getting down quickly

henryinparadise

henryinparadise

Pre-Flight
Joined
Oct 27, 2023
Messages
42
Display Name
Henry
Its quite common in the western US to land at an airport that is in a valley surrounded by tall mountains. Recently I had to drop 5 k ft in about 5 miles. I circled to land but it got me thinking.
Im a retired engineer flying an RV7.
I did a mathematical study and a practical flying study to determine weather its better to "get down (at a high speed) and then slow down". vs "slow down then get down".
So far my results are inconclusive. Any thoughts?
 
I spent 20 years flying around Colorado.. I never did a study, but both I and my passengers felt much more comfortable slowing down before going down when there wasn't time or space for a normal descent. Part of it was descent rate, but part of it was also turn radius since you either need more room or a steeper bank angle for a high speed descent.
 
In situations like that, flying an Archer, i would slow down until at/below Vfe, extend full flaps and continue on down at that speed. Yes this frequently involved retracting the flaps to first or second notch as required, when at/near pattern altitude.

-Skip
 
At the speeds we fly, even in an RV, the time difference from slowing further out is inconsequential, but it affords more reaction time as you get closer to the airport, traffic, and terrain. Your passengers will appreciate the lower descent rate.
 
If you are restricted by terrain a good option is to reduce RPM to reduce power first, thus preventing shock cooling during descent, then slowdown and enter a descending turn with about 45-60 degrees of bank. You can spiral down with comfort and less noise. Monitor the MP and adjust the throttle as required to keep the MP from going up as you lose altitude. If your airplane is slippery like my Mooney it doesn't want to slow down, but if you reduce power and enter a gentle climb first it will slow easily. Then keep a relative high AOA as you descend and it will drop fairly easily. I hope this helps.
 
henryinparadise said:
Its quite common in the western US to land at an airport that is in a valley surrounded by tall mountains. Recently I had to drop 5 k ft in about 5 miles. I circled to land but it got me thinking.
Im a retired engineer flying an RV7.
I did a mathematical study and a practical flying study to determine weather its better to "get down (at a high speed) and then slow down". vs "slow down then get down".
So far my results are inconclusive. Any thoughts?
Click to expand...
Are my eyeballs floating?
 
Yellow arc to approach would mess with my flow and I'd be all out of sorts.
 
henryinparadise said:
Its quite common in the western US to land at an airport that is in a valley surrounded by tall mountains. Recently I had to drop 5 k ft in about 5 miles. I circled to land but it got me thinking.
Im a retired engineer flying an RV7.
I did a mathematical study and a practical flying study to determine weather its better to "get down (at a high speed) and then slow down". vs "slow down then get down".
So far my results are inconclusive. Any thoughts?
Click to expand...

First you should define what "better" is. Better for safety, better for efficiency, better for passenger comfort?

For descending into a valley in mountainous terrain, I would argue that it is better from a safety perspective to come down in a high-drag configuration so you have more control of your airspeed and turn rates. I remember doing such a descent into Angel Fire, NM on a ski trip many eons ago. Due to weather, I had to wait until closer to the airport until commencing the descent - probably about 4000 ft drop within a few miles. Gear down and full flaps brought me down like an elevator (this was a 210), and we landed uneventfully.

In flat terrain, I rarely slow down for descents unless there are other factors (such as turbulence). This lets me at least partially recoup the energy that I gave up during the initial climb.
 
Another way to look at it. Keep yourself in the glide ring as much as possible. Don't descend so early and quickly that you can't make the runway should the engine quit.
 
IMO for steep descents, it is safer to slow down before you start, especially if spiraling down.
 
The way we used to descend a 182 after dropping skydivers was to put it in a full slip and about a 45-60 degree bank. I would normally be loading the next lift by the time the last jumper made it to the ground. Unfortunately one guy did beat me to the ground.
 
henryinparadise said:
Its quite common in the western US to land at an airport that is in a valley surrounded by tall mountains. Recently I had to drop 5 k ft in about 5 miles. I circled to land but it got me thinking.
Im a retired engineer flying an RV7.
I did a mathematical study and a practical flying study to determine weather its better to "get down (at a high speed) and then slow down". vs "slow down then get down".
So far my results are inconclusive. Any thoughts?
Click to expand...
I would say that this greatly depends on what your goals are and thus what you define as "better", as well as what aircraft type you're flying.

Some aircraft-specific examples:
* A DA40 does a really nice falling-leaf stall with about a 1500 fpm descent rate and a low forward speed - Excellent if you're trying to make an off-field forced landing.
* A TBM with proper motivation can get on the ground from FL310 in five minutes, which is a very nice feature if you're on fire or you otherwise need to get on the ground NOW.
* My Mooney does high-speed descents very well - Pull power, pull back on the yoke briefly to get below the Vlo(ext) speed of 140 KIAS, and then push the nose down to accelerate back up to the Vle of 165 KIAS. With all that drag and trying to go that speed, frankly I've never dared get it all the way back up to that speed because you literally don't see any sky out the front any more, or even close!
* Are you flying an airplane that can spin well? For certain scenarios, a spin will get you down nice and fast without carrying a ton of extra energy that needs dissipating before landing.

