Power-Off Glide descent rate

[Jaybird180]

I have a chart in my POH that tells me the distance I can travel at best glide per 1,000 foot altitude loss but is there any published data or way of calculating the descent rate (fpm)?

Of course I could go up and experiment, but right now this is just a mental masturbation question that may or may not (legal disclaimer) be used for future flight planning purposes.

 

[olasek]

Why do you have to know that? Best glide speed is what you need to know and it is in your POH.

 

[Cpt_Kirk]

Probably faster than you would prefer.

 

[COFlyBoy]

How about this formula:

Descent Rate = 1000 feet / (Glide Distance / (Glide Speed/60))

Where:
Glide Distance is in nm
Glide Speed is in nm/hour
Descent Rate is in feet/minute

 

[flyingriki]

Don't forget it's different with the prop windmilling or stopped.

 

[Dead Stick]

Why do you have to know that? Best glide speed is what you need to know and it is in your POH.

Because, when it comes to maximizing distance, it all depends upon whether you've got a headwind or a tailwind. The glide speed listed in the POH assumes still air.

 

[dmspilot]

I have a chart in my POH that tells me the distance I can travel at best glide per 1,000 foot altitude loss but is there any published data or way of calculating the descent rate (fpm)?

Of course I could go up and experiment, but right now this is just a mental masturbation question that may or may not (legal disclaimer) be used for future flight planning purposes.

If you want minimum sink speed, this speed is not published but is approximately halfway between stall and best glide speed. If you just want descent rate fpm during a max glide, you can calculate it using the glide ratio and airspeed as stated above.

 

[poadeleted20]

If you know the glide ratio in nm per 1000 feet, and you know the glide speed in knots, it's just a bit of math to compute descent rate in feet per minute. Just remember that you need to use true, not indicated airspeed, and while IAS for best glide is a constant, TAS for that IAS (and likewise the descent rate in ft/min) is higher at higher altitudes.

1. Convert TAS in knots to nm/min by dividing by 60.
2. Multiply speed in nm/min by altitude loss/nm (inverse of glide ratio in nm/1000 feet) to get descent rate in ft/min.

So, if your glide ratio is 5nm/1000 feet lost, and your glide speed is 75 knots, then the airspeed is 1.25 nm/min. Multiply by 1000 and divide by 5, and you get 250 ft/min. And remember that best glide range is not the same as minimum sink rate, which will occur at a lower airspeed by also result in shorter glide distance.

 

[olasek]

The glide speed listed in the POH assumes still air.

Wrong, best glide speed is your indicated speed so wind really has nothing to do with it.
Of course your glide range will depend on wind.
Frankly I see zero sense in engaging in such computations while having emergency. You should already know where your wind is and pick direction of your glide accordingly (and based on other factors like where a hospitable terrain is) but you still want to maintain the same best glide speed.
Your glide speed may depend however on weight, this would be the only reason to make a small adjustment to that number.

 

[MAKG1]

How about this formula:

Descent Rate = 1000 feet / (Glide Distance / (Glide Speed/60))

Where:
Glide Distance is in nm
Glide Speed is in nm/hour
Descent Rate is in feet/minute

Glide speed is GROUND SPEED, not Vg. Good luck with that if you don't know the wind.

 

[dmspilot]

I have a chart in my POH that tells me the distance I can travel at best glide per 1,000 foot altitude loss but is there any published data or way of calculating the descent rate (fpm)?

Glide speed is GROUND SPEED, not Vg. Good luck with that if you don't know the wind.

Why would he need to know the wind?

 

[Hank S]

Why would he need to know the wind?

Distance covered = ground speed [nm/hour] * time

If an airport is 15 nm away when your engine quits, whether you reach it or not may depend on which way and how hard the wind is blowing. The closest airport may be unreachable, while one further away downwind is within glide range.

Set speed using airspeed; calculate range using groundspeed.

Groundspeed = corrected true airspeed + wind speed [using vector addition].

 

[Clip4]

For a real world application, the number you want is ground speed, which is available on your gps.

 

[dmspilot]

Distance covered = ground speed [nm/hour] * time

If an airport is 15 nm away when your engine quits, whether you reach it or not may depend on which way and how hard the wind is blowing. The closest airport may be unreachable, while one further away downwind is within glide range.

Huh? Did you actually read the question?

 

[dell30rb]

Jay it depends on airspeed.

Highest power off descent rate will probably be at Vne. Descent rate will also be pretty high if you pull the yoke back and stall it.

If you want to calculate the FPM at best glide airspeed that's just some simple math (assuming you have the altitude loss per 1000 feet as mentioned earlier)

 

[Matthew]

Why don't you just try it and find out?


Efit: ok, just re-read the 2nd part of your post. Never mind.

