Air Density and Pattern

This article is written by our valued CK Aero contributor Earl Haury

Have you noticed that some days your airplane has more or less power than others? That it’s more or less responsive? The variable property causing this is air density.

Full scale pilots are very aware of air density, expressed as Density Altitude (DA), and its effect on aircraft performance. Low DA provides good performance in terms of thrust and lift while high DA does just the opposite. A fully loaded aircraft may have no problem at sea level DA and yet may be unable to get off the runway at altitude.

I find density altitude numbers difficult to relate to pattern, low numbers are good and high bad, but the numerical range is large and somewhat unwieldy. An easier metric is simply air density (AD) as expressed in grams per cubic meter of air and seems to better fit the narrower altitude range pattern aircraft operate in.

What determines the density of air? Temperature and pressure, with consideration for humidity. How is it calculated? The equations for calculating air density are well known but not something most of us have thought about since our last physics class. Warmer air is less dense. The lower the pressure (baro) the less dense. The more humidity the less dense – that’s correct, air with humidity is less dense than dry air (until liquid droplets form). Putting all the variables together to calculate air density has required some math skills. Now it’s as easy as an app on your phone. The AutoDens app does the job effortlessly by obtaining current data for your location from the closest airport and automatically making the calculations.

Most understand the difference between “good” and “bad” air relative to the output of engines. Engine performance is directly proportional to the amount of air the thing can ingest, low air density means less output and vice versa. When running glow I tried to establish a performance “standard” with the engine of the time and reduce throttle on high AD days and add “liquid performance” (nitro) on low AD days to stay close to that standard level of performance. The goal being to ensure similar airspeed with the same throttle management through the pattern.

I expected these air density driven power variations to go away with the advent of electric power. Didn’t happen! Initially it wasn’t clear what was going on, as the effect of AD was mask by the motor performance changing from beginning to end of the pattern as the batteries discharged and voltage declined. When I began using the Tech-Aero Throttle-Tech, to ensure consistent performance throughout a sequence, I was surprised to find AD effects actually stayed pretty much the same as with glow. It’s apparent that the AD driven thrust variations are much more an effect of prop performance than engine performance.

So, how can we take advantage of this information? Just like with glow, we can establish a “standard” performance level at our “normal” air density conditions. Performance can then be increased or decreased to meet that standard as AD changes. Expect the changes to vary depending upon what the “normal” AD is for your location and your airplane’s thrust / drag profile. Available data suggests that thrust is most affected, with control sensitivity next, and finally drag.

OK, if you’ve stayed with this, here are some numbers. Note that the range is only 100 gr/m³, so an app capable of providing an accurate number is essential. These ranges were derived on the gulf coast at near sea level and with little experience at the lowest AD. Consider these a starting point and work out what works best for your location.

The overlap in the AD numbers accounts for wind, move toward more thrust as the wind increases. “Less Pitch” is 1” less than “Std” while “More” is 1 ½“ more than standard. “Less Power” is up to 10% less throttle curve max than “Norm” where “Norm” is 5% less than “Max. These ranges are valid for glow also, the specific engine / prop / fuel changes would need to be defined.

Managing thrust to provide a constant speed range in various AD situations will minimize the variation in airplane feel, mAh used (E), and flight times experienced across the AD range. At the extremes it’s probably beneficial to also adjust control travels. Expect spins & snaps to require more or less control travel at the AD extremes. We compensate pretty well in normal maneuvers but often a couple of percent change in expo will improve feel. If your airplane requires significant changes it may be worth the effort to have another program in the TX with optimum settings for the extremes.

Is this of value? Everyone is free to decide for themselves. With no adjustments an airplane may be fast and touchy in high AD conditions, while it will feel slow and sluggish at the other extreme. Generally, I find only seasonal changes are necessary on the gulf coast. However, at the Nats this year (2014) the AD changed dramatically throughout some days. Morning numbers were at the high (1200) end of the range and late afternoon numbers were approaching the low end (1120). I think it worthwhile to be aware how much AD can vary airplane performance and to be prepared to compensate.