Best Glide Speed

[av8trflying]

Peoples

Just wondering what everyones opinions are on best glide speed for twins.

In some POH that i have looked at there is no glide speed stated.

Some people are of the opinion that a glide speed is not necessary in a twin.

thoughts?

[ZEEBEE]

If you really need it, give or take, it should approximate Vx ie. best Angle of Climb speed.

Close enough if you've got both of them feathered and the gear up.

If not, look for a padock behind you :eek:

[The Green Goblin]

Close enough if you've got both of them feathered and the gear up.

If not, look for a padock behind you

I'd think carefully about that :}

A 150 has a glide ratio of about 7/1 a 747 around 17/1 something like a PA31 will even see over 12/1 with the engines not feathered!

Things are not what they always seem :p

[Exaviator]

Depends on the twin - what you are really looking for is Vmin Drag for your particular aircraft.:)

[Oktas8]

Have flown a variety of two, four and six seat aircraft. Every single engine aircraft had a best glide speed of somewhere between Vy-10 and Vy itself.

Twins - Vx is very slow unless configured for landing (in which case Vx is about right). If you're going for best range clean and with propellers feathered, try somewhere between Vyse and Vyse+10.

If you wish to calculate your own best glide speed, glide with zero thrust on both engines and experiment with the speed until you get minimum sink rate. This is close to the minimum power speed. Best glide speed (minimum drag) will be about 30% faster than min power speed for most light piston engine aircraft.

All this is from memory without books in front of me. Happy to be corrected if wrong!

Cheers,
O8

[Mainframe]

Av8trflying

Most aircraft can be ball parked at best glide simply by maintaining the cruise attitude existing prior to power loss.

Having done something to ball park best glide, you may then have time to look in the AFM or QRH to find the speed appropriate to the operating weight.

Yes, the speed varies with weight.

Power plus attitude plus configuarion equals performance.

Lose power, some loss of tail down force, change in thrust drag couple etc.

Result, the nose drops.
Simply get it back where it was when everything was working.

The correct "ATTITUDE" can be either the "n fingers" attitude relative to the dash if VFR, or the cruise pitch attitude (maybe 1 or 2 degrees nose up) if IFR.

Double engine failures were normally not considered possible until the Whyalla Air Chieftain tragedy.

Fuel exhaustion can yield a double engine failure.

Next time you fly, note the cruise attitude (deck angle, Attitude indicator or dashboard to horizon reference).

Twin or single, maintaining that attitude afterwards will buy you time while you get sorted out.

[ZEEBEE]

I'd think carefully about that

A 150 has a glide ratio of about 7/1 a 747 around 17/1 something like a PA31 will even see over 12/1 with the engines not feathered!

Things are not what they always seem

Goblin It was in jest, but Vx is still supposed to be the best L/D point on most twins unless they're massively overpowered.

I guess wind comes into it as well ie if one is forced to glide into a strong wind (to say, get to land rather than water) then a faster speed might be in order to minimise the time in the wind stream.

[Cap'n Arrr]

I'm with Mainframe on this one. I don't think I've ever flown a plane where holding the standard cruise attitude didn't get you close enough to best glide.

Someone once told me it's something to do with aircraft being designed to be efficient in the cruise, but anyway it's worked everytime I tried it.

Cruise attitude + Engine/s out = Approx Best Glide:ok:

[Lightning Mate]

Mainframe and Oktas8 have given you the best advice on this thread.

VX is the speed for max. gradient climb and for a piston/propeller aeroplane it is minimum power speed (VMP). Do not use this for max. gliding distance - it is too slow.

Best glide range (still air) is achieved at best lift/drag ratio. In level flight this equates to minimum drag speed (VMD). In a glide descent, best L/D ratio speed is not the same as VMD because lift will be less than weight.
However, at shallow glide angles the two are very close.

In still air, best range for a piston aeroplane is VMD, so this will give you a very close approximation to your best glide speed.

Stay safe,

LM

[Joker 10]

Blue Line works well

[tea & bikkies]

Yes would have to agree, best L/D ratio will give best glide speed. This typically occurs at 4 deg AoA. The wing is bolted on to the fuselage at 4 deg, so the deck angle is close to level in the cruise.
Yes, best glide speed does vary with weight. However if the level attitude (maybe 1deg nose down) is maintained in a glide the speed takes care of itself regardless of weight.
Picture an aircraft gliding...they don't glide nose down or nose up.
As stated before cruise attitude = best glide speed:)

[SNS3Guppy]

In most light airplanes, the sea level best rate of climb speed (Vy) will closely approximate your best glide speed (farthest distance traveled compared to altitude lost). At the same time, a second important speed (seldom taught) is minimum sink speed...which provides you the longest time aloft in a glide. In most light airplanes, minimum sink speed will closely approximate the sea level best angle of climb speed (Vx).

[Brian Abraham]

Figures you can take to the bank, ie from the flight manual - Cessna 404. Vy (blue line) 109KIAS, Vx 105, best glide (props feathered) ranges between 120 at 8400 pounds (MAUW) and 103 at 6300 pounds. 10,000 feet will give you no wind range of 22 nautical miles.

[tea & bikkies]

Yes, exactly Brian, Glide distance (range) will not vary with weight, the speed will, if you are heavy you just cover the range quicker:E

[MakeItHappenCaptain]

Baron 58 Glide speed specified at 120 KIAS
Best Angle 86 and 96 OEI

???????

[Tankengine]

Tea and bikkies :
when was the last time you glid level?!!:}

[ForkTailedDrKiller]

Hmmm, for what its worth, the Best Glide Speed at MAW (105 kts) in the V-tail is much closer to the Best Rate of Climb speed (96 kts) - NOT the best Angle of Climb speed (78 kts).

