Another question for the experts

[JimNM]

Ooops, I did it again. I got a question buried deep in a thread with a different topic. I re-submit the question, and ask for the accumulated expertice to hold forth.


I think the most under-rated aspect in our discussions is ALTITUDE-DENSITY . I am in no way an expert, however, I understand A-D as the realationship between barometric pressure, temperature, humidity and altitude, to wit:

1) The lower the barometric pressure...the lesser the lift

2) The hotter it gets, the more room there is between the air molecules... yielding less available lift

3) I am not sure which way humidity impacts the formula :(

4) Altitude - no brainer. The higher the altitude, the less dense the air is... less lift.

My base altitude in the dry New Mexico desert is at about 4500 ft. I have much less air density than those who fly at sea level, so I must use a larger prop to get the same thrust (read this as "lift" in the A-D discussion). If anyone can expand my understanding of Altitude-Density, I would be very grateful.

Thanks again for playing

Fly'em High
JimNM

[Blackbird]

Pressure altitude and temperature are the primary things that increase density altitude. So, if you are at a high elevation, the higher the temp the higher the density altitude. Humidity has very little effect on density altitude.

Where you are, when the temp goes up, the plane thinks it is at a higher altitude and performs as if it were at a higher altitude.

[Ollie]

The vector sum of lift and drag is equal to weight. This relationship is maintained independent of altitude. The loss in lift due to air density decrease is regained by an increase in air speed. The consequense of this airspeed increase in a straight glide is an increase in sinking speed and the decrease in duration that goes with it. If that were not bad enough, circling flight suffers even more. Because of the airspeed increase, the size of a circle associated with a given angle of bank also increases. If the circle size associated with a given airspeed is maintained then the angle of bank must increase to maintain the centrifugal force associated with that airspeed and size circle. The increase in bank angle results in an additional sinking speed penalty for circling flight.

The increase in airspeed mentioned above also affects the stalling speed and therefore the landing speed. The kenetic energy that is dissipated in landing increases with the square of the airspeed.

You can overcome the penalties associated with airspeed increase by choosing models with a lighter wing loading to maintain the same airspeed you would have at a lower altitude.

It is interesting to note that the maximum service ceiling of the U2 was determined by the increase in stalling speed with altitude. It also happens that the speed of sound decrease with altitude. At the U2's service ceiling the stall speed was within ten knots of the speed of sound and a deviation outside that ten knot window resulted in a stall.

[Sparky Paul]

Density altitude usually is of interest on a high-hot day, when contemplating aircraft gross weight, power available and runway length.
A hot day here in the Antelope Valley (2500' ASL) often results in the 12,000 foot runways at Plant 42 being TOO SHORT for a wide-bodied transcontinental airplane which if it flew at Oh:Dark:Thirty could take off without exceeding runway length limits.
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For electrics, it's of no particular interest to the motor, but the prop and wing will be affected, although not as much as a similarly powered i.c. plane would be.
Longer takeoffs and higher flight speeds, as Ollie says.
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ps: An article in Q&EFI about flying at 15,000 feet in Nepal.. worth reading.

[JimNM]

Thank you for the information. I had to read Ollie's post three times before I understood it, despite the fact that it contained all the answers I asked for!

I am aware of the "law of the squares" or is it the "law of scale"- where if you take a wing that is 1/2 the scale of another, the smaller will only produce 1/4 the lift, and a wing twice the size will produce 4 times the lift... again, a little knowledge is a dangerous thing.

I seem to remember in the recent past a few times when the airport in Phoenix was shut down due to excessive heat and the resulting Altitude-Density problem. My question stems from the fact that most of the ARF electric gliders that I've seen come with an 8x4 prop on a 540 or 550 motor. I have gone to a 9x6 prop on the stock motor for almost twice the volume moved per revolution (I think..., again a little knowledge....) I understand that due to the ditribution of the population in the U.S., it probably makes better business sense to outfit the ARFs to perform best at or near sea-level.

Thanks again for the info.

Fly'em high (at least as high as you can :) )
JimNM

[Ollie]

The electric equipped ARF's are powered by motors that are determined by minimum acceptable performance at a minimum price. If performance at 5000 feet is more important to you than money, then you can improve performance by replacing the propulsion system with a low weight brushless motor. The higher effenciency of the brushless motor will allow lower weight bateries. Using nickel metal hydride instead of nicads will save some more weight. The lower power and wing loading will give you better climb and glide performance than the stock units have at sea level.