Travels With MIRAGE
Experimenting at High Altitude with the Goldberg Mirage 550
Bernard Cawley Jr.
We took a three-week vacation to the sunny Southwest (New Mexico and Colorado) this past summer to visit family and friends. We had room to take along two planes with the hopes of getting some flying in.
We selected my wife Avis' faithful Goldberg Mirage 550 (well into its third year of frequent flying) and my newly completed Idealair Elf 1-20E to come along. The Mirage, which normally doesn't use a folding prop, lent itself to more propeller experimentation, so it received most of the attention and got most of the airtime.
Why was prop experimentation so important? Our destinations ranged from about 3,000 feet to a bit over 7,000 feet above sea level, and experience from a prior trip told me that with correct prop choices an electric would have virtually sea-level performance at higher altitudes.
I wanted to approach the subject a little more scientifically this time, so I did a number of rpm, current, and thrust tests in several locations. I hope to write a piece that analyzes high-altitude propeller selection and other flying considerations, but I still have much to do and learn.
This article describes the places the Mirage flew, a little about how it flew, and some test analysis I did to try to understand why it flew the way it did.
First, a bit about the plane
For those experienced in electric power who want a frame of reference: the Carl Goldberg Mirage 550 was basically stock, built by my wife Avis while I was working on my '89 Nats Taylorcraft. The only airframe modifications are a stouter, but still fixed, tailwheel and a replacement fiberglass cowl from Fiberglass Master.
The power system is a recent Astro 05 Cobalt, which runs on quite an assortment of six- (yes, only six) cell packs, though most flights were made using two Sanyo 1400 SCR packs. The power is controlled by a Flightec SEC-I throttle and my current sea-level prop choice is an 8 x 4 APC.
When flying from grass or sand, I remove the 2-inch Electra-Lite wheels that came in the kit and put on 23-1/4-inch Lite Rites. All-up weight with the big wheels is 47 ounces, giving a wing loading of 14.6 ounces per square foot. I brought along APC props ranging from 8 x 4 to 8 x 6 and 9 x 4 to 9 x 6. As it turned out, the Mirage flew the different altitudes best with an 8 x 5 APC. More on that later.
Where We Flew
We flew the Mirage at five different locations:
- Las Cruces, New Mexico — elevation approx. 4,000 feet
- White Sands National Monument, near Alamogordo, New Mexico — approx. 4,000 feet
- Carlsbad, New Mexico — approx. 3,300 feet
- Santa Fe, New Mexico — approx. 7,000 feet
- Chatfield State Park, south of Denver, Colorado — about 5,300 feet
The Las Cruces flying site was a schoolyard about the size of two soccer fields, surrounded by obstacles. It was a damp, gusty evening, shortly after a thunderstorm, and the temperature was probably in the 80s.
These flights were the first of the trip. Based on my initial tests, I had decided to use an 8 x 5 prop. Because of the close quarters and not knowing what to expect of this plane at such an altitude, the first flight was nerve‑wracking. After a hand-launch and some climbing, flying was a bit shaky at first, but soon became comfortable. After some successful touch‑and‑gos, an ROG (rise‑off‑ground) from the grass, and some mild aerobatics, I was confident that flying at higher altitudes would be just fine.
We had the most fun with the Mirage at White Sands National Monument on one windy evening. Once again it had recently rained, the temperature was probably in the low 80s, and it was an hour or so before sunset. We flew the Mirage from the top of a dune near one of the picnic areas.
The wind was strong enough that takeoffs from the dune were made with almost no ground roll, and touch‑and‑gos were done at almost full throttle. The flights were shorter than usual, but full power (and lots of down trim) was necessary to make much progress upwind. However, loops and Immelmanns were as easy as back home at sea level.
We got a number of favorable comments from other people in the picnic area—many of whom did not know that model airplanes could be flown with electric motors. I have found that people really like quiet, clean models, and will recount stories or comment about noisy ones they had been exposed to before. It’s a great way to get more interest, as long as the flight area is clear of people.
In Carlsbad, my brother‑in‑law took me to an organized flying site outside of town. The field had two paved runways, a place to park, and lots of open space. It was a very windy morning, and no one else was flying. Takeoffs into the wind were quick. Landings were scary; when done right there was almost no ground speed, much like at White Sands, but this time over a much harder surface.
