,000 ft, I would not back off to 6.5-second laps at sea level to keep the number of molecules/sec. constant, or even to 5.8-sec. laps to keep dynamic pressure constant. An interesting thing about stunt is that both lift and "centrifugal" force in a maneuver increase with the square of speed, so turning capability is pretty much independent of speed (but not with density, hence the grass stains). Flying faster won't help. Gravity stays about the same with altitude, though, so a little extra speed in thin air will reduce its effect on the last hourglass corner. That may be the effect you see with your RC plane: the relative effect of gravity on maneuvers would be more pronounced with higher wing loading.

Quote from: Rudy Taube on December 16, 2010, 02:01:35 AM

This is really IAS (Indicated Air Speed), this is how fast the plane THINKS it's going. If you could talk to your plane and ask it how fast it was going your plane would count the # of molecules of air going over it in a given time and tell you its speed.

A couple of things are wrong with this. The first is no big deal, but indicated airspeed is what the cockpit indicator reads. Given a perfect airspeed indicator and perfect static and total pressure sensor locations, it is the same as equivalent airspeed if the airplane isn't going very fast. Equivalent airspeed is proportional to the square root of dynamic pressure. None of these is proportional to the number of molecules of air going over a wing (or a plane) in a given time. I presume you are extrapolating from your full-scale experience, where the airplane pretty much feels the same at any altitude for the same indicated airspeed. You'd land at the same indicated airspeed at any airport elevation, for example.