Originally Posted by
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.
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