Another Stupid Question (No3) Best Glide Speed

[piperboy84]

My question is regarding how do I calculate or demonstrate the Best Glide Speed when in the takeoff configuration ( if indeed there is a difference). From the POH it lists 83MPH in a clean (Vs1) config.(I have figured a glide ratio when clean off 7.5:1 based on actually trying it in the air with engine idle) The reason I am asking is I was running engine failure on takeoff scenarios/plans at my grass strip factoring in obstructions at the end of the field and again at about 500 feet passed the end. Based on these obstructions I have identified a few areas to set her down if there is a failure from takeoff to about 200agl, however I want to quantify how the published best glide speed/glide ratio differs with TO flaps to see if they afford me the ability to reach the preferred landing spots. The plane is a maule mx-7-180A with flaps range as follows:
-7 Dgrs. (Cruise) , 0 Dgrs. (Normal flight), 24 Dgrs.(Takeoff) 40Dgrs.(Landing) 48Dgrs(Landing)

Any ideas? or am i looking at this the wrong way?

[Flamin_Squirrel]

Not quite sure if I understand your question completely, but flaps reduce L/D ratio. Therefore you'll always glide further clean.

[piperboy84]

yeah in looking at it now it's not worded very well, to cut to the chase, how to i figure my best glide speed in the TO flap config of 24 degrees or do i stick to the VS1 speed of 83MPH as per the POH regardless of flap configuration

[BackPacker]

From a theoretical standpoint it's probably very interesting to figure out Vbg with some sort of flaps down configuration, based on Vbg(clean). But as FS said, she'll definitely glide better with the flaps up. So if you need the best glide performance, pitch for Vbg and raise the flaps immediately.

In practice however, you have so little time in an EFATO situation that trying to break a world gliding record is not a good idea. Stuff the nose down, aim for something that is within easy reach and relatively free of obstacles, and set her down there. The objective is to survive. Bonus points are only given away for controlling drag (through flaps and sideslipping) so that you don't overshoot your chosen field. In contrast, gliding at exactly Vbg0 or Vbg1 into a tree or other obstacle only gets you an AAIB report.

In that respect, learning to play with the drag, at engine idle, so that you arrive exactly where you want to arrive at the runway from a high approach, is probably a much better investment of your time. And I have never flown a Maule, but from what I've seen it looks like she can generate a lot of drag in a sideslip.

[Flamin_Squirrel]

Out of interest, what's prompting you to TO with flaps? Unless you've got a particularly short or soft takeoff run, flaps up is generally best, I believe.

[n5296s]

For sure Vbg with flaps is not equal to Vbg clean. If it's not published for your aircraft then the only way to find it is to go up there and measure it.

Set up the configuration you want to test, pull the power, hold a steady speed and time the descent over 500 feet (helps to have a passenger who can take care of this). Repeat for a range of speeds, separated by say 5 knots. Afterwards sit down with a calculator and work out the glide ratios, plot them and interpolate to find the best.

Tedious though makes for some interesting flying.

In my 182RG, Vbg clean is about 85 knots, Vbg dirty is in the 70-75 range.

Note also that the curve is pretty flat close to Vbg, so there's no point in obsessing to the nearest knot.

[BackPacker]

FS, there are several aircraft types for which taking off with one or two stages of flaps is recommended, or even mandatory, depending on the exact situation.

Especially short-field performance is greatly improved by adding the appropriate stage of flaps. But they may also be used to prevent tailstrikes for instance.

[gasax]

Given that a Maule glides with 'brick like tendancies' I suspect the question is somewhat academic - the difference is 'not far' to 'no distance at all'.

I have a fair bit of time on the inspiration to the Maule the Pa22 and gliding is a long way from its forte. The Maule from the couple I have flown is little better - but it has manual flaps and putting them away takes no effort.

But realistically if you have a power failure on takeoff you will land ahead-ish. The distance will be anything from just over the fence to a mile away. The key to getting away with a power failure is a quick reaction and not pushing it. Once you are over 800 odd foot the options start to open up - but up to 500ft sort out where you can go - and if you have to fly aggressively to get there.

[piperboy84]

Out of interest, what's prompting you to TO with flaps? Unless you've got a particularly short or soft takeoff run, flaps up is generally best, I believe.

2 reasons really, 1. The POH says this is the correct method, and secondly when I was buying the Maule from the Maule factory, Mr Ray Maule took me for a test flight and he used 24 Dgrs. (the first positive stage) and that was on a runway of about 6000ft, so I figured that was probably the best way to fly it.

