L/D Max in a Twin

Acestick

Who's on First??
Joined
Nov 28, 2001
Posts
50
Total Time
700
Here's a doozy...

What is best glide speed in a twin engine airplane? I know that many of the Piper POH's don't include a best glide speed, as they assume that one of the engines will always be operating at any given time. But supposing it did happen, should the aircraft be flown at Blueline speed? I know this speed allows for the least drag and best ROC when the other engine is feathered, and it also provides the least rate of descent when an engine fails above the single engine absolute ceiling. So does it also mean that it will allow for least rate of descent with both engines feathered?

I've been told to aim for blueline, and some have even said that they would rather not think about that scenario. But if anyone knows give me a holler...
 

A Squared

Well-known member
Joined
Nov 26, 2001
Posts
3,006
Total Time
11000
Seems to me that the Cessna twins had a best glide airspeed in the POH, though I don't seem to have one around anymore.

Don't confuse Minimum sink with Maximum Glide.

Maximum glide gives you the most horizontal distance travelled for each foot of altitude lost. This occurs at L/D max, or minimum drag airspeed.

Minimum sink glide speed gives you the lowest rate of descent, this will be slower than minimum drag airspeed.

You wrote:
>>>I know this speed (blueline)allows for the least drag and best ROC when the other engine is feathered.

Not exactly, best rate of climb will be faster than the minimum drag airspeed. Best single engine rate of climb airspeed will be lower than normal betst rate of climb airspeed.

Having said that, if you don't have a published best glide speed, blueline will get you in the ballpark, you'll be a little fast, but that's ok, it will give you less time to contemplate your fate <g>
 

avbug

Well-known member
Joined
Dec 14, 2001
Posts
7,602
Total Time
n/a
As a general rule, Vy will give you best glide, and Vx will give you minimum sink. This isn't accurate, but close enough in a pinch. Recognizing that both speeds are dependent on power, and come together at the absolute service ceiling, it's not a perfect soloution, but it's close enough.

If applying this to a twin, use Vy and Vx, not Vyse and Vxse.
 

A Squared

Well-known member
Joined
Nov 26, 2001
Posts
3,006
Total Time
11000
Avbug,

>>>>If applying this to a twin, use Vy and Vx, not Vyse and Vxse.

Not sure I agree with you on this. Both Vy and Vyse will be greater than V-L/dmax, and Vy will be greater than Vyse. Therefore, Vyse will be closer to V-L/D max. It seems to me that Vyse will be closer to the best glide speed, unless I'm missing something here.

regards
 

avbug

Well-known member
Joined
Dec 14, 2001
Posts
7,602
Total Time
n/a
I'll have to get back on that, but you may be right.
 

hyper

We got "change" alright.
Joined
Nov 26, 2001
Posts
469
Total Time
6000
That may be a starting point for a AFM lacking sufficient data for the scenario, but there's probably more support for even faster speeds. Consider these two examples; BE-76 with Vy and Vyse @ 85kts yet the best glide is published 95kts and the BE-58 has Vy 104 and Vyse 100 while best glide is 120kts. Seems like there is enough variance with just those two examples that you could even say adding 10-20kts to your Vy would give you best glide :confused:
 

A Squared

Well-known member
Joined
Nov 26, 2001
Posts
3,006
Total Time
11000
Hmmmm...scratching my head on this one. Your data
certainly seems to back up Avbug's position. maybe I need to dig thru Kershner and Aerodynamics for Naval Aviators again.
 

Eric

See you in the Wasatch!
Joined
Jan 6, 2002
Posts
205
Total Time
??????
The best of a bad situation.....

What is everyone's opinion on just pitching for the level flight attitude? I have been told that this will give you L/Dmax.
 
Top