 

[coloradobluesky]

Post deleted.

 

[Bill Watson]

Wrong, best glide speed is your indicated speed so wind really has nothing to do with it.
Of course your glide range will depend on wind.
Frankly I see zero sense in engaging in such computations while having emergency. You should already know where your wind is and pick direction of your glide accordingly (and based on other factors like where a hospitable terrain is) but you still want to maintain the same best glide speed.
Your glide speed may depend however on weight, this would be the only reason to make a small adjustment to that number.

Well, that's not quite correct.

Yes, best glide speed is effected by weight; I think any number quoted in the 'book' will be for full gross. For lesser weights (think no fuel, pilot only) the best glide will be at a lower airspeed. I'll guess that the range from full gross to no fuel/pilot only will be about 10 knots in most SELs (please, someone run the numbers).

Knowing what the wind is doing is important since your glide over the ground will be significantly effected by a significant wind.

But your minimum sink speed is lower than your best glide speed - again think 10 knots. Downwind glides will be longer if go slower than best glide because you will sink slower and be carried a bit by the wind. Conversely, upwind glides will be longer if you go a bit faster than best glide because the wind will have less time to effect you.

I don't do these calcs any longer... in fact I stopped when GPS glide computers became available for gliders in the late 80s. But who has time or bandwidth anyway when the vibrator up front goes quiet?

Best glide typically falls somewhere between Vx and Vy (actually I don't understand why Vx doesn't equal min sink and Vy doesn't equal best glide but that's not what the 'books' show). Gliding downwind, slow down towards Vx but don't go lower. Going upwind, speed up toward Vy but don't go further unless the wind is really blistering. I've never checked those numbers against a specific airplane but spent a lot of time gliding into 'ports and fields. I'll bet those thoughts will serve you well as a ROT.

Someone want to run some numbers?

 

[Blatham489]

I'd say know your plane, and over estimate the worst case descent rate and come up with something simple so that in the face of an emergency you have a simple answer to "how long do I have until I'm on the ground and how far can I go until then?" For the Cherokees I rent, I figure 1mile per 1000ft above 1000ft AGL and 1minute for each of those. At that point, I'm setting up for landing no matter where I am.

5000AGL when it gets quiet => 4000ft to play with = 4 minutes to solve my problem, 4 miles to get to an alternate site and then set up for landing.

Anything I've ever read written by someone who really had an emergency indicates your mind functions at a pretty low level. Keep it simple. Hopefully I never have to test my theory.

 

[poadeleted20]

Wrong, best glide speed is your indicated speed so wind really has nothing to do with it.
Of course your glide range will depend on wind.

Well, maybe not all that wrong. Your "best glide speed" in the book give you the best still air range. However, with a head or tailwind, you will cover more ground at a higher or lower IAS than the book best glide speed. The computations are a bit complicated, but it's possible to do it with enough time (which you won't have in the actual event).

Your glide speed may depend however on weight, this would be the only reason to make a small adjustment to that number.

Not "may" -- "does". Best still air glide IAS goes down by the square root of actual gross weight as you get below max gross weight. Some POH's have the numbers precalculated for weights less than MGW, some don't.

 

[poadeleted20]

For a real world application, the number you want is ground speed, which is available on your gps.

Flying a ground speed equal to your book best glide speed will not get the job done. Think about what would happen in a 172 with a 65 KIAS best glide speed trying that with a 30 knot tailwind.

 

[Hank S]

"Best Glide" is a number on your airspeed indicator--nail it and you will glide the longest distance; miss it and you will be through gliding in less distance. If you have a GPS, it will give you your groundspeed, which will determine whether you can reach your desired landing area or not.

"Minimum Sink" is an unpublished number on the airspeed indicator that will let you glide for the longest amount of time, but a shorter distance than at Best Glide. It can be useful to troubleshoot your issues AFTER REACHING a spot where you can land by flying at Best Glide.

Caveat emptor, YMMV, swim at your own risk, not responsible for damage or loss of personal property, etc.

 

[oldShar]

Someone want to run some numbers?

I've done some based on trig and POH numbers and come up with figures varying between about 750 for aircraft like the -172 and RV12 to as much as 1300 for the Arrow. FWIW -- haven't verified any tho'

 

[Jaybird180]

Got around to working the math
iPad won't allow me to post both pics in same post

 

[Jaybird180]

Working the numbers

 

[Jaybird180]

Here's why I did this:
I am planning a trip that has a 17nm water crossing. At the time I asked, I forgot that it only needed to account for 1/2 the distanc to glide power off.

I figured the engine is pretty reliable but if I climbed up to a ridiculous height then I should plan a descent rate that less that the power off best glide, just in case the engine quits during the descent.

At 769fpm power off descent I figure that anything less than that is free fuel, so why not.

