Lift flap on the C152

[GWHIZ]

What constitutes lift flap on the C152?
Is it just 10 or 10 and 20?

[DB6]

How very topical, I was just about to write a post on the very same topic. 10 - lift, 20 - lift + drag, 30 - drag. For an idea of how much of each think back to the early lessons on use of flap and remember the lurch you get when you retract the flaps from 20 to 10 etc. I was wondering the same thing as a checklist we use says 'retract drag flap as soon as possible on the go-around, the remainder at a safe height'. What is lift flap and what is drag flap ? In this case I have decided drag flap is 30 as so much lift is generated at flaps 20 that immediate retraction would cause unacceptable sink, however on e.g. a short field takeoff, lift flap would be 10 only since 20 would cause too much drag in that case. That doesn't really help, does it ? Oh well.
Cheers DB6

[This message has been edited by DB6 (edited 04 May 2000).]

[chicken6]

I'll go with 10deg. As DB6 said, 20 slows you down too much, and if it was 20, why wouldn't they say 20 for a short field takeoff in the manual? I've discovered that 10 (and first notch in a PA28) actually helps range in a glide as well.

Safe flying

[Tinstaafl]

I must differ with respect to Flap 10 increasing range in a glide!!!!

Flap selection changes the lift:drag ratio adversely. This in turn REDUCES the glide range of the aircraft.

Easily demonstrated by flying straight & level at whatever speed requires minimum power (min thrust to be precise but harder to find easily in a piston type), & then selecting flap. You will need more power to maintain S&L. This because there is an increase in drag -ie a reduction in lift:drag ratio

NOTE: Very, very calm/smooth conditions are best for this

This IAS can be found by maintaining S&L & gradually reducing power a bit at a time, wait until IAS is stabilised & then reducing again.

Eventually you will not be able to maintain a constant IAS without descending.

After stabilising at this just found min. pwr S&L IAS, select 10 deg flap. You will need more power to maintain S&L flight. If in doubt repeat with the next flap setting.

The definitive proof is to look at a set of Lift/drag curves with & without flap. They very clearly show an adverse effect.

BTW: If it was correct that flap selection increases the glide performance, then this would also mean selecting flap to maximise cruise range

[This message has been edited by Tinstaafl (edited 05 May 2000).]

[chicken6]

Thanks for the bite!

I've got a renewal coming up soon and wanted someone else to explain something to me for a change.

How about if I change the word "range" to "endurance"? I honestly have found that 10deg in a glide approach reduces the ROD. Can someone else talk me through the theory of this. I think drag is irrelevant (to throw the cat amongst the pigeons!)

Safe flying

------------------
Confident, cocky, lazy, dead.

[Checkboard]

If the lift/drag ratio was better at 10° flap, then the flap would be welded in that position by the manufacturer!

If you are staying aloft in an unpowered glide longer with the flap out than clean, then the glide speed you are using when clean is incorrect.

[chicken6]

Thanks for the replies, but don't you understand? My brain hurts! http://www.pprune.org/ubb/NonCGI/frown.gif

Please E-X-P-L-A-I-N S-L-O-W-L-Y using theoretical terms though, so I've got something to "hang my hat on" as my Chief says.

As far as I can tell, using 10deg increases L more than it increases D, therefore increasing (L/D) both range and endurance. If my premise is wrong, explain my measurements some other way. Don't just rubbish it. For reference, I'm looking at Kermode (9th ed.) p. 176 in the 'Gliding and Landing' chapter. Can't find what happens to L/D as Fowler flaps are extended by 10deg intervals, which seems to be the crux of the debate.

Sorry for hijacking your thread GWHIZ ;)

------------------
Confident, cocky, lazy, dead.

[GWHIZ]

No problem Chicken6!

The way I see it is that 10 is lift flap, 20 is an intermediate stage providing both some lift and increase in drag, and 30 is drag.

So what do you all do in a go around after an approach using 30 degrees?

[Runandbreak]

"Full Power, prevent excessive pitch, roll and yaw, drag flap up(i.e. flap from 30 to 20),check you have a +ve rate of climb (and clear of terrain)then flap to 10" and so on.

However, flap 30 is not going to kill you in the climb if it is left down. The old Cessna 172 used to have 40, which really needed to be watched when you applied go - around power at low IAS, it also gave mega drag and needed to be retracted asap!

[DB6]

Chicken6, I think what you may be seeing is roughly this. The best glide speeds for flaps 0 and flaps 10 are different. In a C152 the figures are (roughly) 70 kts clean and 65kts with flap. Thus if you are gliding at slightly lower than your optimum clean gliding speed and you lower 10 flap, you would probably see the RoD decrease a little. Try increasing your gliding speed a little and repeating the experiment. If your RoD increases this time you've nailed it. If not then I'm buggered if I know.
Cheers DB6

[This message has been edited by DB6 (edited 07 May 2000).]

[redsnail]

Checkboard, have a close look at the Islander's flaps when they are "fully" retracted..... Sorta explains the Islander completely.... :)

[stussy]

Chicken6...a bit hard to explain how lift/drag ratio alters when flap is lowered but take it from me it does...alot.

