December 7th, 2014
Total Hours: 116.2
This week I did some high altitude training. I had three flights for a total of about 3.5 hours, enough for a taste of what it’s like, but leaving plenty of room to keep learning. You may be wondering what high altitude training is about and it is simpler than some may imagine. Because of the small piston engines that power our training helicopters we are greatly affected by changes in altitude. This is normal for any engine, but I would wager that most are not taxed to the point of our little R22! The point of high altitude training is to experience first hand what happens when one of these small engines is taken into the thin air of the mountains.
What kinds of altitudes are we talking here? Not as high as you might guess. We flew approaches to spots in the 5,000 to 6,000 foot range and that was plenty to see the affects of high altitude on the engine. There are a couple of interesting things that happen flying in thin air. The most important is that the engine cannot produce as much power. Under normal circumstances when we increase the pitch of the rotor blades the engine adds more power to compensate for the increased drag. We have to be careful in power limited situations because if we ask for more power by increasing pitch it may not be there. If we lift the collective and increase the rotor blades’ pitch it is similar to applying a giant brake to the engine. At sea level the engine can spool up and there is no problem, but at altitude things are different. If we don’t have the power the engine and rotor RPM will begin to droop.
The danger of loosing RPM is that we can get into a stall. But not the kind of stall you’re thinking of! What’s interesting is that we wouldn’t stall the engine, like you might if you bogged down an automobile engine, instead we would stall the rotor blades and this is a very bad situation indeed. While an airplane can recover from a stalled wing once the rotor blades of the helicopter are fully stalled there is no recovery. This is one of the few situations where you could literally drop like a rock from the sky. Now we can start to see the importance of recognizing power limitations and the caution that must be applied when flying at high altitude. At sea level we know that there will be enough power for almost any kind of maneuver we want to accomplish, but with altitude we must make an accurate assessment of how much power is actually available. We also use different approach techniques to mitigate the lack of power in the engine.
In the thin air we try to keep a little more airspeed throughout the approach. Without diving too deep into the aerodynamics suffice it to say that with higher airspeed less power is needed to maintain flight. But once we start to slow down the power has to come in or we’ll start to sink. By carefully juggling our deceleration with the increase in power we can judge the conditions throughout the approach. Before committing to the landing we leave ourselves an out by increasing airspeed and climbing. Each morning that I had high altitude training we flew to the southeast side of Hualalai, the large volcano behind my house. It is a bizarre landscape of pukas (vents), lava rocks, and forest. Our first spot was a large open area of fine gravel bisected by a road. The approach is flown uphill which changes all the normal sight pictures. Anytime you fly to a new area the sight picture is different, but this area seemed to have more illusions than average. Trying to develop an accurate picture in a new place is one of the many challenges of flying!
The thing I liked best about altitude training was that I could actually see some of the aerodynamic principles in action. The two that are really made apparent are ground effect and effective translational lift. Ground effect happens when we are close to the ground, as the name would imply, and it provides a boost in lift. Basically the airflow through the rotor is changed because of the way it impacts the ground and this improves the aerodynamics of the blades. Ground effect begins when we are within one rotor diameter of the surface, about 30 feet in the R22, but even at sea level we don’t feel it much until we are within 5 to 10 feet. The closer we get to the ground the stronger the effect. In the high altitude environment as we slowed down through the final ten feet the helicopter started to sink because we no longer had the power or the airspeed to keep it up. At first it seemed as if we would have to perform a run on landing but in the final feet ground effect became strong enough and we settled into a shallow hover just above the surface. Amazing!
Effective translational lift, or ETL, is another condition that provides a boost in lift. Just as in ground effect ETL creates improved lift due to a change in airflow. But this time the change comes from forward motion instead of proximity to the ground. After settling into our shallow hover we needed the boost from ETL to assist in climbing out. As I mentioned the terrain at our first spot sloped gently uphill so as we gained altitude the ground followed along below us! When we begin to move the forward motion spoils ground effect causing the helicopter to try and sink, but we need to get enough speed for ETL to climb so once again it is a balancing act. Gently coaxing the helicopter forward, perhaps with the occasional tap of a skid on gravel, we would hit ETL and suddenly leap into the air. It is really fantastic to see aerodynamic principles so predominantly on display!
I had expected to enjoy my high altitude training but I was filled with even more excitement than I had predicted. I returned to the airport with a big grin on my face every time! It’s hard to explain just what struck a cord with me up there, but it definitely happened. I’ll certainly be trying to get in some more hours of high altitude flying when I can. Adding a sweet end to what was already a wonderful week of flying I took a trip out to the valleys of the Kohala Mountains this morning. Finally the weather was perfect without a single cloud in the sky when we arrived. First we searched for a bomber that made a forced landing back in 1941. A wild piece of wreckage in a tight ravine it is miraculous that no one was killed in the accident. After finding our bomber we spent a while just flying around the valleys soaking in the big drops and long waterfalls. I couldn’t think of a better way to end the week!