At the time I asked the question, I also had a long day at work and was spent. The math was pretty simple but I couldn't see that I had everything I needed and was overthinking the problem.

 

[Bill Watson]

Funny thing, after flying sailplanes exclusively for 10-15 years, and then going back to airplanes, I was constantly doing real time estimates of where I could glide to in case of a failure. I avoided certain straight line courses because of water or forests. Because I had landed off field more than a few times, my thinking always extended down to the landing which made many situations worse.

There was no "Eureka" moment but slowly it dawned on me that the engine was a lot more reliable than the next thermal. Additionally, the amount of time I was concerned about engine failure was a lot less flight time than the time I spent worrying about the existence of the next thermal.

I got comfortable with short overwater jaunts and such and forests no longer send me into final glide calc mode. The NYC Hudson River corridor flight still gives me the chills because it's probably the longest, lowest flight over unlandable territory that I 'routinely' make... I'm still trying to identify places to land but know all of them can suck!

Strangely enough, having built a kit plane and now flying and maintaining same, I've become even more comfortable with the engine (and the airframe). Sh*t can and does happen... I still worry about failures just after takeoff at my home field. But most of my flight time is at altitude with a loafing, well instrumented engine flying over landable ground.

So now when I head out for the Bahamas I no longer get any over-water-roughness. And the water is warm. And I can swim. And I have preservers and a raft. And the new ELTs really work well.

Fly on!

P.S. As an experienced glider guider, I realize that I can 'see' my glide path pretty accurately in the typical light plane simply because it's coming down so fast. That is, stuff that is moving up the windscreen is beyond my glide range, stuff moving down is within my glide path. That works quite well even at altitude. That doesn't work at all for a high performance sailplane where it is impossible to 'see' one's glide path, even on short final.

The effects of weight on L/D (i.e. the square root law) are hard and fast but are easily compensated for by gut. Just add or subtract a few knots... <5 for normal weight ranges.

The effects of wind, especially 20-30 knots winds, at altitude where they will impact you for a relatively long time, are quite significant. Especially when the difference between up wind and downwind is 40 to 60 knots (think of the wind as a percentage of your glide speed). I think about it every time I fly down the east coast to and from FL and request a 'cut the corner' out over the Atlantic. It's pretty easy to be high and yet completely out of range of the quite visible land that appears nearby when a stiff westerly wind is blowing. And the Atlantic can be cold.

 

[flyingron]

For olasek, if you're unconvinced, think of this thought experiment:

What happens if the headwind equals your book best glide speed. Where do you end up?
What happens then if you fly ten MPH over your best glide speed?

 

[Jaybird180]

For olasek, if you're unconvinced, think of this thought experiment:

What happens if the headwind equals your book best glide speed. Where do you end up?
What happens then if you fly ten MPH over your best glide speed?

1. going down right over the spot the engine quit + a little forward movement as the windspeed reduces as altitude is lost
2. You glide less distance.


What's your point

 

[flyingron]

Think harder. If you're best glide speed is 40 knots and there's a 40 knot headwind, you will (neglecting wind speed changing) hit the ground directly below where you start.

If you fly at 50 knots, you may indeed hit the ground sooner but at least you'd have moved upwind a bit.

From Aerodynamics for Naval Aviators (which also has the info C'Ron posted on Gross Weight effects) http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80t-80.pdf:

However, when a wind is
present, the optimum gliding conditions may
not be accomplished by operation at (L/D)ma.
For example, when a headwind is present,
the optimum glide speed will be increased to
obtain a maximum proportion of ground dis-
tance to altitude. In this sense, the increased
glide speed helps to minimize the detrimental
effect of the headwind. In the case of a tail-
wind, the optimum glide speed will be reduced
to maximize the benefit of the tailwind. For
ordinary wind conditions, maintaining the
glide speed best for zero wind conditions will
suffice and the loss or gain in glide distance
must be accepted. However, when the wind
conditions are extreme and the wind velocity
is large in comparison with the glide speed,
e.g., wind velocity greater than 25 percent of
the glide speed, changes in the glide speed must
be made to obtain maximum possible ground
distance.​

 

[brcase]

I seem to recall the rule of thumb for gliders is to add or substract 1/2 the headwind or tailwind (no less than minimum sink speed) to your best glide speed.

So for your 40 knot best glide in a forty knot headwind you should use a 60 Knot glide speed for best forward progress. I am sure someone could to some math to see if this is close or not.

Brian

 

[Bill Watson]

I seem to recall the rule of thumb for gliders is to add or substract 1/2 the headwind or tailwind (no less than minimum sink speed) to your best glide speed.

I remember the same.

 

[flyingcheesehead]

Man... Lots of misunderstandings in this thread. Our collective reading comprehension sucks.

