By Steve Krog, EAA 173799
This piece initially ran in Steve’s Traditional Teacher column within the December 2021 problem of EAA Sport Aviation journal.
We’re all conversant in the time period density altitude, or at the very least ought to know of it. However seldom will we take it severely sufficient to truly take the time to calculate its impact on our flight on a heat day. As a substitute, one would possibly assume, “It’s a bit heat and humid immediately. I suppose it can take a bit of extra runway to take off.” That’s in regards to the extent of it for a lot of pilots, sadly, when contemplating density altitude.
What precisely is density altitude? How does it have an effect on plane efficiency?
And why ought to a pilot be involved about it?
Within the easiest of phrases, density altitude is stress altitude adjusted for nonstandard temperature. Stress altitude can simply be discovered by setting your delicate altimeter to 29.92 inches within the Kollsman window after which studying what the altimeter is indicating. Utilizing that altitude and correcting it for the nonstandard temperature, we are able to discover immediately’s density altitude.
In immediately’s flying atmosphere, it’s fairly straightforward to amass the density altitude as a result of most all of the airports which have automated floor observing system or automated climate observing system companies embrace the density altitude on the finish of every recorded briefing.
Understanding density altitude is essential for a lot of causes. It impacts engine, plane, and propeller efficiency. Mixed, they will create an uncomfortable scenario if the pilot has not taken this severely into consideration.
Right here within the comparatively flat higher Midwest nation, density altitude is generally a nonfactor besides in the summertime months. It may idiot many people on a sizzling humid day. We’re on common solely about 1,000 toes above sea degree. However then we’ll have a stretch of 85-95 diploma Fahrenheit days, often round a vacation, when associates are on the town and wish an airplane trip. Let the enjoyable start.
Essentially the most simplistic analogy I’ve provide you with when explaining density altitude to college students is as follows. Assume for this dialogue that at sea degree and 59 levels Fahrenheit each cubic foot has 100 molecules of air in it. The plane we’re flying is a Piper J-3 Cub with an 85-hp Continental engine. The propeller is steel, 71 inches lengthy with a 45-inch pitch, generally known as a 7145. In keeping with the producer, the engine will ship 85 hp when turning at 2575 rpm at sea degree and 59 levels Fahrenheit.
Let’s assume the present temperature is 90 levels Fahrenheit. Because the air is much less dense, there are solely 75 molecules of air in every cubic foot, however our plane must expertise 100 molecules per cubic foot to carry out as desired. Consequently, the airplane’s efficiency will probably be considerably affected on this scenario.
If the engine can not get 100 molecules of air per cubic foot and is simply getting 75 molecules per cubic foot, it won’t be able to offer a full 85 hp. The fuel-to-air combination generated by the carburetor will probably be on the wealthy facet due to the shortage of sufficient air, additional lowering horsepower. We could solely have the ability to generate 70 hp. How does that have an effect on every little thing else? Efficiency is decreased. If in case you have a gas combination management in your plane, it could possibly and ought to be adjusted for density altitude.
Subsequent, let’s contemplate the propeller in easy phrases. If it can not get a full 85 hp delivered to it to acquire 2575 rpm, the rotation pace is barely much less. It may’t transfer the plane ahead 45 inches with each revolution (underneath excellent temperature and rpm settings). The plane is being pulled by way of the air at a slower-than-anticipated airspeed. To take care of a continuing 60 mph climb, the pitch angle must be lowered to a lesser angle. By decreasing the nostril, you’ll achieve airspeed however now the speed of climb is decreased.
In case you attempt to keep the same old climb pitch angle, the airspeed will endure and doubtless solely present 50 mph. At this airspeed, you’re in a nose-high angle nearing the again facet of the ability curve, which means you can not climb as a result of steepness of the nostril. Now you’re nostril excessive, can not climb, and presumably approaching the stall pace of the plane. That is the place power-on stalls happen.
By decreasing the nostril to reestablish a 60 mph climb, chances are you’ll quickly discover the airplane is simply capable of achieve altitude at just some toes per minute. Once more, that is essential to know and acknowledge if departing from a runway with obstructions of any variety on the departure finish.
Plane efficiency is affected by all of the above. If the airplane must have 100 molecules of air per cubic foot shifting over and underneath the wing to generate sufficient raise for flight, the one means to offer that a lot airflow is to maneuver quicker by way of the much less dense air. In a high-density altitude scenario, your airspeed indicator could solely be exhibiting 40 mph when your true groundspeed is 50-55 mph in a no-wind situation. The identical sensation can happen when touchdown with a high-density altitude.
