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Flaps - Lift and Drag

lr172

Well Known Member
I am familiar with the short wing RV flaps and know that the flaps deflected equal to a fully deflected aileron will provide mostly all lift, with very little drag. As I think about ways to improve my slow flying for OSH, I wonder if the same applies to the 10/14 wing or if the optimal position is different or possibly not exist at all.

Hoping Scott can provide some guidance on where the cut off point between all lift and lift+drag occurs with that wing, if it even exists at all. I presume there are significant differences with a fowler flap arrangement compared to the barn doors.

Larry
 
Flaps will increase drag at all positions, but as you point out they provide a lot more lift than drag for the first 5-15 degrees of deflection depending on the aircraft configuration. On many aircraft, reflexing them a couple of degrees can reduce drag in cruise, too.

That's why we use a small amount of flap for takeoff. I expect the best bang-for-the-buck position is the takeoff position.
 
I am familiar with the short wing RV flaps and know that the flaps deflected equal to a fully deflected aileron will provide mostly all lift, with very little drag. As I think about ways to improve my slow flying for OSH, I wonder if the same applies to the 10/14 wing or if the optimal position is different or possibly not exist at all.

Hoping Scott can provide some guidance on where the cut off point between all lift and lift+drag occurs with that wing, if it even exists at all. I presume there are significant differences with a fowler flap arrangement compared to the barn doors.

Larry

Sorry Larry, I don't have any definitive info to share.
I do know that at -3 degrees reflex, the flaps (and ailerons) are top loaded slightly at high speed cruise AOA.
At 90 Kts flying the OSH arrival they probably aren't.

It has always been my practice to have flaps at least at zero degrees for slow flight and if flying an airplane without a flap indexer (able to set the flaps at any value I choose at the moment) I always use about 10 degrees for the 90 kt slow flight regime on the OSH arrival. There is no noticeable handling difference, but it trims the airplane to a more nose down attitude for better over the nose visibility and potentially better engine cooling.

Maybe Steve (airfoil designer) has a better estimate the amount of flap deflection that would produce mostly additional lift with little to no additional drag.
 
Sorry Larry, I don't have any definitive info to share.
I do know that at -3 degrees reflex, the flaps (and ailerons) are top loaded slightly at high speed cruise AOA.
At 90 Kts flying the OSH arrival they probably aren't.

It has always been my practice to have flaps at least at zero degrees for slow flight and if flying an airplane without a flap indexer (able to set the flaps at any value I choose at the moment) I always use about 10 degrees for the 90 kt slow flight regime on the OSH arrival. There is no noticeable handling difference, but it trims the airplane to a more nose down attitude for better over the nose visibility and potentially better engine cooling.

Maybe Steve (airfoil designer) has a better estimate the amount of flap deflection that would produce mostly additional lift with little to no additional drag.

Thanks. I tried about 5-10* the other day at 80 knots and it seemed to help but couldn't really be sure if it was an improvement. Hopefully Steve will chime in. Want everything working for me, as I have had times where the Fisk line seems to get slower than I would like.
 
Also consider the advantage of the nose pitch with the use of flaps for greater visibility.

My last trip to OSH found me behind a 150 with 1st position flaps half way in. My bird wasn't happy. I regretted not passing the idiot
 
I’d go with full flaps. More flaps will always reduce the stall speed (and - as others have said - will allow you to fly with less of a nose-up attitude). Yes, they will also increase the drag, but RVs have plenty of power to handle that (I say as someone who inadvertently took off with full flaps once…).

Some quick googling reveals that the increase in lift coefficient from flap deflection (according to papers like this) looks something like the following. This of course depends on the airfoil and its angle of attack and so on.

4iZJRW9.jpg


For a plain flap, you see that the curve bends around 12 degrees. For the first 12 degrees, your CL increases by about 0.6, and then for the next ~24 degrees, it increases by a further 0.4. So although this will depend on the airfoil, the kind of flap, and so on, you can see that somewhere around 10 to 15 degrees, you start needing a lot more flaps to get only a little more lift.

For a slotted flap, the trend is a little more complicated… but the “diminishing returns” effect is still there: Because dynamic pressure is proportional to the square of the speed, the initial increase in CL (going from 0 degrees to 12 degrees) reduces your stall speed a lot more than the second increase in CL (going from 12 to 35 degrees) even if the added-CL-per-added-degree of flap does not get shallower (as is closer to the case in slotted flaps).

A quick, oversimplified example: A 23012 airfoil stalls when the CL is about 1.5. For an airplane weighing a little over 2000 lbs and with 126 ft2 of wing area, that works out to about 68.4 mph. (This is only a little off from an RV-14. Again, general ballpark, not exact numbers). If 12 degrees of flaps increases the CL by 0.6 to about 2.1, that would allow for a stall speed of 57.8 mph, a reduction of 10.6 mph. If going to 35 degrees of flaps increases the CL by another 0.4 to about 2.5, that would allow for a stall speed of 53 mph, a reduction of only 4.8 mph. (Note that this assumes 100% span flaps, e.g. ailerons drooping along with the flaps, or full-span flaperons… So, again, the lift increase in an RV will be smaller than this).