Once landed, the plane often blew over on one wingtip. My brother‑in‑law got a little buddy‑box time, but the gusts made it difficult to know whether he or the wind was commanding the plane.
Our next destination was Santa Fe. Some phoning got me in contact with the Royal City R/C Club, and soon I had directions to their flying field south of town. We (my two brothers, my wife, and a couple of our kids) visited the field on two evenings, both times in the last couple of hours before sunset.
The RCRCC field has a long, narrow paved runway, a pit area with tables, and a windbreak shelter. It is in a slightly hilly area with lots of wide-open spaces. On both visits we saw no other modelers.
At over 7,000 feet above sea level, approach speeds were noticeably higher, and takeoff rolls, when not assisted by a headwind, were longer than I was accustomed to. Once in the air, the Mirage handled just about as I expected. Flying was gentle and predictable.
Thanks to the higher-pitch prop turning faster in the less-dense air, mild aerobatics were performed much as they would be at sea level (see test results). I have had problems with accelerated stalls (stalls in a turn, resulting in a snap roll to the outside) on previous trips with other planes, but the Mirage, no matter how hard I hauled it around, refused to let go and snap out.
The most elegant flying site we visited, and the only one where we flew with other modelers, was inside Chatfield State Park, near Denver, Colorado. This field, built and maintained by the Jeffco Aeromodelers, has two expansive paved runways, pit tables, flight stations separated by frequencies, lots of airspace, and a large covered area with picnic tables. Spectators are welcome in the picnic area.
Because it was a busy field, I ran into the biggest technical problem of the trip. Ostensibly due to interference generated within the audio portion of television station channel 4, channels 20 and 21 have been banned in the Denver area. Guess which channel the Mirage is on? Why, 20, of course. So that's why I couldn't find my flight station!
In the process of finding this out, the safety officer offered to shut the whole field down for a while so I could fly, after having come so far. Talk about accommodation! I was rather embarrassed to be the cause of others having to wait, but at least I knew they wouldn't be waiting on me trying to get my engine started.
I got some favorable comments about how well our little electric flew—apparently the myth that electrics don't work at high altitudes was prevalent in Denver, too.
Tests
Because I was curious, or perhaps just because I'm an engineer, I started doing static tests with various propellers at each location, using a 30A panel meter for current measurement and my trusty AccuTach I for rpm. The purchase of a Normark digital fish scale allowed thrust tests via the hook-the-scale-to-the-tailwheel method.
In the tests I found something that was a bit surprising at first. As I expected, the motor ran faster at higher altitudes and therefore at a bit lower current, because of the less-dense air. To my surprise, the measured thrust stayed nearly the same regardless of altitude (at least to the precision of the scale, which reads to the nearest ounce).
All tests were done indoors with the temperature in the low 70s, with the power system mounted in the Mirage, an ammeter between the battery and the speed control, using one of two 1400 SCR battery packs. It should be noted that not all tests with the same size prop were made with the same individual prop. We tried to take readings that were representative of the middle part of the run; however, a voltmeter was not used.
Conclusions
In corresponding with Ed Westbrook, who does much experimenting and theorizing about electric "propeller twisters," I learned that these results should not be surprising. In fact, they simply confirm what the Mirage was trying to tell me as she flew: the power system will deliver about the same amount of power to the air — with an appropriate prop choice — and general flight performance will vary little, regardless of altitude. Ed summed up his analysis and my experience succinctly:
"Moral: when operating at higher altitude, load for current with increased pitch... let the rpm fall where it may, and thrust will take care of itself."
As mentioned, flights tended to be shorter, but I think this was due mostly to fighting wind — having to use more power to go where I wanted to go. Flying under windy conditions sometimes made landings a bit dicey, and I did have to do some repairing, especially after one botched landing in Santa Fe that knocked the whole tail section off. (Not the most elegant demo of RC flying for my brothers.) I was glad that APC props were available in Alamogordo and Denver.
All in all, the flying was so successful that on a future trip I'd like to try float-flying from high mountain lakes (but I have to try floats here first). I'd also like to try to fly from the meadow atop Wolf Creek Pass in Colorado, which is over 11,000 feet above sea level. Based on how the flying worked this past summer, I have no doubt it will be fine even there.
Next time you go on a trip, even to the high country, take an electric plane along - and enjoy!
Transcribed from original scans by AI. Minor OCR errors may remain.
