Density Altitude

[clear_prop1961]

I want to cross the Rockes... most likely the southern route.... this summer to go to Blakesburg in my A65 powered Taylorcraft. I want to practice high density altitude take off's so the other day I loaded up the aircraft to a weight within about 20 lbs of the load I expect to carry and took off from a near sea level (200msl) paved airport (6000ft length) at about 77F temp with only 1900rpm. I took off fine (less then a 1000ft) but with only 150 to 200fpm climb rate... the question is what was my density altitude I was simulating? What would be my density altitude if I used 1850rpm or 1950rpm?

I assume there is a relationship between engine rpm (manifold pressure) and density altitude.... is that true?

I would be interested in everyone's thoughts on the subject.
Jerry

[M Towsley]

Re: Density Altitude

Jerry,

A friend of mine, Robin Reid, brought my L-2 back from San Jose many years ago but I am unsure which route he took. I know mine had a 65 with the wrong prop, (it was for an 85). He made it during the summer. I can put you in-touch with him if you don't get any answers or you want more. He has done the route with small aircraft many times.

[otrcman]

Re: Density Altitude

Hi Jerry,

You are on a good track with your tests. And I applaud you for giving this some thought in advance rather than going into the trip unprepared. That's the old Cal Poly learn-by-doing spirit. (Inside joke. We're both Cal Poly grads.)

If you do some climb tests at a various altitudes you should be able to plot the results and extrapolate climb rate for even higher altitudes. Ideally, if you expected density altitudes in route as high as 10,000 feet, then you would do a climb test to 10,000 in advance to see what your ROC is. Then you can come back down to near sea level and experiment to see what RPM gives you the same ROC as you experienced at 10,000. That would be the RPM for your takeoff tests.

The other part of the density altitude equation is increased true airspeed. So you will not only have a reduced climb rate, but also a higher true speed. The net effect makes for much longer takeoff rolls and much lower climb angles.

To simulate the true airspeed effect, you might experiment with taking off downwind and even making approaches to landing in the downwind direction. For guidance on just how much of a tailwind you need for the simulation, you can pick a worst case density altitude and then calculate your true speed for that situation. Say, for instance, that you expect to experience a 10,000 ft. density altitude. Your true airspeed at a 60 mph IAS approach speed would be around 72 mph, so a 12 mph tailwind would put you in the ballpark.

One final note: The density altitude on takeoff is only the beginning of the problem. If you take off at a 10,000 ft. density altitude airport, you still need to climb to a cruising altitude. Let's say you need to be 4000 agl for comfortable terrain clearance. That 4000 agl now becomes 14,000 msl. Can you do that ?

Dick

[skip]

Re: Density Altitude

I posted last month about a ferry flight I did of an A65 BC12-65 to New Mexico.

http://vb.taylorcraft.org/showthread.php?10548-41-Deluxe-Ferry-Flight

This is right through the area that you will go if you take the "Southern Route".

You didn't specify exactly the route, but the first passes that you hit will either be La Veta pass south of Pueblo CO, or across Glorieta pass east of Santa Fe NM. Both are over 9000' MSL.

I had no trouble on a warm day (High 70s, low 80s) going to 10500 over Glorieta pass.

After I delivered the '41 Deluxe to my friend, he worked with an instructor to make sure he was sharp and up-to-date in the plane. His instructor is an experienced vintage taildragger pilot mostly in things like Cubs and Champs, but he wanted to see how this Taylorcraft did at gross at our altitudes. So he and I and fuel (right up to 1200#) went flying from our home field of Los Alamos NM (7200 MSL). In short the plane handled the altitude beautifully! Better than Cubs in my opinion. Doing T&Gs at Espanola NM (5800' MSL) he started by doing stop, taxi back to end, go, patterns, but after a few he just rolled to a stop (about 2000' - easy on the brakes), go (off in 700'-800') with 300-400 feet altitude at the threshold on a 5000' runway. It was comfortable. Going to 8500' for airwork was no problem. Just hit the best rate airspeed for your plane and let it go up.

I am not sure how valid the reduced power practice is at low altitude. In my experience, the high altitude characteristics of the plane are similar to the low altitude characteristics with everything stretched out by the thinner air. In particular, the perceived airspeed will be higher looking out the window than at sea level but as long as you keep an eye on the Air Speed indicator and don't let your sea-level instincts get you too slow, you will be fine. Running at low power just doesn't give this same effect and feel. I know that when I get own to sea level I tend to look out the window coming down final and think "Geez! I could get out and walk faster than this!" but the feel of the plane and the performance all fit as long as I fly the proper airspeeds.

While I like the C85 in my Tcraft at these altitudes, I am quite impressed at the capabilities of the standard A65 planes. Come on out. Watch the airspeeds and talk to some of the locals when you stop for fuel. It's a lovely part of the country for flying a Taylorcraft.

[skip]

Re: Density Altitude

I really like Dick's suggestion to use a tailwind rather than reduced power for practice. My experience is that the main PERCEIVED difference at high altitude is that stretching our of the airspeed due to the thinner air. Your instincts near the ground will be that you are going way too fast and need to slow down. Thus, pulling back, getting slow and behind the power curve, stalling and spinning. The tailwind practice will come about the closest to this effect that you can get at sea level.

Thanks Dick! I'll remember that one.

[otrcman]

Re: Density Altitude

Skip,

You reminded me of one more consideration when flying at high density altitude. I was so focused on performance that I forgot all about that "perception" thing. Yes, looking out the window at high density altitude gives the impression that you are flying too fast. Watch that airspeed indicator !

There was a recent account of a fatal accident (June 2014 Sport Aviation) in which the NTSB finding was that the pilot got too slow and stalled on approach to a high altitude airport.

The other perception problem involves turn radius. Since your true airspeed is higher, all of your turns will require more distance as well. Where this can get you into trouble is flying too close to the runway on downwind. If you stay close to the runway as you normal do you may find that you have to turn much more sharply to line up with the runway. Bad idea ! So just pretend that you're flying a Bonanza instead of a Taylorcraft and keep those patterns a little wider and bank angles safe & shallow.

Dick

[astjp2]

Re: Density Altitude

Smiley Creek Idaho, over 7k' field elevation and 85* day, I had to fly out alone in the afternoon and put the GF at the time in a friends 182, I almost didnt make it over the 8400' pass, caught a downdraft and was blown into the treetops...so low I could not bank...Tim

[clear_prop1961]

Re: Density Altitude

Dick & Skip- Thanks for the helpful input. Now when you suggest making a downwind approach I assume you don't suggest I land with a 12mph tailwind but just an approach to landing to get a feel for the ground speed on high altitude landings? A few months ago I did do a climb test but I only when up to 7000msl and I did not check the air temp to get a better feel for my real density altitude. At 7000 ft I was still climbing at about 200fpm Maybe I'll do it again. What I would like to know is what is my safe density altitude limit without the risk of trying it beyond the limit. Also of course I need to consider winds aloft.... as they can cause serious down drafts. What is the rule of thumb here?

Marty- I know Robin Reid very well I will give him a call... I did not think to talk to him.

Tim- I talked to a guy about a year ago who was flying a 65hp homebuilt aircraft at 5000msl (I don't remember if he told me the density altitude) and 1500agl and a down draft took him right into the ground. Shoulder harnesses saved him and his passenger. I believe he said he was flying in Montana. Anyway it can happen.
Jerry

[otrcman]

Re: Density Altitude

Yeah, rate of climb gets pretty academic when you are in serious sink. At that point you need to be a glider pilot and know where the lift is and where the sink isn't.

Dick