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-   -   Slat drag on the DC-9 / MD-XX series (https://www.pprune.org/tech-log/497734-slat-drag-dc-9-md-xx-series.html)

hikoushi 11th Oct 2012 02:17

Slat drag on the DC-9 / MD-XX series

A quick question someone with a background in aerodynamics and experience on DC-9 airframes may be able to help with. In the MadDog, I fly regularly with a gentleman who insists that slats do not create any drag; he also insists that his copilots extend them any time we are below 235 knots "for safety reasons". Even if we are 20 miles out on an unrestricted visual approach. Even if the FMS-targeted CLEAN maneuvering speed is below 220 knots; even if the aircraft weighs 90,000 pounds on approach. Even if the "foot" is at 180. Even if we are 30 miles out and ATC asks us to slow to 230 on descent, at 90,000 pounds with a clean maneuvering speed of 218 knots.

Now, I'm not one to criticize someone for being extra conservative, but as this personal technique was not briefed at any point prior in the flight it resulted in a well-planned "idle to 1000 ft" visual approach getting completely screwed up and was a bit irritating. Even knowing the score on later legs, it remains a bit annoying. As the slats in the DC-9 series obviously create quite a bit of drag at 220-240 knots, we got way low way early and burned a TON of extra fuel. He did not accept the explanation that the slat drag botched the approach, which was planned to be in a clean configuration for as long as practical (per the typical approach profile that most folks here use).

Have of course come to a gentleman's agreement with him that if I can disprove him, he owes me drinks on our next trip. If he can disprove me I owe him. Have demonstrated the contrary position thusly: Set up a clean descent at flight idle at 220 knots; noted the descent rate (1200-1300 FPM). Had him extend the slats. Let the aircraft stabilize at the SAME airspeed. Noted increased descent rate (1700-2000 FPM).

No dice. Tried again on climbout my next leg. Left the slats extended for climbout and selected 230 knots climb speed. Note climb rate appx. 2000 FPM (forget the weight on that leg). Had him retract the slats. Stabilized in climb, CLEAN, still at climb lim and 230 knots, noted improved climb rate of 2700 FPM. Still no dice, insisted that there must be some other factor influencing the climb rate other than the slats. He plans to demonstrate a counterargumentative technique on a later flight, which I await in curiosity (weather intervened and he was unable to find conducive conditions to his demonstration on our last day). However, I believe that I have convincingly demonstrated that he is completely wrong. And I want my beer.

Somebody help me out here. Charts? Diagrams? Technical explanations? For any model DC-9, MD-XX, 717, jeez ANYTHING that might get through to this gentleman? Slats, as you well know, primarily serve the purpose of keeping boundary layer flow attatched at high angles of attack / low airspeed. Their primary function at HIGH airspeed is... nothing. They just stick out into the airstream and create... DRAG. They work GREAT for slowing down in the MD. Any additional lift they do create at high speed is of course balanced by induced... DRAG. How do you explain this to a brick wall? There are beers and Starbucks riding on this; it's important. Help a brother out. Thank you.

stilton 11th Oct 2012 07:36

Fascinating, I have flown with people as stubborn as this. Once they believe in an idea you can't confuse them with mere facts.

With your demonstration of climb and descent rates clean and with slats extended you would think that would convince him but I understand, some people can't be told or shown.

My own pet theory with this chap is his incorrectly perceiving the characteristic MD, almost vibration free and extremely smooth in flight characteristics with slats only extended as a 'no drag added' configuration.

I was very surprised, on transitioning to the MD80 from the B727 how smooth and vibration free the effect of extending slats only was.

But it doesn't mean there's no additional drag, and you conclusively proved this :E

VC9 11th Oct 2012 07:41

You are dealing with an F___wit here. Don't bother.

MD80rookie 11th Oct 2012 08:22

Aaaaahh these funny, inspiring colleagues...

Here we go: if your Vp is reduced (hence your margin to stall is increased) by extension of slats, obviously you have created lift. If you create lift, you generate drag...

Next time, descend to Initial Approach altitude. Level off at this altitude at lets say 230 IAS and make a note of the EPR when the engines are stabilized. Extend the slats and note how the EPR increases by 0.15 - 0.20 depending of weight.

Capt Claret 11th Oct 2012 09:10

Somebody help me out here.
Not possible, from what you've described. He sounds like he'd argue that water's not wet, and the earth's flat, if you gave him a chance.

Personally I only take slats (B717) passing about 3000' AAL, approaching about 210 kias, unless I've mucked up the approach, or ATC request reduced speed and I want to slow down fairly quickly.

MD80rookie 11th Oct 2012 09:22

Awaiting your results!

With slats only, center of pressure moves fwd on the MD80. Level flight, 210-230 IAS you will end up with about 5 ANU compared to only 2-2,5 ANU for a clean aircraft.

hikoushi 11th Oct 2012 09:36

MD80Rookie, intriguingly enough that level flight demonstration is exactly the technique my colleague has offered to use as a demonstration of HIS point. He claims that the N1 will not change in level flight at 230 with the slats extended or clean. Obviously this does not make a sensible argument as, like you stated, either the power will eventually stabilize at a slightly higher value to maintain the same speed or, (with the autothrottle off) the speed will eventually bleed off a bit. Either the result of..... DRAG.

After he demonstrates his bit, I plan to demonstrate this to him and beat him using his own trick. I await the demonstration enthusiastically, and will share the results. I REALLY want my free drink.

misd-agin 11th Oct 2012 17:39

Straight and level flight clean at a given speed. Set and stabilze the power to hold speed. Turn a/t's off. Extend flaps. The plane will slow.

No matter how many times you show him he's clueless he'll never believe you.