You may notice a reduction in ROD when 10 flap is lowered , but I guarantee that your airspeed reduce aswell...and that is why we lower flap, so that we can land at a lower airspeed.

What will be altered is your glide angle, which steepens when you lower flap, alter your glide speed from defined best glide and your glide angle will steepen.

Basically we always try to fly at our best lift to drag ratio.

A PA28 for example has an optimum l/d ratio of approx 10:1..we can prove this thus.
ROD in a glide at best glide 700fpm approx
Best Glide speed = 73kts
Using PFM maths (73 * 6080)/ 60=7397.3
7397.3/700 = 10 LD ratio = 10.....
L/D ratio S+L is thus
MGW=2400lbs
Engine HP(max)=160 * 2.2 (lbs of thrust or something)=352
lbs of thrust at cruise pwr is 75% of max
so = 264lbs, in the cruise s+l @ 100kts
Drag = thrust therefore drag = 264lbs
@mgw S+L lift must = weight so lift equals 2400 lbs...Drag is 264lbs 2400/264 approx 9
So L/D in S+L flight is 9:1

So I hope you can see from this that if you start sticking flaps and stuff out then your L/D ratio is screwed and you will not achieve optimum perf.

Cheers

[chicken6]

DB6, stussy et al

When I first came up with this problem I was teaching forced landings and thought to myself "what are the different glide speeds for best range and endurance?". Not that you would really want to glide for best endurance most of the time, but anyway....

Here are some actual measurements from yesterday triggered by DB6's comments.

(C152, no flap, D.Alt 2500')

Gliding at 60 knots gives ROD 570fpm, angle=5.4deg

Gliding at 65 knots gives ROD 650fpm, angle=5.6deg

Book figure for engine failure (best range)is 60. I haven't yet had the opportunity to find a speed for a lower ROD, or to measure the ROD with 10deg at various speeds - I've been either trying to talk a student down to the ground or just concentrating on whether or not that stupid bloody cow's going to go back into it's own paddock!

Safe flying all, back in the weekend hopefully with more data. This is good fun!

------------------
Confident, cocky, lazy, dead.

[chicken6]

another quick point,

Book figures for engine failure without power are 65kt clean, 60 "flaps DOWN" (their emphasis). This is in the Procedures section, there are numerous other references to 60 being the "maximum glide" or "best glide". Anybody know why there are two different figures for apparently the same thing?

------------------
Confident, cocky, lazy, dead.

[stussy]

just a quick point re my earlier post..
If you read it and thought it made no sense at all, then you were right.
The first figure, using rod and distance over the ground actually gives you the glide angle..not l/d ratio.
But if you now compare this angle to the l/d ratio you can see that they are very similar proving that your best glide angle is achieved at best l/d ratio.
Sorry about the confusion

[DB6]

We could fast be approaching the 'I'm buggered if I know' stage on this one.
Cheers DB6

[stussy]

Chicken 6, The two figures that you have are both correct.

Your 'maximum glide' distance would be 65kts clean because that gives you your best l/d POSSIBLE. The 'best glide' is the speed that you should fly once you have your flaps set at 10 to achieve the best l/d ratio for THAT CONFIGURATION.(I am assuming that in your a/c ops manual it states there being a max glide and a best glide speed).

I'm not too sure but I think it gives you the performance graphs for these calculations in the back of your ops manual and it may make it clearer once you have looked in there.

Cheers

[DB6]

Here's a thought though, Chicken6. When these test pilot chappies arrive at 'best glide' figures, do they actually switch the engine off ? I am assuming you didn't when you did your tests, so there may be a discrepancy there. Anyway I shall watch with interest. I tried it yesterday, by the way, while trying to show a student the effect of flap on the descent, and of course the descent rate seemed to reduce with 10 flap (I think we hit some rising air). Certainly increased with 20 though so she was convinced in the end.

[Oktas8]

If it's not too late to add a reply:

Book figures for best glide speed are quoted for max all up weight. As aircraft weight decreases, so will the IAS for best glide range.

Also, best glide speed decreases as altitude increases.

So if you go up to 3000 feet or so at less than MAUW, best glide speed will be several kts less than the book figure.

To add something else to the thread - has anyone else here been embarrassed when teaching best rate versus best angle of climb attitudes, and found that there is little or no difference in rate of climb when using book airspeeds?

[This message has been edited by Oktas8 (edited 13 May 2000).]

[4478]

I think the easiest way to explain what happen with flaps is as follows:

Selecting flaps creates more drag. Ich think all agree to that. That means a clean aircraft has less drag than with any configuration!
An aircraft with more drag needs more energy.
The energy comes from the engine during climb and cruise and from the component of weight along the gliedpath during powerless descent.
That means that an aircraft with flaps and max power can climb only with a lower ROC or a lower climb angel compared to an aircraft without flaps. During cruise the powersetting is higher, therefore also the fuelflow and the range decreases with flaps and during glide, the glidangel must be steeper with flaps as well as the glidratio becomes worth due to the higher drag with flaps.