Does best glide change depending upon configuration?

Challenged

Challenged

Pattern Altitude
Joined
Apr 4, 2011
Messages
1,901
Location
Louisiana
Display Name
Display name:
Challenged
How much does best glide speed change if you have the landing gear down in a retract, or one notch of flaps, etc... I imagine this information is specific to each airframe, but thought I'd raise the question at least.
 
Last edited:
Best glide speed is also minimum drag speed, or L/D max. With gear extended, best glide speed is slower in order to reduce the parasite drag.

Speculating, since flaps would also increase lift and induced drag, they might not change best glide speed that much. Of course, they will decrease glide range substantially even if the proper airspeed for best glide is not changed substantially.
 
Challenged said:
How much does best glide speed change if you have the landing gear down in a retract, or one notch of flaps, etc... I imagine this information is specific to each airframe,
Click to expand...
It is, so there's no specific answer to your question as asked. But it is always reduced when you lower gear and/or flaps due to the leftward shift in the drag curve.
 
Considerably!

Drop the gear and you are going to reduce your glide, only drop em when you're landing assured (know how long it takes to drop gear too). Flaps, depends, on a caravan some folks have fond that having a little out increases the range a slightly.

If you have a CS prop, try a simulated engine failure and pull the prop all the way back and see the diffrence that makes --> in some engine failure models this may not be possible, but still something you should try.
 
More drastically than change of speed, deploying gear or flaps will drastically reduce your glide RATIO
 
Speed. He's looking for best glide speed, not range, in higher drag configs. I'm curious about that too.
For instance, say your retract gear doesn't come down, you circle around handcranking it down, which may take a few minutes, then all of a sudden the engine quits.
What's best glide speed with gear down? ETC?
 
How do people deal with this issue for the commercial PTS maneuvers?

When I was taught the steep spiral, my CFI told me to lower the gear and make a descending turn at best glide speed. And he told me the number for best glide. I checked, and the best glide was specified in the manual for gear up, not gear down. So the airspeed he specified didn't seem to me to be what I should really use. I'll bet a lot of commercial students encounter a similar dilemma.
 
nosehair said:
Speed. He's looking for best glide speed, not range, in higher drag configs. I'm curious about that too.
For instance, say your retract gear doesn't come down, you circle around handcranking it down, which may take a few minutes, then all of a sudden the engine quits.
What's best glide speed with gear down? ETC?
Click to expand...
No way to know for sure if it's not in your POH unless you do your own flight testing. And in that practical test scenario, the examiner has no better idea than you what that speed is with gear down if that's not covered in the POH.
 
Sounds like I'll need to go and do some flight testing. I'll report back if I feel like I get some solid numbers.
 
Ron Levy said:
It is, so there's no specific answer to your question as asked. But it is always reduced when you lower gear and/or flaps due to the leftward shift in the drag curve.
Click to expand...

Always? The POH for my CTSW lists best glide as 62 knots at 15° flaps.
 
The more streamlined it is the better it flys. Ie: mooney verses archer, etc. anytime drag is introduced glide ratio is shortened. Pretty standard stuff without a lot of hyperbole.
 
NoHeat said:
How do people deal with this issue for the commercial PTS maneuvers?

When I was taught the steep spiral, my CFI told me to lower the gear and make a descending turn at best glide speed. And he told me the number for best glide. I checked, and the best glide was specified in the manual for gear up, not gear down. So the airspeed he specified didn't seem to me to be what I should really use. I'll bet a lot of commercial students encounter a similar dilemma.
Click to expand...


Gear down is for RUNWAY ASSURED.

Engine failure

Pitch for best glide (based on weight, heavier = faster, though range won't change)

Direct nearest landing site

Trouble shoot to a restart or secure engine

Go to the approach end and gently circle to the left

When you see you're going to make it drop gear and bring her in, slip if needed.

Land
 
Jimmy cooper said:
The more streamlined it is the better it flys. Ie: mooney verses archer, etc. anytime drag is introduced glide ratio is shortened. Pretty standard stuff without a lot of hyperbole.
Click to expand...

Is it possible that on some aircraft flaps add more lift than they introduce drag?

I'm just trying to understand why my POH lists 62kt @ 15° as best glide.
 
bullwinkle said:
Is it possible that on some aircraft flaps add more lift than they introduce drag?

I'm just trying to understand why my POH lists 62kt @ 15° as best glide.
Click to expand...

Not ever. Without resorting to a lot of math, any time you hang anything out in the breeze, drag increases.
 
bullwinkle said:
Is it possible that on some aircraft flaps add more lift than they introduce drag?
I'm just trying to understand why my POH lists 62kt @ 15° as best glide.
Click to expand...
That is airfoil and speed dependent. The CTSW (at least the one I flew a few years ago) has a glider airfoil - the flaps are actually reflex in cruise. That lowers drag at cruise speed and AOA. Slow down and you move to a less desirable portion of the drag curve in that configuratiion. Your airfoil at 15° acts similarly to most other airfoils at 0° flaps at lower speeds.
Without doing a bunch of flight testing (as mentioned in an earlier post), the info in the POH is all we have.
 
DGlaeser said:
That is airfoil and speed dependent. The CTSW (at least the one I flew a few years ago) has a glider airfoil - the flaps are actually reflex in cruise. That lowers drag at cruise speed and AOA. Slow down and you move to a less desirable portion of the drag curve in that configuratiion. Your airfoil at 15° acts similarly to most other airfoils at 0° flaps at lower speeds.
Without doing a bunch of flight testing (as mentioned in an earlier post), the info in the POH is all we have.
Click to expand...