[Genghis the Engineer]

Firstly, please stop with this "stupid question" crap - it's a question, and as fair (or not!) as anybody else's.


In the climb or landing configuration, with no thrust, the glide performance will be poorer than in the cruise configuration (flaps and gear up in other words). However, also it is transient (as speeds, flap setting, and gear setting) change, so will glide ratio. Further, what are you going to do with the number? Planning any kind of glide is really not feasible at low level after an engine failure: it has to be responsive.

In the real world, after an engine failure on climb out or approach, the only think that you can sensibly do is judge your glide with the "constant aspect" method. If a point is going up in your point of view, you aren't going to make it - putting the nose down or flaps up *might* improve things a bit, or they might not. If the point is going down in your field of view, you're going to fly over it - but if it's a good place to land, then you can use flaps, sideslip, or S-turns (in that order) to lose height.

If a point is staying still in your field of view, then that's where you're going!


It is possible to calculate, or more reliably measure, the glide performance and best glide speed at different flap settings. But really, what's the point? What are you meaningfully going to do with that number? If the engine stops, you're quite busy enough and aren't going to be doing mental arithmetic with semi-useless numbers about glide performance.


Also if you are close to the ground, then the only real speed to be flying is your approach speed because you need to be set up for the best possible landing. Use of a best glide speed is seldom wise, bar increasing speed a little to stretch a glide and clear a hedge.

G

[Pilot DAR]

If there is an approved takeoff flap setting, you should use it. The airplane will fly better, and the landing gear will take less wear and tear.

As for the glide, don't necessarily apply the best glide technique, configuration, and speed to EFATO. in that situation, you work with what you have, and get back on the ground safely. If you are so certain that EFATO is a real risk, a long runway is your best safety feature.

John Farley, in his excellent book, advocates getting it back on the ground, not stretching the glide. Avoid planning to make the most of the glide distance when it quits, leave that for the sailplane pilots, or powerplane pilots flying just a bit too far off shore when it quits.

For leaping out of your little runway in the Maule, pick the suspiciously nearer plot of land, and figure out how to get into that with a big sideslip, getting more flap applied as you need it. Don't stretch the glide to get down, if you mis judge, you have no options. You're better to roll through the far hedge at 20kts 'cause you could not stop, rather than bash through the nearer hedge at 70kts, 'cause you could not make the field as you intended!

[foxmoth]

If there is nothing laid down I would try as a starter looking at the relationship between stall and best glide speed, i.e. if stall clean is 60 and best glide clean 1/3rd more at 80, for a flap down stall speed of 48 you would be looking at at glide of 64. This might not be totally correct and might be worth trying it at differing speeds to find the proper speed, but at least this gives a figure to start with.
Having said that, where you will end up is probably going to be more affected by the wind and as said by others in this thread, get the nose down, look where the aircraft is going and do your best to put it in the best area available.

[FleetFlyer]

I have found through reading lots of POH's and also through aircraft I've flown, that there is a close correlation between best climb and best glide speed for any given configuration.

This means that in my A/C, if I know no different, I'll always pick 55kts with flap and 62kts without flap to achieve best glide should the donkey quit.

I suspect that the correlation is a common one. Perhaps Pilot DAR or Genghis can verify/bust this?

[Flaymy]

It is far from a stupid question, but also one easily answered if you really are curious. Genghis has a good point, in the event you should be concentrating on the picture, but if you are pre-planning for potential failure it might be useful to know how urgent cleaning up after T/O is to avoid any point where you have no options, and what sort of glide you can manage.

Take a buddy up flying (head in the cockpit always have a second pilot on board to look out for you!) and try it. Set up a glide in T/O configuration at published best-glide speed. Note rate-of-descent speed against IAS. Do the same for 5 kts below, and then (at a decent altitude) 10 kts below. For completeness you might try 5 kts above too, although best glide with flap will be slower than without (flap increases parasite drag more than induced drag; increasing parasite drag reduces speed for lowest drag, and so best lift/drag ratio).

Using 100 ft/minute = 1 kt you can then work out the glide ratio at each speed. If you are really interested go back to school days and plot the points on graph paper, speed across and glide ratio up the side and draw a smooth curve through them. Then you will have best glide speed and glide ratio.

Then forget the precision with which you know this. You can probably work this all out to the nearest knot, but if you try to fly that after an engine failure you will crash with your eyes on the ASI.