Once more, the airspeed could point out a correct 60 mph on strategy, however your groundspeed is considerably to considerably larger.
Excessive-density altitude considerably impacts the efficiency of the plane programs and, due to this fact, your complete plane.
For instance, let’s use a typical coaching plane that has a 2,300-pound gross weight at takeoff. The surface air temperature is 86 levels Fahrenheit, the density altitude is 3,000 toes, the floor wind is gentle and variable, and the runway is turf.
Making use of this information to the plane efficiency chart for calculating takeoff distance, the plane will want an estimated 1,185-foot floor roll. And to clear a 50-foot impediment, the space will increase to about 2,115 toes. However these numbers apply to a tough floor runway.
Discovered within the advantageous print accompanying the efficiency chart, there’s a assertion that claims, “For operation on a dry, grass runway, enhance distances by 15 %.” Including this determine, our takeoff roll now turns into 1,363 toes, and we’ll want an estimated 2,432 toes to clear a 50-foot impediment. However the turf hasn’t been mowed in additional than per week and is about 6 inches tall.
The turf runway we intend to make use of is 3,000 toes lengthy and has excessive pressure energy strains on one finish. In case you intend to take off towards the ability strains, you’ll nonetheless have 568 toes of runway to spare with out factoring in a security margin for the taller grass. There isn’t a true rule of thumb for working in taller grass aside from it can have some influence on the bottom roll. A very good, protected, and proficient pilot will often add 15-20 % to the bottom roll to compensate for the taller grass. As a great, protected, and proficient pilot, would you proceed with the takeoff on this scenario?
Floor winds are gentle, considerably variable, and seem like barely favoring a departure towards the ability strains. For security, although, you determined to take off in the other way to keep away from the obstruction. After finding out the floor wind, it seems you may have an estimated 4 mph tailwind on takeoff. What does that add to the bottom roll distance?
As soon as once more, the advantageous print states, “For each 2.5 mph of tailwind, add 10 % to the bottom roll.” On this instance, you may have an estimated 4 mph tailwind. For security causes, it’s best to add 20 %, giving a floor roll of 1,636 toes.
Are there some other elements a great pilot ought to contemplate earlier than making the choice to depart? Take into account this:
The printed information discovered within the plane pilot’s working handbook (POH) was decided by utilizing a brand-new plane — airframe, engine, and propeller. The plane you’re utilizing is sort of 40 years outdated, the engine has a bit of greater than 1,000 hours for the reason that final main overhaul, and the propeller has been reconditioned at the very least as soon as. One can assume the empty weight of this plane can be fairly totally different than a brand new airplane of the identical make and mannequin.
Many pilots I do know will, after calculating the takeoff distance utilizing the efficiency charts, additionally apply their very own established rule and add it to the efficiency of the plane. Some add 10-15 % to the calculated figures whereas others add 25-30 % for a bigger security margin. A lot relies on the place you’re doing most of your flying. If within the Midwest flatlands, 15 % could also be a great quantity. However should you’re flying in hill nation, 25-30 % could be a a lot safer quantity to make use of when calculating takeoff distances.
Neglecting to contemplate plane efficiency could cause an incident — or worse. An acquaintance of mine realized this lesson someday final summer season when he made an early morning touchdown at his deliberate vacation spot. He supposed to depart for his dwelling base within the early afternoon. Earlier than taking the time to calculate the required runway, he assumed he may simply make it.
Fortunately, this particular person was working towards the rule of selecting a “no go” level on the runway. The primary try was aborted on the “no go” level. Giving it some thought, he determined to strive a short-field takeoff. That made little distinction and was shut down once more on the “no go” level. The third try was a mix of a turning transfer and a never-stopping mix of a short- and soft-field takeoff. Once more, “no go.”
At that time, my acquaintance made two sensible choices. First, he retrieved the POH and decided that the plane would by no means get airborne underneath the present circumstances. And second, he parked the airplane and waited till early night after the temperature had dropped after which made a protected departure.
There are small airports that, underneath sure circumstances, help you land safely however is not going to have sufficient runway to permit for a protected takeoff. Take a second and know what you’re entering into earlier than one thing unexpected occurs.
Preserve flying and fly safely.
Steve Krog, EAA 173799, has been flying for greater than 4 many years and giving tailwheel instruction for practically as lengthy. In 2006 he launched Cub Air Flight, a flight coaching faculty utilizing tailwheel plane for all major coaching.