Thanks, I found out that the CTLS with the same wing has a best glide in the POH listed as 71kt at 0 flaps, which sounds more realistic. Some other CT owners also say 0 or even -6 flaps will work out better.
 
Shepherd said:
Not ever. Without resorting to a lot of math, any time you hang anything out in the breeze, drag increases.
Click to expand...

Then why do manufacturers recommend increased flap settings for performance takeoffs if they increased drag more than they did lift? Wouldn't flaps, up to a certain point, give you more of a benefit than a drawback?

I was told, in the Arrow, that flaps set to 25 degrees would give more lift than it would drag which was why the POH recommended that setting for both short and soft field takeoffs. It was explained that it was the same logic with flaps being set to 10 degrees in a 172.
 
Cpt_Kirk said:
I was told, in the Arrow, that flaps set to 25 degrees would give more lift than it would drag which was why the POH recommended that setting for both short and soft field takeoffs. It was explained that it was the same logic with flaps being set to 10 degrees in a 172.
Click to expand...

However, in older 172s, the manual says not to use flaps for a short-field takeoff over an obstacle.
 
NoHeat said:
When I was taught the steep spiral, my CFI told me to lower the gear and make a descending turn at best glide speed. And he told me the number for best glide. I checked, and the best glide was specified in the manual for gear up, not gear down. So the airspeed he specified didn't seem to me to be what I should really use.
Click to expand...

I'm not sure I understand his reasoning. Aside from the gear issue that you mention, best-glide speed (like stall speed) would increase 40% in a 60-degree-bank spiral descent. Plus, the usual reason for using best-glide is to maximize your glide range, which is not a consideration during an emergency spiral descent. The goal there is just to get down as rapidly as possible, at the spot you're currently above.

On the other hand, you do want an airspeed between (load-adjusted) Vs and Va. So the default best-glide speed might just serve as a familiar pre-computed airspeed that happens to be in the right ballpark.
 
acrophile said:
best-glide speed (like stall speed) would increase 40% in a 60-degree-bank spiral descent.
Click to expand...
Stall speed does not increase in a 60 degree bank descent. Stall speeds only increase in constant altitude banks. Holding back pressure during a steep turn puts the extra load and consequent increased stall speed, as if you added extra weight.

Bank it over to any bank but don't hold back pressure and stall speed remains the same as level flight. Of course, if you do pull back, the stall speed will increase according to the amount of wing load you put on it.
But the bank itself does not increase the stall speed.
 
nosehair said:
Stall speed does not increase in a 60 degree bank descent. Stall speeds only increase in constant altitude banks. Holding back pressure during a steep turn puts the extra load and consequent increased stall speed, as if you added extra weight.

Bank it over to any bank but don't hold back pressure and stall speed remains the same as level flight. Of course, if you do pull back, the stall speed will increase according to the amount of wing load you put on it.
But the bank itself does not increase the stall speed.
Click to expand...

dmspilot said:
What is the load factor in a 2000fpm stabilized straight descent?
Click to expand...

Nosehair, I think dmspilot is trying to get you towards correcting what you said.

Your statement:

Stall speed does not increase in a 60 degree bank descent. Stall speeds only increase in constant altitude banks.
Click to expand...

... Not true. Stall speeds for a steady-state, vertically unaccelerated descent are the same as they are for level flight. For the stall speed to not change, load factor has to not change, and a load factor of 1 in a 60-degree bank is going to result in only half the lift necessary to keep the plane level or in a steady-state descent.

If you don't believe me, go flying, climb high, trim out to your desired speed (preferably well below Va to aid in recovery from the maneuver!), and bank over to 60 degrees, keeping the plane coordinated, without pulling back. You'll find that you accelerate downwards and you'll quickly be unable to maintain a load factor of 1, thus your stall speed will increase in the bank.
 
flyingcheesehead said:
Nosehair, I think dmspilot is trying to get you towards correcting what you said.

Your statement:



... Not true. Stall speeds for a steady-state, vertically unaccelerated descent are the same as they are for level flight. For the stall speed to not change, load factor has to not change, and a load factor of 1 in a 60-degree bank is going to result in only half the lift necessary to keep the plane level or in a steady-state descent.

If you don't believe me, go flying, climb high, trim out to your desired speed (preferably well below Va to aid in recovery from the maneuver!), and bank over to 60 degrees, keeping the plane coordinated, without pulling back. You'll find that you accelerate downwards and you'll quickly be unable to maintain a load factor of 1, thus your stall speed will increase in the bank.
Click to expand...

I may be confused, but I thought that was his point, it wasn't the bank itself that created the issue, but remaining in level flight while in the bank that provided the issue since that is where you have to increase the load factor.
 
Henning said:
I may be confused, but I thought that was his point, it wasn't the bank itself that created the issue, but remaining in level flight while in the bank that provided the issue since that is where you have to increase the load factor.
Click to expand...

The misconception is that a descent will lower the load factor. It won't. The load factor will be the same in a steady-state descent as it is in level flight.
 
flyingcheesehead said:
The misconception is that a descent will lower the load factor. It won't. The load factor will be the same in a steady-state descent as it is in level flight.
Click to expand...

Right, the load factor can be 1g as long as you're good with the plane dropping out of the sky.
 
You must log in or register to reply here.
Back
Top