I am a model airplane fan, in La Paz Bolivia, I tell you this since La Paz is located at 12006 ft above sea level and have problems with park flyers, Please let me know if you have or can modify one of your models with a 30% stronger motor ( 20% is the power loss due to then thin air at this level).

From : Don Stackhouse

I just got a similar question from another flier who lives at an altitude of 7000' msl, and I remember thinking when I read it "it could be worse, at least he doesn't live in La Paz!" I got quite a chuckle recalling that thought when your question arrived only a day or so later!

The density ratio for 12K ft. is about .69, so a gas engine would have only 69% of the power at full throttle in La Paz that it has at sea level. The density ratio between La Paz and our flying areas here in Ohio (about 1000' msl) is about .71, only slightly better. However, an electric motor does not suffer from that problem; it can make the same power at 12,000' msl that it can at sea level. The problem for an electric is mainly a matter of the propeller size, particularly its pitch.

The required prop pitch is related to true air speed. At 12,000' msl your true air speed is about 1.2 times the true air speed at sea level, so you need a prop with 20% more pitch to get it to apply the same load to the motor, and therefore make the motor pull enough amps to make the same power as at sea level. You can also compensate for this by adding cells to increase the RPM, but there are other problems to this approach. Because power absorbed by a prop is proportional to the cube of the RPM, you can't increase RPM that much without resulting in a huge increase in power draw long before you get enough "pitch speed" to fly the airplane at a comfortable true airspeed at that altitude.

Also, because the true air speed is related to the square root of the air density, the increased pitch alone does not entirely compensate for the thinner air. You also need more diameter (probably about another 10%) if you want to maintain the same efficiency.

So, the bottom line is, you need more pitch, you need a little more diameter, and it may help to add cells to the battery as well. At your altitude I'd suggest doing all three. You can also go to one of our twin-motor powerplants like the MPS-2A, which already comes with the bigger prop. In addition, by dividing the power up between two motors instead of one, the motor efficiency improves, so you get more power out to the prop for the same power coming in from the battery.

The other factor here is that 20% increase in flying speed. this also means a 20% increase in landing speed. It's a good idea to pick a plane with excellent low speed performance and handling, and a reasonably slow landing speed, so that it won't have to be blazing fast on final approach at your altitude.

So where does that lead us? It suggests a Roadkill model with plenty of wingspan and plenty of power, that has a demonstrated ability to carry weight well at lower altitudes. Two that come to mind are the J-3 Cub and the Lockheed Model 10 Electra. Both can be upgraded with MPS-2A power systems for an extra $8 per powerplant when you order the kit. I'd also recommend getting a 3-cell 250 mah Lithium-polymer pack for the Cub, or a 3-cell 700 mah pack for the Electra.

The Curtiss-Wright Junior is another possibility. It handles weight quite well, and is nearly as efficient as the Cub. My only reservation is that the pusher motor mounting on the Junior only has enough clearance with the tailboom for the 5-4.3 prop on the MPS-1A, not the 6-5 prop we use on the -2A system. Going to a 3-cell pack might give you enough extra RPM to take care of the problem that way, especially if you upgrade to what we call an MPS-2B (that's a twin-motor system but with the small 5-4.3 prop instead of the 6-5 prop we use on the -2A).

Just to put those options in perspective, at our altitude the MPS-1A on a 2-cell 250 mah battery pulls about 5-6 watts, and turns the 5-4.3 prop at about 5500 RPM. The MPS-2B with that same 5-4.3 prop and a 3-cell 250 mah pack pulls about 13-14 watts, a little more than twice the power, and turns the prop at 7600 RPM. That's a 38% increase in prop RPM, which equates to an increase in power at the propshaft of 2.6 times. Therefore the motor efficiency must be about 13% better, since the power from the battery only increased 2.3 times. I've been flying the MPS-2B and a 3-cell 250 mah pack in my Fokker Triplane for a while now, and it makes a whole new airplane out of it. I usually fly the Triplane with this setup at half throttle, and even for wild maneuvers I rarely use more than 3/4 throttle.

The Junior with this setup should handle your altitudes acceptably even though it still has the small prop. One other option if that still isn't enough is to cut down a 6-5 prop to 5", so you have the correct pitch for your altitude but the prop tips still clear the tailboom.

