Engine failure best glide speed descent rate

Any DA40 related topics

Moderators: Rick, Lance Murray

User avatar
Rick
5 Diamonds Member
5 Diamonds Member
Posts: 1575
Joined: Tue Jul 06, 2010 10:09 pm
First Name: Rick
Aircraft Type: DA40
Aircraft Registration: NONE
Airports: KROA
Has thanked: 107 times
Been thanked: 297 times

Re: Engine failure best glide speed descent rate

Post by Rick »

It seems like the best configuration will be the one with the lowest rate of descent, won't it? The glide distance will always be affected by the wind relative to the direction of travel. But your rate of descent at best glide should be constant and independent of wind direction, so once you know that, you can look at ground speed and altitude AGL and know how far you will go (in that direction) (assuming winds don't change on the way down).
Roanoke, VA (KROA)
User avatar
astaib
5 Diamonds Member
5 Diamonds Member
Posts: 616
Joined: Mon Feb 26, 2018 9:00 pm
First Name: Arnaud
Aircraft Type: DA40
Aircraft Registration: FGNJX
Airports: LFPX
Has thanked: 244 times
Been thanked: 51 times

Re: Engine failure best glide speed descent rate

Post by astaib »

Rick wrote: Thu Oct 01, 2020 5:10 pm It seems like the best configuration will be the one with the lowest rate of descent, won't it? The glide distance will always be affected by the wind relative to the direction of travel. But your rate of descent at best glide should be constant and independent of wind direction, so once you know that, you can look at ground speed and altitude AGL and know how far you will go (in that direction) (assuming winds don't change on the way down).
My idea was to calculate the 8.8:1 or 10:1 value of my plane.
Arnaud
DA40 Star 180 / 40.026 / 2001
Wingtip, landing and taxing LED (Whelen)
Skitube
GNS430 NON-WAAS
Steam gauges
Non certified ADS-b
User avatar
Soareyes
4 Diamonds Member
4 Diamonds Member
Posts: 306
Joined: Fri Feb 28, 2020 12:47 pm
First Name: Dan
Aircraft Type: DA42-VI
Aircraft Registration: N518R
Airports: KINF
Has thanked: 231 times
Been thanked: 191 times

Re: Engine failure best glide speed descent rate

Post by Soareyes »

"It seems like the best configuration will be the one with the lowest rate of descent, won't it?"


Depends on whether you want maximum range or maximum time. Minimum sink rate and best glide ratio are not the same speed.

As usual, Bold Method does a good job on the subject. They say, "Minimum sink is always slower than best glide, because it's the point on the power required curve where the least amount of power is required. Keep in mind, though, you're going quite a bit slower than your best glide speed, and that can significantly impact your glide range." https://www.boldmethod.com/learn-to-fly ... -to-do-it/

On a graph of L/D vs. speed, a little fast is better than a little slow. The L/D drops off more steeply on the slow side than on the fast side. That means if you can't hold speed perfectly, 5 kts faster will penalize your L/D less than 5 kts slower.
L-D.JPG
Speed is reserve energy. If it looks like you are going to land short of your chosen field you can convert some speed to distance. If you are going to overshoot you can drop flaps and/or side slip.

Minimum sink is pretty slow. Trying to hold a speed just above stalling while troubleshooting and maneuvering to an off field landing runs a risk of getting distracted and stalling.

Best L/D in an avgas plane is with throttle forward and prop lever back. Not sure if the position of the power lever would make any difference in a diesel. Anything in the POH about that?
Current: DA42-V1

Previous: Hang gliders, Paraglider, DA40(x3), Cessna 150 Aerobat, SR22
User avatar
Boatguy
5 Diamonds Member
5 Diamonds Member
Posts: 1828
Joined: Mon Jan 22, 2018 4:48 am
First Name: Russ
Aircraft Type: DA62
Aircraft Registration: N962M
Airports: KSTS
Has thanked: 1329 times
Been thanked: 1163 times

Re: Engine failure best glide speed descent rate

Post by Boatguy »

Soareyes wrote: Thu Oct 01, 2020 8:02 pm Best L/D in an avgas plane is with throttle forward and prop lever back. Not sure if the position of the power lever would make any difference in a diesel. Anything in the POH about that?
The Power/RPM curve in the AFM suggests that having the power lever set at 20% would be the most "feathered" position. This is not marked on the console and obviously would have to be established with the engine running, then marked by the owner.
User avatar
Rich
5 Diamonds Member
5 Diamonds Member
Posts: 4592
Joined: Mon Aug 30, 2010 12:40 pm
First Name: Rich
Aircraft Type: DA40
Aircraft Registration: N40XE
Airports: S39 Prineville OR
Has thanked: 145 times
Been thanked: 1180 times

Re: Engine failure best glide speed descent rate

Post by Rich »

I'm not about to try and second-guess the AFM on anything about based on tests with the engine at idle. In that state, you're still putting a certain non-zero amount of energy into the system. For minimum sink, though, I consider it reasonably valid to seek the IAS at which the minimum power is required for maintaining altitude (at a specific weight, of course).

For calculating the geometry of sink rate at best glide, consider this; It's really a matter of ground speed, not indicated airspeed. What does that mean? Even in the simple case of no wind, ground speed is true airspeed derived from calibrated (not indicated) airspeed adjusted for density altitude. According to the airspeed calibration graph in the AFM, for a DA40-180 with flaps up, 70 KIAS = 74 KCAS. At 4000 ft. DA, that works out to 79 KTAS. At 6000 ft DA it's 81 KCAS. The angle is unaffected, but your vertical speed will be higher than if you used 70 as the speed and even higher as DA increases.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
User avatar
perossichi
3 Diamonds Member
3 Diamonds Member
Posts: 188
Joined: Tue Sep 18, 2018 4:05 am
First Name: Peter
Aircraft Type: DA40
Aircraft Registration: N925RH
Airports: KVNY
Has thanked: 31 times
Been thanked: 75 times

Re: Engine failure best glide speed descent rate

Post by perossichi »

On landing in Lake Michigan

The thought of making a water landing in a fixed gear airplane scares me. The thought of sinking inverted inside the DA40 canopy and fumbling for the red rear door hinge release handle and climbing into the back seat to get out sounds very challenging as well.

I’m often fly fishing for Steelhead in Michigan in a river just east of the narrow point in the lake and I’ve told myself that I’d fly around the bottom of the lake rather than at the “waist” if I ever try flying from LA to Michigan.

Even in summer Lake Michigan is cold away from shore. Very cold
Sold 2002. Powerflo, Hartzell composite two blade, 530W/430, 345 transponder.
User avatar
Rich
5 Diamonds Member
5 Diamonds Member
Posts: 4592
Joined: Mon Aug 30, 2010 12:40 pm
First Name: Rich
Aircraft Type: DA40
Aircraft Registration: N40XE
Airports: S39 Prineville OR
Has thanked: 145 times
Been thanked: 1180 times

Re: Engine failure best glide speed descent rate

Post by Rich »

Today I confirmed what I've seen before. In my plane at idle power and 60 KIAS the RPM is ~1350. Whether the prop control is full forward or back makes no difference in this RPM nor does it produce any perceived change in the glide path. Remember moving the prop control does not directly affect the propeller pitch. It merely changes the RPM set point. In this case the above RPM is below the minimum RPM limit hence it doesn't change. I daresay with a non-firing but freely rotating engine I wouldn't expect the situation to be any different.

I have the MT with a Woodward governor. With a different setup YMMV, but it does illustrate that you may or may not be able to rely on getting any benefit in glide angle with prop adjustment in this situation.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
Post Reply