Mountain Flight Planning

[tonycondon]

I think Tony would have a better answer for this than me, it's almost a glider question. In fact, I bet the guys who get the most performance out of their airplanes in the mountains are the glider guys. I played with ridge lift some, but I didn't encounter a situation like was described in this question. Part of the course was about avoiding those situations in the first place.

To determine the best speed to fly through a downdraft you first have to know the glide polar (sink rate on the vertical axis vs. airspeed on the horizontal) for your airplane. then, depending on the strength of the downdraft, you select your speed. basically, the stronger the downdraft, the fast you need to go. But, in low performance gliders (and airplanes) the polar drops off pretty steeply with speed. So there gets to be a sort of natural speed that it really isnt worth exceeding. In my glider that seems to be about 70 mph.

The only way to figure this out for sure in your airplane would be to either climb to altitude and do some testing, finding sink (or climb) rate vs. airspeed. Or you could probably figure out a reduced power setting at lower altitude that would simulate the performance you would see up high. that would probably get you close enough.

don't forget to slow down in the lift after you dive through the down.

 

[gismo]

I think Tony would have a better answer for this than me, it's almost a glider question. In fact, I bet the guys who get the most performance out of their airplanes in the mountains are the glider guys. I played with ridge lift some, but I didn't encounter a situation like was described in this question. Part of the course was about avoiding those situations in the first place.

Funny you should mention soaring experience as a boon to mountain fliers. My first experiences on my own in the mountains (I had taken some dual out of Aspen one winter before venturing into the tall rocks by myself) occurred soon after I had gotten a glider rating and I was able to put some of my glider experience to good use, especially WRT finding and flying in lift as a means of comspensating for the headwinds heading west.

In the glider I was taught to prepare a table of "speeds to fly" after preparing a plot of the glider's vertical speed vs airspeed and applying some geometry. This table gave the ideal IAS for a range of sink rates. I've never seen anyone create such a table for a powered airplane but the same technique ought to work.

Yep, I had a thread about this a few months ago, and put together a radius vs. speed vs. bank angle spreadsheet for kicks. I think it was 74 degrees with the stall warning screaming to get the best turn for normal category, and utility category only allowed another degree or two.

I think you can see that this method would significantly decrease the canyon width required for a retreat compared to a 45 degree bank but it's something that should be practiced in less hostile conditions before employing it in the mountains.

I'm not sure how effective those would really be when you're already on the edge of the performance envelope. IE, you wouldn't get much vertical going before you had to give it back up, and you might not have enough to help the turnaround maneuver in any of the cases above. It'd sure be interesting to see how an Extra performs at 15,000'!

That's way I included the caveat about room ahead and below. In almost any airplane you can acquire sufficient energy to fly an Immelman by diving under power if you have enough altitude to give up plenty of room in front of you. If you are already close to the end of a canyon a pullup into an immediate turn is the only hope unless you had enought energy (airspeed+power) to go vertical and even then you will extend closer to the rocks ahead than you would with a max performance climbing turn.

 

[flyingcheesehead]

Funny you should mention soaring experience as a boon to mountain fliers. My first experiences on my own in the mountains (I had taken some dual out of Aspen one winter before venturing into the tall rocks by myself) occurred soon after I had gotten a glider rating and I was able to put some of my glider experience to good use, especially WRT finding and flying in lift as a means of comspensating for the headwinds heading west.

In the glider I was taught to prepare a table of "speeds to fly" after preparing a plot of the glider's vertical speed vs airspeed and applying some geometry. This table gave the ideal IAS for a range of sink rates. I've never seen anyone create such a table for a powered airplane but the same technique ought to work.

Sounds to me like the beginnings of figuring out the glide polar Tony is talking about!

This is one of many things that glider pilots learn that apply to powered aircraft. Gotta go get that rating...