Glide AoA vs. Drag

[ECAM_Actions]

Assume you have two aircraft that are identical, but one has higher drag than the other (let's say due to appendages to the fuselage).

What is the effect on best-glide AoA/speed in this situation?

[RetiredF4]

I take a guess:
AOA and speed stay the same, however the ratio NM per 1.000 ft will change, or as shown on the instruments the descent rate will increase.

But it´s just a guess out of expierience from inside an old ugly F4.

franzl

[Captain Sand Dune]

appendages to the fuselage wuold increase zero loft drag only. The effect on the total lift curve would be to move the EAS for best L/D left. Therefore I reckon the EAS for best L/D would decrease, with a corresponding increase in AoA.

[ECAM_Actions]

@RetiredF4: This was my thought too, the reasoning being that (as far as I'm aware) the best L/D is based upon the lifting surfaces best L/D for a given mass, and the extra drag incurred simply means more gravitational potential energy needs to be traded to attain best glide speed, thus a reduced pitch angle (and as you say, reduced glide distance), but otherwise the same AoA and glide speed is required.

@Captain Sand Dune: I considered this, but the effect could be to increase drag as a result of increased alpha, which may not necessarily mitigate the drag induced from the extra appendages despite the lower airspeed, resulting in a net increase in drag and further reduced glide distance as a result of the lower forward speed.

I wonder, because given a set drag of the fuselage, etc.. the only variable that can be optimized is the L/D of the lifting surfaces, whilst reducing speed alone is the only way to reduce induced drag from everything else (zero airspeed being ideal), but would then be sub-optimal for the lifting devices.

:hmm:

[Pilot DAR]

but one has higher drag than the other (let's say due to appendages to the fuselage).

You're thinking something like this?

http://i381.photobucket.com/albums/oo252/PilotDAR/Jims%20DAR%20Testing/CopyofIMG_4778001.jpg

I have been doing extensive flight testing on this Caravan, though not for glide speeds, just characteristics, among all the other requirements.

Among many things, I have learned during repeated climb tests at different speeds, that the best rate climb speed has been reduced 11 knots by the drag of the mod. The 20 flap best rate climb flown at the flight manual speed, no longer met the minimum climb requirements for the aircraft's certification basis. Flying climbs at lesser flap settings did meet the requirements, though this was operationally undesirable.

Flying the best rate climb at 80 knots did get me the climb performance required to show compliance to the certification basis. This slower climb speed did, however, introduce other challenges, one of which was demonstrating a land back from an engine failure at 50 feet after takeoff. The slower speed, coupled with the added drag, resulted in a quick deceleration, and a return to the runway that brought helicopter autorotations to mind! It was also required to fly an approach and landing at Vref -5 kt, which meant an approach flown at 70 knots, with the stall warning sounding the whole way down final, and very little let to flare with.

Interestingly, when the survey bird is winched down about 10 feet, the drag is reduced, resulting in a 3 knot increase in speed, all other things being equal. The operational deployment is about 300 feet though, so the drag of the rope has reduced performance again by that point.

Though I did not do direct AoA and glide drag comparisons between the pre mod and post mod aircraft, I'm sure they are not the same....

[QJB]

Agree with Captain Sand Dune,

Assuming an increase in parasite drag due to the "appendages" a reduction in EAS would be required to reach the new best glide speed (L/D ratio). Therefore the new best glide would happen at a lower IAS (increased angle of attack), and since the overall drag of the aircraft has increased it's glide performance would be reduced.

J

[ECAM_Actions]

@Pilot DAR: Thanks for that! Interesting! Is it a thrust vs. drag problem then, hence why the slower airspeed (compared to the configuration without the sensor package) provided a higher rate of climb?

Interesting too that the glide speed would be REDUCED with a higher drag config.

Is there any calculation that can be done or is it simply a case of test flying and seeing what happens?

[Pilot DAR]

ECAM,

The difference in climb performance/best rate speeds was determined by actual test results. I suspect that as I know we increased the drag of the aircraft, and the drag increases as a square of the speed, the faster I fly it, the more more drag the aircraft has, and the performance is thus reduced accordingly. A climb speed of less than 80 KIAS did not result in any greater improvement, and results in too little a margin to the assigned minimum maneuvering speed for the modified aircraft.

The calculation of the drag is possible, but exceeds my skills. My flight test analyst does this for me - I just fly!