Now, for a few scenarios:
* IFR, you probably don't want to be in an overly big hurry to hurtle toward the ground when you can't see it. VFR, not as big of a problem.
* If you're on fire, you need to get on the ground NOW no matter what. That descent needs to be quick!
* If you have a medical emergency on board, you might want to consider a fast descent to a bigger city airport that's 20 miles in front of you instead of spiraling down to Podunk Muni directly below you where you'll be on the ramp WAY before an ambulance can make it there.

Given that it sounds like your goal was to get to an airport with terrain nearby as expeditiously as you can, it sounds like a high descent gradient is the goal and you're not really under a time crunch. In an RV, you really only have flaps for drag, or possibly constant speed prop. I'd probably put in full flaps and then fly at Vfe, maneuvering as necessary to get where you need to be.
 
Interesting study I’ve found it really depends on wind and terrain. In calm air, “get down fast then slow down” can work, but in turbulence, slowing first gives better control.
 
Emergency descents in all the transport category jets I’ve flown have you doing it at red line.

If you’re high on an approach you pretty much gotta slow first to mitigate the down range travel, then you can catch up, going down… if that makes sense!
 
henryinparadise said:
Its quite common in the western US to land at an airport that is in a valley surrounded by tall mountains. Recently I had to drop 5 k ft in about 5 miles. I circled to land but it got me thinking.
Im a retired engineer flying an RV7.
I did a mathematical study and a practical flying study to determine weather its better to "get down (at a high speed) and then slow down". vs "slow down then get down".
So far my results are inconclusive. Any thoughts?
Click to expand...

Depends on the scenario, and also, what you are flying. Some airplanes can go down and slow down at the same time. I thought my old plane couldn't do this, until I flew a 737-900ER, which really can't in comparison. If you can't do both at once, I'd say I'm in the camp of go down first (because you probably are for a reason), and then slow down, which normally can be done quickly in most planes in level flight. In larger, slicker, swept wing jets, you "go down" much faster at high IAS than you do at low speed with boards out (save throwing the gear down too). People say this is because of added induced drag, but I don't know if I believe that. You just go down at a better rate with higher airspeed associated with a more negative flight path angle that buys you that increased descent rate. But I could certainly be wrong. Perhaps an aerodynamicist will chime in
 
Yes to all the above. I guess kinda obvious, but backing up some to mitigate ahead of time. Even in areas with terrain, one usually has a valley on one side or so to maneuver in. With ATC one can default to ‘unable’ as needed, obvious I realize. If VFR, no need to enter the pattern at 1st opportunity. I don’t get overly wrapped up over ‘shock cooling’ but still want to treat the engine favorably.

Taking it to the extreme, 1000’s AGL at a few miles, one has to maneuver anyway, just enjoy a few more minutes of flight time.
 
Tools said:
Emergency descents in all the transport category jets I’ve flown have you doing it at red line.
Click to expand...
if we’re moving from normal descents for terrain to emergencies, emergency descent confuguration recommendations (if the manual even covers the subject) vary from Vne clean to Vfe or Vle with gear and or premiminary flaps deployed.
 
I read long ago that in a Connie you could enter downwind at 18K and make the landing. IDK if true?
 
All depends on the goals as other have stated.
When I want to descend into a steep valley, my preference is to not make supper rapid descents for passenger (and pilot) comfort reasons. Therefore, I like to do gentle S turns, slips and occasional circle descents; trying to maintain a 500fpm descent rate but keep the lateral dimension to a minimum.
If I ignore descent rate; power off, 60 degree steep banking S turns at max possible drag that is 2x stall speed is what a former USAF pilot taught me. With a normal buffer of 1.3 and the 40% increase due to bank angle, he said 2x stall speed in that configuration is what to aim for.

BWTHIK, I am just some guy on the net?

Tim
 
Get down, get down……




giphy.gif
 
ASE has a steep approach. In the CE-550 I used to dangle the Dunlaps, landing flaps, and deploy the cheese graters. All that drag got me down as needed and I was at a proper speed to land.

IMG_6238.jpeg
 
35 AoA said:
In larger, slicker, swept wing jets, you "go down" much faster at high IAS than you do at low speed with boards out (save throwing the gear down too). People say this is because of added induced drag, but I don't know if I believe that. You just go down at a better rate with higher airspeed associated with a more negative flight path angle that buys you that increased descent rate. But I could certainly be wrong. Perhaps an aerodynamicist will chime in
Click to expand...
Not too tough to calculate, if you have vertical speed and airspeed numbers for a fast descent and a slow descent. You could probably even avoid the trig if you have both sets of numbers.

keen9 said:
4g 360s at maneuvering speed with IDLE power will get you down fast.
Click to expand...
8g 360s will get you down REALLY fast. Once...