[BackPacker]

Good overview of all V-speeds, and why Vx and Vy are all related to a bunch of other speeds including Vbg here:

Tutorial: airspeed and the properties of air (http://flysafe.raa.asn.au/groundschool/umodule2.html)

Scroll down to para 2.9.

[englishal]

Vy is best glide (or minimum drag) ;)

[Genghis the Engineer]

Vy is not equal to best glide, but they are very close. In the absense of any other information, using Vy to glide at is unlikely to give you any more problems then you had already.

They are not the same because Vy is also affected by the relationship between engine efficiency and speed.

G

[Genghis the Engineer]

It is far from a stupid question, but also one easily answered if you really are curious. Genghis has a good point, in the event you should be concentrating on the picture, but if you are pre-planning for potential failure it might be useful to know how urgent cleaning up after T/O is to avoid any point where you have no options, and what sort of glide you can manage.

Take a buddy up flying (head in the cockpit always have a second pilot on board to look out for you!) and try it. Set up a glide in T/O configuration at published best-glide speed. Note rate-of-descent speed against IAS. Do the same for 5 kts below, and then (at a decent altitude) 10 kts below. For completeness you might try 5 kts above too, although best glide with flap will be slower than without (flap increases parasite drag more than induced drag; increasing parasite drag reduces speed for lowest drag, and so best lift/drag ratio).

Using 100 ft/minute = 1 kt you can then work out the glide ratio at each speed. If you are really interested go back to school days and plot the points on graph paper, speed across and glide ratio up the side and draw a smooth curve through them. Then you will have best glide speed and glide ratio.

Then forget the precision with which you know this. You can probably work this all out to the nearest knot, but if you try to fly that after an engine failure you will crash with your eyes on the ASI.

The test techniques you describe are referred to as "sawtooths", and are a standard flight test technique. As you've described them however, they won't work all that well.

You need to use a large altitude bracket, and climb/descend through that bracket. So, typically I might start at 1500ft, start the stopwatch through 2000ft, time to 2500ft, then level at 3,000ft, before starting my descent again. So you need around 1500ft to play with, to get data across 500ft. The reason is straightforward - it takes that long to get the conditions properly stabilised.

I would not use the VSI as my primary data source: I would always use the altimeter and stopwatch. I'd also have 1013 set on the altimeter I'm using for the test to simplify standardisation to standard density altitude, since I then only need to adjust for temperature.

For glides, plot two graphs - RoD against speed, which will give you min sink at the minimum (which is of little use to anybody but a glider pilot trying to catch thermals) and glide angle against speed, which will give you Vbg at the maximum. Doing it properly, also reduce IAS to CAS to TAS, DO NOT calculate angles using IAS, or you can be moderately out simply because you were too lazy to do a bit of maths. But do use IAS on the X-axis (we'd use CAS or EAS on most test programmes, but that's a different environment).


I'd also emphasise absolutely the importance of having somebody else doing lookout. Some flight test teams use "I have lookout" / "You have lookout" to formalise whose job it is, because flying accurately enough to get good numbers whilst also managing the aeroplane and looking out is bloody difficult for an experienced Test Pilot on their own, and beyond the abilities of most PPLs. So don't try.

G

[BackPacker]

(which is of little use to anybody but a glider pilot trying to catch thermals)

When you're out of gliding distance of land, and the engine quits, the sea is the same everywhere - wet and cold. If there are no ships nearby which might be able to rescue you, you'd better try to maximize your time in the air (minimum sink) so you can get the mayday out, with a position report as accurately as possible, so that the SAR helicopter is on its way before you splash.

[Genghis the Engineer]

When you're out of gliding distance of land, and the engine quits, the sea is the same everywhere - wet and cold. If there are no ships nearby which might be able to rescue you, you'd better try to maximize your time in the air (minimum sink) so you can get the mayday out, with a position report as accurately as possible, so that the SAR helicopter is on its way before you splash.

No darned way am I making an ocean crossing low enough that I don't have a good several minutes available to make that call and position fix, whatever happens. From 2000ft, I should always have at-least 3 minutes, and most times I'll be higher than that. And I carry an ELT, or at-least a PLB.

Realistically, if I can't get it out in 2 minutes, another 2 is not going to make any real difference.

I take your point of-course - although a couple of minutes less in the sea is probably more useful, and I can also use the VSI to fine tune min-sink, which will be around Vy.

G