Continuing the comparison, the MPS-2A with its 6-5 prop pulls about 18-19 watts at about 5100 RPM. Note, we cannot compare the power output directly through RPM comparisons because the props are not the same size. However, we can compare pitch speeds. The stock single motor system has a theoretical pitch speed of about 22 mph, which by the time you allow for about 60% prop efficiency equates to a flying speed of 13-14 mph. The twin motor system with the small prop (the MPS-2B) has a pitch speed of 45 mph, but we should probably only use about 50% prop efficiency, giving us an approximate airplane airspeed of about 23 mph. This is an airspeed increase of about 70%, more than enough to compensate for the 20% that results from your thin air. Note, these are very crude comparisons, but enough to indicate that you should have plenty of power, even in your thin air.

The MPS-2A with the large prop has a pitch speed of 35 mph, but its efficiency (due to the increased diameter) should be more like about 70%, for an airspeed of about 25 mph. Either way, you should have enough power and pitch speed to fly the plane, even in La Paz.

The Junior, despite its limited prop diameter, has about 2" more span than the Cub and probably the best low-speed performance and handling of any of the single-motor Roadkill Series models. Even with the smaller prop it should do well at your altitude.

The best RK Series twin engined model for efficiency, excellent low speed performance, excellent handling and extremely wide speed range is the Electra. It also is surprisingly nimble, with rock-solid stability, but a roll rate and a minimum turning radius that rivals the A6M2 Zero. With a pair of MPS-2A powerplants and a 700 mah 3-cell pack it should perform very well at your altitude.

The Cub, Junior and Electra have all been flown successfully in winds of at least 10-12 knots at our altitude. At your altitude that would equate to about 12-15 knots.

Another option for outdoor flying would be the big B-17 with FOUR MPS-2A powerplants and a 3-cell 1200 mah battery, but that's really starting to get expensive. However, the big Boeing even with just the stock MPS-1A single-motor powerplants and a 2-cell pack has plenty of power and truly outstanding low-speed handling. However, its top speed is limited and so is its roll rate (on the other hand, its turning radius is very tight, it just takes it a relatively long time to roll into or out of a turn), so it isn't a good choice for small indoor flying sites like a basketball court, or for outdoors if the winds are more than about 8 knots (although at your altitude that might equate to more like 10 knots).

Among the WW II warbirds, the A6M2 (long-wing) Zero and the F4U Corsair seem to be the best for weight carrying ability, efficiency and wide speed range. Either of these with an MPS-2A and a 3-cell 250 mah pack might be a viable choice for you.

The P-38 also has excellent reserve power and a fairly wide speed range. With a pair of MPS-2A powerplants and a 700 mah 3-cell pack (the same setup I recommended above for the Electra) it might do very well for you. The only drawback is that it tends to land a bit faster than the Electra, so at your altitude the landing gear might take a beating if you don't have a smooth place to land. Top speed of the P-38 is similar to the Electra, both of them have among the fastest top speeds of the entire Roadkill Series.

I would recommend you stay away from the P-51, the Me109 and the P-40. These have fairly high landing speeds, and the effects of altitude would make that worse. The Spitfire is somewhat better in this regard, but still is probably not as good a choice as the others I mentioned above.

The WW I series RK models are a bit of an unknown in my mind. They are as a group somewhat draggier than the others, due to all those wings and struts they have to haul through the air. The Triplane in particular seems to be especially sensitive to weight; its short wingspan gets into induced drag problems if you try to make it support too much. On the other hand, the reserve power with the MPS-2B and the 3-cell 250 mah pack is almost enough to make it capable of hovering at full throttle. My guess is that at your altitude it should do OK. The Camel and the Jenny both have more wingspan and less stuff hanging out in the breeze, so they should do somewhat better. The Jenny's low speed performance is similar to the Cub except that it cruises at a substantially higher throttle setting due to the drag of all those wings and struts. With an MPS-2A or -2B and a 3-cell 250 mah pack it should do OK, although flight times will be shorter than for the Cub or the Junior. In general these three models don't like a lot of wind, although I've flown the Triplane with the 3-cell pack in 10 knots. The Jenny's upper limit with a 2-cell pack seems to be about 7-8 knots at our altitude, although a 3-cell pack should improve that significantly.

So, obviously there are a lot of possibilities that should work. Unfortunately we don't have an opportunity to test these ourselves at your altitude, so we have to rely on engineering theory for our predictions. However, in recent years we've been very successful with that method, most of our designs these days seem to perform as predicted right from their first tests. Obviously we can't afford to make outright guarantees, but I'd say the odds look very good for the ones I recommended above. I don't see it as a question of "if" they will fly, but rather a question of just how much reserve performance they will have.

Don Stackhouse
DJ Aerotech