Even in a back-of-the-envelope calculation, the “diminishing returns” are clear. You get about two thirds of the benefit (stall speed reduction) from the first one third of flap travel.

(Note that max aileron deflections are around 15 or 17 degrees, right?)

What about the drag? That’s even harder to quantify, because although the wings generate 100% of the lift (or pretty close), the rest of the airplane contributes a lot of drag. So if the drag of the wing – or the drag of the flap surfaces – goes up so-many percent, what is the increase of the drag of the overall airplane? That would be tough to calculate. But even just looking at the flap, as this study does…

7rzdKnw.jpg


Interestingly, the increase is minor for the first ~12 degrees, then gets steeper, i.e. the opposite of the lift trends. That’s not surprising.

It’s also worth pointing out that the loads on the flaps are lower at 10-15 degree deflections than they are at 30 to 40 degrees. (The flap changes the airflow around the entire airfoil; The flap surface itself is not the source of all this extra lift, especially at relatively small deflection angles). So if your flaps are at 30 degrees or more and you exceed the max flap speed, you will be more likely to do damage than if your flaps are at 10 or 15 degrees. (The max flap speed assumes that the flaps are all the way down. If they are at only 10 degrees, then you can typically go at least a little faster before the loads on the flaps get too high).

All that having been said: On the approach to Oshkosh, there are many good reasons to use full flaps, and only weak reasons (saving 0.1 gallons of fuel, reducing the chance of damage if you exceed the max flap speed) for using less-than-full flaps.
 
I’d go with full flaps. More flaps will always reduce the stall speed (and - as others have said - will allow you to fly with less of a nose-up attitude). Yes, they will also increase the drag, but RVs have plenty of power to handle that (I say as someone who inadvertently took off with full flaps once…).

Some quick googling reveals that the increase in lift coefficient from flap deflection (according to papers like this) looks something like the following. This of course depends on the airfoil and its angle of attack and so on.

4iZJRW9.jpg


For a plain flap, you see that the curve bends around 12 degrees. For the first 12 degrees, your CL increases by about 0.6, and then for the next ~24 degrees, it increases by a further 0.4. So although this will depend on the airfoil, the kind of flap, and so on, you can see that somewhere around 10 to 15 degrees, you start needing a lot more flaps to get only a little more lift.

For a slotted flap, the trend is a little more complicated… but the “diminishing returns” effect is still there: Because dynamic pressure is proportional to the square of the speed, the initial increase in CL (going from 0 degrees to 12 degrees) reduces your stall speed a lot more than the second increase in CL (going from 12 to 35 degrees) even if the added-CL-per-added-degree of flap does not get shallower (as is closer to the case in slotted flaps).

A quick, oversimplified example: A 23012 airfoil stalls when the CL is about 1.5. For an airplane weighing a little over 2000 lbs and with 126 ft2 of wing area, that works out to about 68.4 mph. (This is only a little off from an RV-14. Again, general ballpark, not exact numbers). If 12 degrees of flaps increases the CL by 0.6 to about 2.1, that would allow for a stall speed of 57.8 mph, a reduction of 10.6 mph. If going to 35 degrees of flaps increases the CL by another 0.4 to about 2.5, that would allow for a stall speed of 53 mph, a reduction of only 4.8 mph. (Note that this assumes 100% span flaps, e.g. ailerons drooping along with the flaps, or full-span flaperons… So, again, the lift increase in an RV will be smaller than this).

Even in a back-of-the-envelope calculation, the “diminishing returns” are clear. You get about two thirds of the benefit (stall speed reduction) from the first one third of flap travel.

(Note that max aileron deflections are around 15 or 17 degrees, right?)

What about the drag? That’s even harder to quantify, because although the wings generate 100% of the lift (or pretty close), the rest of the airplane contributes a lot of drag. So if the drag of the wing – or the drag of the flap surfaces – goes up so-many percent, what is the increase of the drag of the overall airplane? That would be tough to calculate. But even just looking at the flap, as this study does…

7rzdKnw.jpg


Interestingly, the increase is minor for the first ~12 degrees, then gets steeper, i.e. the opposite of the lift trends. That’s not surprising.

It’s also worth pointing out that the loads on the flaps are lower at 10-15 degree deflections than they are at 30 to 40 degrees. (The flap changes the airflow around the entire airfoil; The flap surface itself is not the source of all this extra lift, especially at relatively small deflection angles). So if your flaps are at 30 degrees or more and you exceed the max flap speed, you will be more likely to do damage than if your flaps are at 10 or 15 degrees. (The max flap speed assumes that the flaps are all the way down. If they are at only 10 degrees, then you can typically go at least a little faster before the loads on the flaps get too high).

All that having been said: On the approach to Oshkosh, there are many good reasons to use full flaps, and only weak reasons (saving 0.1 gallons of fuel, reducing the chance of damage if you exceed the max flap speed) for using less-than-full flaps.

Thanks for the detailed explanation. Very helpful and much appreciated.

Larry
 
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