Will electric indoor models can fly at 4500' msl altitude?

From : Don Stackhouse

Doug, the answer is definitely a qualified "yes".

Two main factors come into play here, true air speed and Reynolds number.

An airplane's stall speed, and the full-scale concept of "indicated air speed" (the speed that shows on the airspeed indicator in the cockpit), is based on dynamic pressure, which is linear with air density and proportional to the square of airspeed. At 4500 feet your air density is only 87% of sea level air density, so your model has to fly 7% faster to get the same dynamic pressure and indicated air speed. This means that your stall speed (in visual terms, which is a function of true air speed) will be 7% faster at your home in Colorado than it would be in, say, Long Beach or Miami. Your prop pitch (which is also a function of true air speed) will need to be 7% higher. You will fly to the edge of the parking lot in 93% of the time that it would take if you moved that same parking lot to sea level, and you will have to turn around that much sooner. Because of the thinner air, you will also need more room to complete that turn.

The other factor, Reynolds number (typically referred to by modellers as "scale effect"; abbreviated "Re"), will also tend to reduce your model's performance. Reynolds number is air density times model size times speed, divided by air viscosity. As you go up in altitude, air density decreases, but viscosity stays about constant. This means that as you go up higher, Reynolds number decreases, which hurts performance. Drag gets worse and the wing's max lift coefficient decreases. Models that had marginal performance at lower altitudes might not be able to fly at all. One specific example of this I know of is the Great Planes "ElectriCub". Here where I live, at about 1000' msl, it's a marginal performer if built absolutely stock and with the supplied powerplant. On hot, humid days (which lowers the air density and therefore has the same effect as raising the altitude) it gets even worse. Some folks in the Denver area have reported that it doesn't fly at all at their altitude. Also, that temperature issue can be quite significant; a model that has trouble on a hot, sticky day in August might do significantly better on a cold, dry day in February. When Joe and I are doing performance tests of new kit designs, we have to carefully take this into account.

The fact that your model has to fly faster in the thinner air helps offset the decrease in air density, so the Reynolds number effects are not as pronounced as they might be otherwise. Still, the true air speed in flight will generally change with the square root of the air density change, while Reynolds number is linear with air density, so there will still be a drop in Reynolds number. L/D will decrease, and max lift coefficient will decrease (which means that stall speed and minimum turn radius will increase). Generally speaking for small, slow models, thinner airfoils with less camber will suffer less from this than thicker or more highly cambered airfoils. At low Reynolds numbers, the air generally seems to prefer thin airfoils with modest amounts of camber.

Now, the good news. The amount of degredation due to airspeed and Re related effects is likely to be fairly small at the altitudes you're dealing with. An already marginal performer is likely to have problems, but if you pick a model that is known for having very good performance at sea level, it will probably still have reasonably good performance at your altitude. Just pick a good performer and you shouldn't have much trouble.

You can find more discussion of the effects of Reynolds number and other aeronautical phenomena in the "Ask Joe and Don" section of our website.

Regarding your efforts to get permission to use the local school gym for indoor flying, have you tried putting on an airshow for the local elementary or middle school? Some friends and I do an outdoor show every year for the school where my wife teaches, and we have done it for other schools as well. We get about four to six pilots and about one or two dozen models for static display, then fly about six of those in a 30-minute airshow. We divide up the school into groups of about three to five classes at a time, then do a series of thirty-minute shows with ten minutes between till we've performed for all the students. Seven shows later, four to six very tired pilots have just demonstrated a cross section of what's possible in R/C models to a total of about 600-700 people, including teachers and passers-by. Some of those folks later show up at local flying fields and hobby shops.

We structure the shows around a theme, so that each show is a 30-minute science lesson in some facet of aeronautics, such as powerplants, ways of making lift, or different ways of controlling an aircraft. It's carefully planned to have genuine educational value, it's not just a circus act. BTW, this is an important feature to include in your show if you really want to do well with the school; curriculums are so full of things they have to cover in a limited school year that they don't have time for purely entertainment programs that don't also have true educational value.

Once you "break the ice" with something like this, you might have more luck arranging with the school for use of the gym after-hours for indoor flying. Of course you might be able to get even more support from them if you can put together some sort of extra-curricular activity for the students as part of the deal. You get to fly, the students get a regular program to learn about aeronautics, and the school gets a unique and interesting activity that improves student interest in science and math, and that looks good on the evening news. Everybody wins. Yes, it's more work and commitment than just showing up whenever you feel like it at some large indoor space. Trust me, the rewards of inspiring young minds are well worth the effort. BTW, those young minds will go home and get their parents thinking positively about model aviation, which might help the next time you're approaching the city fathers about permission to fly at the local park.

Don Stackhouse
DJ Aerotech