Wrong, best glide speed is your indicated speed so wind really has nothing to do with it.

Wind has nothing to do with Vg. It has a LOT to do with where you'll end up. Vg lets you travel farther THROUGH THE AIR. How far you travel across the ground will change with wind, and since we generally have a spot on the ground where we want to end up, wind must be considered.

Glide speed is GROUND SPEED, not Vg. Good luck with that if you don't know the wind.

Careful how you say that... Don't want anyone aiming for Vg on their GPS groundspeed!

For a real world application, the number you want is ground speed, which is available on your gps.

Again, let's not lead any newbies into thinking you want to fly Vg on your GPS. Vg is what's on your airspeed indicator. Convert to true airspeed and compare with what's on your GPS to see if you have a headwind or tailwind component.

You'll go farther in a headwind if you fly faster than Vg, you'll go farther in a tailwind if you fly slower than Vg. Strong tailwind, you want minimum sink speed instead of Vg.

Yes, best glide speed is effected by weight; I think any number quoted in the 'book' will be for full gross. For lesser weights (think no fuel, pilot only) the best glide will be at a lower airspeed. I'll guess that the range from full gross to no fuel/pilot only will be about 10 knots in most SELs (please, someone run the numbers).

For a few of the planes I fly a lot, Vg at full gross vs. Vg with just me and an hour of fuel:

Mooney M20R: 91 / 82
Cessna R182: 80 / 68
DA40: 76 / 66

So yeah, pretty close to 10 knots in each case.

Best glide typically falls somewhere between Vx and Vy (actually I don't understand why Vx doesn't equal min sink and Vy doesn't equal best glide but that's not what the 'books' show).

Think about the power vector and how that might change things...

Here's why I did this:
I am planning a trip that has a 17nm water crossing. At the time I asked, I forgot that it only needed to account for 1/2 the distanc to glide power off.

Yep. What changes with wind speed on an over-water glide is not how far you can go, but where the turn-back point is. If you have the full glide polar for your airplane you may be able to do slightly better in strong winds, but it's best to start out by just calculating the change in your turn-back point based on winds and sink rate.

What happens if the headwind equals your book best glide speed. Where do you end up?
What happens then if you fly ten MPH over your best glide speed?

1. going down right over the spot the engine quit + a little forward movement as the windspeed reduces as altitude is lost
2. You glide less distance.

Not so. Assuming a constant wind as Ron implied, you'd go straight down for #1. But, for #2, while you'd get to the ground faster, you would make more forward progress. You'd be gliding through less air, but you'd be gliding across more land.

 

[Bill Watson]

For a few of the planes I fly a lot, Vg at full gross vs. Vg with just me and an hour of fuel:

Mooney M20R: 91 / 82
Cessna R182: 80 / 68
DA40: 76 / 66

So yeah, pretty close to 10 knots in each case.

Thanks much for the data!

Think about the power vector and how that might change things...

That makes some sense but I can't quite think it out fully. Is it the power vector?

 

[redtail]

The NYC Hudson River corridor flight still gives me the chills because it's probably the longest, lowest flight over unlandable territory that I 'routinely' make... I'm still trying to identify places to land but know all of them can suck!

Which is why I wear this. Once I bought 3 of these for myself and passengers, my stress level dropped tremendously and the flight became much more enjoyable.

 

[flyingcheesehead]

That makes some sense but I can't quite think it out fully. Is it the power vector?

I think that's most of it. For example, Vx is generally lower than minimum sink. (I think the difference between them, all else being equal, would be greater the more excess horsepower you have.) But if you take the minimum sink AoA and add power to it, chances are that you can slightly increase the angle of attack and have the slight loss in aerodynamic efficiency more than made up for by the increase in the vertical component of the thrust vector. Same for Vy vs. Vg.

Disclaimer: My degree is in electrical engineering, not Aero-E. So this is only based on my own thoughts on the subject.

 

[Bill Watson]

Which is why I wear this. Once I bought 3 of these for myself and passengers, my stress level dropped tremendously and the flight became much more enjoyable.

I have some very small CO2 vests in plane at all times. Certainly mitigates some risk but ditching is ditching... well you know. Still think that it's a 'safe' flight but with significant risk in case of engine failure. Oxymoron, eh?

 

[redtail]

I have some very small CO2 vests in plane at all times. Certainly mitigates some risk but ditching is ditching... well you know. Still think that it's a 'safe' flight but with significant risk in case of engine failure. Oxymoron, eh?

Yeah I know.

It's just that I've made this flight numerous times without wearing a PFD. After that CFI and her student had to ditch a couple of years ago, I told myself I'd never fly it again unless I wore a "condom"

So having vests in the plane isn't good enough for me, I put it on before I launch on such a low level flight.