Increasing aircraft range

[MachFly]

Just for the record; DA40's Vy & Vbg are exactly the same. I find that a little strange but the glide is still not greater than Vy.

[Stoney]

Just for the record; DA40's Vy & Vbg are exactly the same. I find that a little strange but the glide is still not greater than Vy.

I'm thinking wingload and aspect ratio is going to be the factor here.  For one, wingloading on a P-47 is nearly 3 times that of a Cessna, and the aspect ratio is lower.  Induced drag on a heavier, lower aspect ratio plane is going to be much higher in the glide at slower speeds.  That would cause the drag curve to look very different than on a light, high aspect ratio aircraft.

[dtango]

Since you brought up the 172, it's Vy is 74kts (72kts at 10K) and best glide is 68kts. Reference C-172SP NAV III POH.  

There are two airspeeds that can be interpreted as best glide speed: 1) speed for maximum glide time (minimum drag) resulting in the longest time aloft, 2) speed for maximum glide distance (maximum L/D) resulting in longest glide distance.  They are different things.
  
2ndly the chart I posted is supposedly from the 172 POH and can be found on FIU (FIU 172 Propller Aircraft Performance).  They actually post two charts, and you'll see that Vbg is depicted at different speeds- 65 kts, & ~81 kts.  Note that on the chart I posted there is a Vmd indicated.  Vmd here represents maximum glide time which is roughly 65 kts.  They also quote from the POH that speed for minimum drag (by definition speed for max glide time) is 65 kts.

The question is which of the best glide speeds does the 172 NAV III POH refer to: max glide time or max glide distance?  From the FIU data it would appear to me that it's probably the glide speed for max glide time.

[MachFly]

There are two airspeeds that can be interpreted as best glide speed: 1) speed for maximum glide time (minimum drag) resulting in the longest time aloft, 2) speed for maximum glide distance (maximum L/D) resulting in longest glide distance.  They are different things.
  
2ndly the chart I posted is supposedly from the 172 POH and can be found on FIU (FIU 172 Propller Aircraft Performance).  They actually post two charts, and you'll see that Vbg is depicted at different speeds- 65 kts, & ~81 kts.  Note that on the chart I posted there is a Vmd indicated.  Vmd here represents maximum glide time which is roughly 65 kts.  They also quote from the POH that speed for minimum drag (by definition speed for max glide time) is 65 kts.

The question is which of the best glide speeds does the 172 NAV III POH refer to: max glide time or max glide distance?  From the FIU data it would appear to me that it's probably the glide speed for max glide time.

By "best glide" I meant Vbg.
I actually can't think of a single reason why you would want to stay in the air for longer if it would decrease your glide distance.

I don't have access to the that POH at the moment, if you want I can upload an older copy of the 172S POH from 2003 in PDF. I don't have time to read it right now.

[colmbo]

By "best glide" I meant Vbg.
I actually can't think of a single reason why you would want to stay in the air for longer if it would decrease your glide distance.

Ditching in the ocean...going into a forest....anywhere that the landing is going to be ugly.  More time in the air is more time to work the problem, more time to call for help, more time period.

[colmbo]

The question is which of the best glide speeds does the 172 NAV III POH refer to: max glide time or max glide distance?  From the FIU data it would appear to me that it's probably the glide speed for max glide time.

I've always understood that the glide speed listed in a POH is for max distance, but it may be a compromise of the two.

[MachFly]

Ditching in the ocean...going into a forest....anywhere that the landing is going to be ugly.  More time in the air is more time to work the problem, more time to call for help, more time period.

True

[icepac]

Machfly...I tried with prop windmilling and stopped and it glided further with prop stopped.

Of course, some planes have to go unbelievably slow to get the prop to stop.

Doing this comparison on a Typhoon will be interesting since I think I remember gliding considerably less than a sector from 32,000 feet.

[MachFly]

Machfly...I tried with prop windmilling and stopped and it glided further with prop stopped.

Of course, some planes have to go unbelievably slow to get the prop to stop.

Depending on your altitude it can be worth it. If you know that you will be able to quickly recover from the stall and your above 5K than I say defiantly do it, if not it's questionable.

Doing this comparison on a Typhoon will be interesting since I think I remember gliding considerably less than a sector from 32,000 feet.

That's impossible. If that would be true the Typhoon's glide ratio would be worse than a helicopter's with it's routers blown off, you must be mixing up the numbered or you were gliding at a very bad speed (stalling though the whole glide or 400+mph). If you fly the tiffy a lot perhaps it would be a good idea for you to test for the best glide speed offline or just get the numbered from the POH if you have one (need to make sure that the POH is for the exact same model).

[Krusty]

Anything at 32k will get you about 3-4 sectors at least. I once had to glide from 32k or so in B-17s and made it 5 sectors with engines mostly idled (might have given me an extra 1.5 sectors over eng off)

[CAP1]

I had quite forgotten about that on the clipboard in flight - So bothering about it did not come to mind.

Ink,
It is quite possible that when on a 1/4 tank and trying to scrape home I didn't notice. Hence the question.

I will give it a try and measure the effects. Thanks.

Edit: Did a low level run in a Spit 16 Got 2 1/2 sectors on 0 Boost around 1200RPM at 160 MPH 1.6k Alt
                                                Got 1 3/4 sectors on Full Boost and Max RPM at around 315 MPH 1.6k Alt
                                  Hi level     Got 3 + sectors on 0 Boost around 1600RPM at 160MPH (indicated) and 20K Alt
So yes, makes a significant difference. I ended flights on 7 gallons remaining after a 1/4 tank start (drop tank to get to 20k).

 so you really DO save fuel by flying crazy high......

[MachFly]

so you really DO save fuel by flying crazy high......

"High" is a point of view. For example the Spit14 will go a lot further if cursing at 12K than at 10K. But eventually there is a point of diminishing returns, so you just have to know your plane and what is the optimum altitude for you to fly at.

[Krusty]

Only because the engine is running at almost idle speeds. You'll note the MAP drops and RPM sometimes as well based on how high you are?

Try those same settings at 1000 feet and you might see similar savings.

The real issue is the thinner air means greater TAS and less resistance, but IMO burning off all that extra avgas to GET to the thinner air is more of a waste than you save once you get there.

[Blagard]

The real issue is the thinner air means greater TAS and less resistance, but IMO burning off all that extra avgas to GET to the thinner air is more of a waste than you save once you get there.

I agree with this. Climbing to alt burns a lot of fuel so doing it just to make a high altitude fuel saving will rarely pay off. However, it is worthwhile to start your climb to the destination alt straight away, then you will benefit from lower consumption cruising to get to your target.

Edit: The optimum climb rate is something else again! - Perhaps already covered in the long exchanges earlier in the thread. Is the default autospeed(climb) set for best rate of climb? I know it is not for steepest climb. For the best range for fuel point of view, I am thinking a lower rate of climb/ higher air speed may be necessary

[MachFly]

If only HTC would let us have the winds, we would be able to use them to increase the aircraft range (just got to chose the right altitude).