Maybe don't do 4g in a normal category airplane.

sourdough44 said:
One other thing, it’s best to not scare your passengers to much either, should anyone be onboard.
Click to expand...
This. I always hate it when ATC says to "Expedite your descent through xxxx". Does that mean "Go a little faster" or does it mean "Make your passengers scream now or they're going to be screaming later"? I much prefer it when controller say things like "I need you at FL200 in four minutes". Then I can give them exactly what they need, without any guesswork and without making anyone overly nervous.
 
Ed Haywood said:
Neither. It's not certified.
Click to expand...
I didn't say "certified in the..." From a piloting perspective, those categories are mostly used to refer to G limits and often associated W&B limitations. If you're involved in aircraft certification, different story.

So, I went to look it up. FWIW, the RV-7 POH specifically has weight envelopes in the Normal, Utility, and Aerobatic categories, and the Van's website talks about them as well.
 
flyingcheesehead said:
Not too tough to calculate, if you have vertical speed and airspeed numbers for a fast descent and a slow descent. You could probably even avoid the trig if you have both sets of numbers.
Click to expand...

I mean I can tell you right now that the VSI at a 300+ knot descent at idle is a WHOLE lot higher than an idle descent at 200-210. What I don't know is whether that is purely because of the descent angle, or if there is actually "induced drag" allowing a higher descent angle while maintaining that higher airspeed but not accelerating more. I will say specific to the 73, below 20k ft, clean maneuvering speed + Flaps 5, gives a pretty healthy descent rate. Not as good as 300 knots clean, but still good. If your company allows dual speed brake and flap usage (below the flaps 10 limitation), that could add more, but mine frowns on that.
 
35 AoA said:
I mean I can tell you right now that the VSI at a 300+ knot descent at idle is a WHOLE lot higher than an idle descent at 200-210.
Click to expand...
If you're doing 300 knots and 3000 fpm, that's going to be the same descent angle as 200 knots and 2000 fpm - So, you can use simple ratios to determine whether the descent angle is higher or lower.

35 AoA said:
What I don't know is whether that is purely because of the descent angle, or if there is actually "induced drag" allowing a higher descent angle while maintaining that higher airspeed but not accelerating more.
Click to expand...
As long as both speeds are above L/Dmax, you absolutely have more total drag, but your induced drag is lower and your parasite drag is higher.

Where things may differ is if you're using different configurations at the two speeds (say, clean at 300 vs. flaps 5 at 200). Each aircraft is going to be somewhat different in terms of what is "best" based on the limitations of the various drag devices that may be employed. And now you've got me curious and I'm probably gonna go and test this out in my plane. :D
 
flyingcheesehead said:
Where things may differ is if you're using different configurations at the two speeds (say, clean at 300 vs. flaps 5 at 200). Each aircraft is going to be somewhat different in terms of what is "best" based on the limitations of the various drag devices that may be employed. And now you've got me curious and I'm probably gonna go and test this out in my plane. :D
Click to expand...
In the Aerostar if I wanted to get down fast with maximum VSI, you had to slow down, throw out everything (flaps, speed brakes, gear, engine idle, props course) you could drop like a rock. With everything out, you were looking at scary down angles, where you are only seeing land and no sky even when you tried to duck down a bit to look up, accelerate to max flap speed.

If on the other hand, only speed breaks and engine power changes were made with nose over and accelerate to Va or just under Vne depending on weather conditions you came down almost as fast vertically, but you went a heck of a lot farther!

Depending on your cruise speed would determine the approach to take. Sometimes it paid to slow down because you were just above max flap speed other times you were going like a bat out hell, in which slowing the plane down took forever...

Tim
 
tspear said:
In the Aerostar if I wanted to get down fast with maximum VSI, you had to slow down, throw out everything (flaps, speed brakes, gear, engine idle, props course) you could drop like a rock. With everything out, you were looking at scary down angles, where you are only seeing land and no sky even when you tried to duck down a bit to look up, accelerate to max flap speed.
Click to expand...
Sounds similar to the Mooney's book emergency descent. Speed brakes are good up to Vne (194 KIAS), Vlo is 140 but Vle is 165, and Vfe is only 110. So, book procedure is to pop the speed brakes and pull slightly to get to 140, drop the gear, and then point it at the ground and get back up to 165. At 155, you already can't see the sky no matter how far down you duck your head unless you're looking back.
 
bonefishpete said:
I read long ago that in a Connie you could enter downwind at 18K and make the landing. IDK if true?
Click to expand...
That would play hell on the gearboxes, bearings, and cylinders on the -3350's.
 
You must log in or register to reply here.
Back
Top