Idle Thrust 11th Oct 2012 18:06

A little thread drift here but this reminds me of a captain who I flew with many years ago on the DC-9. When we would get above our descent profile (all in the noggin then - the 3 X Alt rule), he would SLOW from normal IAS of 280 to 250! (clean and idle thrust). Funny we always ended up even higher.

He had a theory that we discussed but I was relatively new to jets, he had plenty of experience and, most important, he was the captain so I just shook my head and watched the ensuing scramble to get down later. I often wondered if he ever got it.

Green Guard 11th Oct 2012 21:06

he would SLOW from normal IAS of 280 to 250! (clean and idle thrust). Funny we always ended up even higher.
It is not funny, it is normal, for most passenger jets.

PS. You obviously have a lunetic on left seat. :}

dhardesthard 11th Oct 2012 23:40

All lift also produces drag.
Due to the fact that the autoslat will only work if you have the slats extended I suspect that he got into the habit of extending the slats once he was below 250 KTS as an extra safety net. Approaching the stall I believe the auto slats will not work if the wing is clean. He is just trying to justify the early use of the slats. All lift produces some drag.

sevenstrokeroll 12th Oct 2012 00:03

I flew the DC9-30 series for 10 years and it was the best, most fun plane i've ever flown.

here is an argument for him. if slats didn't add drag, then why retract them at all? (slat speeds aside of course) Why did the designers spend all the time and money to make the slats retract?

Now, I disagree with you on the idea of flying clean/idle to 1000'...you can get yourself into trouble doing that. What if the engines don't spool up? WE have a mandatory spool up check at 3000' in every jet we fly.

There is nothing wrong with putting the slats out early...oh sure you might have a bit more fuel used. But, you must ask yourself if you have ever been out of position for any reason and had a go around. If the slats had been out, would things have been different.

In 37 years of flying, I've had one perfect descent situation from cruise. ONE. (in this I mean ATC and everything else worked out). It was going from KCLT to Talahasse florida. I still remember it. But I still spooled up at 3000' because I also know guys who have gotten in big trouble being UNSPOOLED>

so...is there drag...yes

is it wrong...no, it may actually be slightly safer.

should he buy you a beer? yes, but be sure you are at least 8 hours plus prior to the next flight...and it would be healthier if he bought you some orange juice.

The douglas twinjet series (DC9 etc) is a great plane.

Oh, and one more piece of advice...if you are anticipating ice in the descent...start heating the wing up in cruise...even clear of cloud...and the caution lights won't come on in the low rpm range of the engines in descent.

too many people wait till they actuallly get into the clouds/ice before starting anti ice.

grounded27 12th Oct 2012 06:26

Cultural change? I understand the drive to save fuel, older pilots may have a greater desire to stay ahead of their aircraft. Seems there is a competition for idle approach.

stilton 12th Oct 2012 07:13

I don't see anything wrong in being at idle to 1000' (in vmc and starting to spool up at that altitude, no lower) If Atc co-operates and you can manage this from top of descent at a normal descent speed that is very efficient.

But still being clean at 1000' is just not going to work, how are you going to be stabilized, even by 500' ?

That is too late.

hikoushi 12th Oct 2012 09:55

Hey, I'm not that much of a cowboy, gents! To elaborate, in our neck of the woods ATC is not very busy save for certain times of day, we have a great deal of VMC, and fly several short sectors a day. So, the opportunity for an unrestricted descent from TOD to a visual approach and landing is fairly common, at least a couple times a day most of the time. By idle to 1000 of course I do not mean CLEAN until 1000 AGL; just fully configured and spooling up around there as a target. Usually begin configuring / decelerating from 250 around 2500-3000 feet and 10-ish miles out, maintaining idle thrust until the gear is down, then bringing it up to a minimum spool setting around 1500 feet, setting final flaps around 1200 so as to be just powering up and settling onto Vapp at 1000 or so (if it all works out). If it seems like we are going high can always swing the gear earlier, if going low can always hang it on the slats and stretch it out a little with a decelerating glide. And of course that all goes out the window in weather or during rush hour traffic.

Flying a decelerating approach profile to 1000 AGL to join a stabilized final to the runway keeps the fuel burns low and the skill set sharp. It makes the day more challenging due to the "hands-on" engagement with the aircraft required of the procedure, and is very enjoyable to do when it all comes together. It gives a smooth ride with minimal deck angle or power changes, as well as remaining WELL within all stabilized approach parameters. And of course the beer will be on the last day of the trip..... but it sounds like it might be a lost cause, anyway!:ugh:

decurion 12th Oct 2012 14:47

Hi hikoushi;

If you want some slat data for the dc9 please drop me a PM. I Will have something for you.

stilton 12th Oct 2012 20:04

That all sounds like good efficient technique Hk

I try for the same myself but rarely does ATC allow it :ugh:

MD83FO 12th Oct 2012 20:30

as i transitioned to the A320 i realized how much drag the md80 has.
we used to intercept the glidesope in clean config and configure later without the need for speed-brakes. this is not possible on the A320 as the vfe is lower and its hard to decelerate.

misd-agin 13th Oct 2012 15:37

hikoushi - I agree. Great way to fly the airplane as clean, and efficiently, as possible.

MD83FO - Agreed. S80's, with their high slat extension speed, are easy to slow compared to more modern jets. And when it was replacing the 727's it was considered 'slick'. My, how the mighty, or slick, have fallen.

Old school 727's guys were worried about how slick the S80 was. The recommendation was 210 kts at 20 nm. That's laughable today. Most guys maintain 250 kts until 10(slightly tight) or 15 nm. So we're flying slicker a/c at 250 kts closer and closer. :D

Drag is fuel. Fuel is expensive. Clean is better. :ok:

MD80rookie 22nd Oct 2012 13:30

So tell us! How was the taste of success? The taste of beer? :)

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