Dr :8

[tea & bikkies]

Hello Tankengine,

I must admit it has been many years since I glided level:E as funny as that sounds.

In sailplanes, I think they call it zero sink. They also climb:)

Whilst its many years now since I flew gliders, back in the early seventies I can remember gliding in front of a roll cloud in an old Pa28 Cherokee D. Was out with a student and thought it would be fun to turn her to fly along in front of this roll cloud (no associated storm, just a roll cloud, base 3000 top 5000ft).

There was 1000fpm lift with the engine at idle, the cloud stretched as far as the eye could see, orientated east/west moving up the NSW coast. With plenty of lift and options for landing if required, at 5000ft I cut the mixture, stalled (to stop the prop) and glided (surfed this wave), down to 3000, back up to 5000 many times for 40mins, must have looked funny from the ground :ok:.

Was great fun and experience. When we had enough, just started the engine and flew home.

On arrival back in the clubhouse, the CFI accused me of landing somewhere and having my way with the female trainee:ooh:, as the oil pressure VDO did not match up with our departure time...NO he didn't believe my story. George if you are reading this...it did happen:cool:

So in reply, Tankengine that was the last time I glided (level).

T&B

[Tankengine]

Right over the top!:rolleyes:

In gliding flight you are ALWAYS going down [relative to the air]
It is just that some air goes up and down, gliders use this!:ok:

Best speed for a light twin engine out in my opinion would be the speed for maximum glide distance [best L/D] - probably about Vy [blue line]
This would be faster [maybe 5-25kts] than min sink speed [min drag] which would keep you airborne longer but not glide as far! :ok:[prob about best angle of climb speed]

Try googling "Bob Hoover" to see what is possible!:}

PS: Great flight T & Bs!!:ok:
Why don't you try gliding again? @ 50:1 it is great fun!!:}

[hoss]

it's probably 65 knots if your flying an islander;).

[Joker 10]

In an Islander I thought it was measured in Decibels

[tea & bikkies]

Yes have been a fan of the legend of Bob Hoover for years.. what a legend:eek:

At 50:1 you have me drooling, would love to get back into it. All my gliding lately has been with my 4.2 meter RC Discus and the odd matrix item in the sim, double engine failure, which comes around once every 3 years:sad:.

What is gliding worth per hour (including launch) these days..and where around syd, camden maybe?? What types do they have?

Rergards

T&B

[tea & bikkies]

Yep 65kts for an Islander will cover everything:)

[tea & bikkies]

Including taxi...lol

[Hornet306]

Hi T&B - see PMs for information on gliding in Sydney area.

[Cloud Cutter]

I agree with Vy-10 to Vy (To clarify, thats the all-engine Vy, not VySE).

Someone suggested min drag speed - as far as i'm aware, that will give you best glide endurance, but if you're after best glide range, you'd want to go for Best L/D speed, which is slightly faster.

In any case, the actual speed will vary depending on weight, wind component, and drag configuration.

[MakeItHappenCaptain]

Any old hands have the method used to determine all engine out glide speeds, epecially with reference to not so detailed flight manuals such as Pipers? Beech and Cessna manuals are usually pretty good.

Would have thought somewhere around Vx (or Vy), but a Baron for example lists 96 KIAS Vx and 104 KIAS Vy against 120 KIAS Best Glide. Big difference.:confused:

Chasing informed reasoning, not minimum sink guesses or L/D answers because the Baron example clearly is not following the logic.

[djpil]

Basic pilot theory doesn't work very well in real life even for simple airplanes. Try the FAR 23 Flight Test Guide (free online - its an FAA AC) or for something a bit more user friendly:
Propeller Aircraft Performance and The Bootstrap Approach: Performance Basics (http://www.allstar.fiu.edu/AERO/AirPer-BA.htm)

[MakeItHappenCaptain]

Ventus,

So where am I going to find the min sink speed in a flight manual (let's just go on the wild assumption I'm not a complete fcuking idiot who is going to shut down both engines in flight to collect my own figures)?

Min sink is an even less published figure and has minimal relevance to the glide range required in a both engine out situation.

I'm after a reasoning for application to known speeds.

Aside, can you reference the 1.32 figure? I've heard 1.1 quoted before but never been able to source the figure.

[john_tullamarine]

who is going to shut down both engines in flight to collect my own figures

.. which is how one usually does it .. preferably with unfeathering accumulators .. and over a nice long runway suitable for the (potentially expected) deadstick recovery.

First time is a little distracting (for all the usual reasons) but, at the end of the day, it's a risk managed exercise.

.. a much quieter way to fly in the typical GA twin .. until one gets to the higher speed end and the wind noise takes over.

can you reference the 1.32 figure?

Stock textbook stuff eg Peter Swatton (http://books.google.com.au/books?id=VPEgZ49tAFYC&pg=PT191&lpg=PT191&dq=1.32+Vimd&source=bl&ots=mSX1oM_Fdz&sig=zqpKpT4zlF9sy7dAaV5OsBenOWs&hl=en&sa=X&ei=6eHnT5fWBtGuiQeDw4BZ&ved=0CHgQ6AEwBw#v=onepage&q=1.32%20Vimd&f=false) and lots of others.

[MakeItHappenCaptain]

Do thank all who have responded and as clarification, chasing theory that can be passed on to all and sundry.

I must stress I am in no way advocating (as I am sure John is not either) every person go and do a double engine shutdown to ascertain this figure on the remote change they may get a double engine failure, but sure the technique could be applied in anger with enough height available to get the lowest VSI reading and bumping it by 30%.

Sure we've all heard of students who we would be horrified (but not surprised) to find had actually attempted this.:rolleyes:

ps, That text looks pretty good, JT, thanks.