Getting better, Thanks for your post.
Still a few questions lingering in my mind.
I was flying a 2011 Turbo 206 today and reading the checklist it says to the lower the flaps to 10 degrees if we encounter conditions where we need best climb angle to clear an obstacle. Why is this? (note: the online PIM does not mention this, but says to leave flaps up for best angle)
Also on the short field T/O, It says to leave the flaps at 20 degrees until all obstacles are clear.
If Im reading what your saying right... The best optimum takeoff would be to takeoff with flaps, once off the runway, remain in ground-affect until reaching Vx, pull the flaps up, then climb at Vx until the obstacle is clear. Would you agree with this? Why do most different POH's say to leave the flaps down until clear of obstacle? Maybe the first notch of flaps does not add enough drag to justify the increased workload? Or pilots incorrectly letting the plane sink back onto the runway because they dont counteract the change of AOA with back elevator?
I think the answer to both questions is the same. If I read your first question correctly, "lower the flaps to 10 degrees if we encounter conditions where we need best climb angle to clear an obstacle," is referring to clearing an obstacle after take-off, yes? And that is pretty much what your second question is referring to, also, I think, so I'll handle them together:
Based on what you wrote in your second paragraph, I think you've got the basic idea pretty much down pat. Flaps let us take off in a shorter distance, by decreasing stall speed (which also decreases take-off speed... take-off speed being nothing more than 1.3 times stall speed).
If you are trying to clear an obstacle, the angle you need to climb obviously depends on the height of the obstacle and the distance to the obstacle... if the distance is greater, you don't need as steep a climb angle to clear the obstacle.
And I think you've also got the general concept that once you are airborne, flaps up is where you want to be for climb performance (best angle OR best rate.... doesn't matter, because flaps hurt you in either case).
But take-off is a weird case: Let us assume for the moment that we have to pick one condition for the whole process, either flaps up the whole time, or flaps down to some particular setting. Is it better to accept the reduced climb out performance from the flaps in favor of the advantage of short take-off distance, or is it better to take the longer take-off distance in favor of the steeper climb-out angle? Well, the answer to this is VERY situation dependent. It depends on a number of things, including the performance characteristics of the airplane you are in, temperature and density, how far away the obstacle is that you are trying to clear, etc., etc.
In general, if the obstacle is relatively close, and relatively low (say a tree at the end of the runway), you are usually better off with using the flaps. A short take-off distance provides more distance to the obstacle in which to do your climb. If the obstacle is tall, and at a pretty good distance, flaps up is probably better.
(Up to this point, I've pretty much just restated what I said above... and I think you probably already were pretty clear on all of that based on how you phrased your second question. I was just making sure for clarity's sake).
Ok, but why can't we do both? Why can't we take-off with the flaps for the advantage of short take-off distance, accelerate and raise the flaps for the advantage of better climb-out performance. Well, we can... but it definitely complicates things in a number of ways:
The first way that it complicates things is the speed we are at. If we just took off using flaps, and immediately raised them (without accelerating), we would be closer to (and possibly below) stall speed. As we mentioned above, the whole reason flaps got us airborne faster was by lowering stall speed, so that we could take off slower (and therefore SOONER, because we don't have to accelerate as much). So one reason that you might want to leave flaps down until well into the take-off and climb-out is to prevent someone from inadvertently raising the flaps too early very close to the ground, and stalling with little to no recovery room.
This is an OK reason, but probably not the reason that your POM has you leave the flaps alone. There is a better reason:
So we do a takeoff with flaps and let's assume we rotate and take-off at the slowest allowable take-off speed (which we would want to do in the short field situation anyway, since that gives us the most advantage from the flaps, because it's the shortest take-off distance). We recognize that we need to accelerate if we want to raise the flaps. Well, when we accelerate, we do that by reducing our climb out angle for some amount of time. Instead of using our excess thrust and power to climb, we are going to use some of our excess thrust and power for acceleration. Then we are going to spend some time raising flaps and re-establishing a no flap climb.
Well, it turns out, there are a lot of different ways we could do this. We could accelerate in ground affect, or basically climb not at all, get the flaps up, and then re-establish our climb. Or we could climb out at minimum climbout speed with the flaps extended, lower our nose ever so slightly and allow the speed to increase to safe flap retraction speed over a long period of time. Or we could do something between these two.
Well, if we accelerate in ground effect... all we are really doing is eating up distance towards the obstacle. Essentially, we are negating the advantage of taking off with the flaps in the first place. We would have been better off accelerating on the ground. In the air, even in ground effect, we are producing plenty of lift and therefore plenty of induced drag. Certainly because of ground effect the induced drag is reduced, but had we stayed on the runway demanding nothing from wing we probably would have accelerated faster. So if it was advantageous to take off with the flaps due to a low, close obstacle to get that short take-off distance, we don't want to negate that advantage by accelerating in ground effect.
Now, if the scenario were different, say a short runway, but then no obstacles to worry about for a good long while, accelerating in ground effect might make sense. We had to take off with the flaps for the runway length in this scenario, not because of the obstacle clearance problem. Since we aren't worried about the obstacle clearance issue, this method will get us off the ground in the shortest distance, and then accelerated in the quickest time. That way, we can get cleaned up and climb out a Vy for best rate of climb. A good method for getting up to altitude in the shortest time, and hence, the most fuel efficient way to do it. This might also be ok for a short runway, with a tall obstacle at a large distance. You would normally have taken off no flap in this case, but couldn't because there wasn't enough pavement. So now you do the next best thing.
But I'm digressing a little, I think...
Ok, so we've examined one end of our range of options: the option where we use ALL of our available excess power (and the advantage of reduced drag from ground effect) to accelerate only, and then climb. What about the other end of the range of options: where we use most of our excess power to climb, and only use some of it to accelerate? What does this look like? Essentially, in this option, we're going to climb out with the flaps... but then slowly accelerate to flap reduction speed while still climbing, raise the flaps, and keep climbing at Vx for flaps-up.
Well, this case is much harder to describe, because it really depends on how slowly we are "slowly accelerating." I'll come back to this in a minute.
First, let's look at what is happening while we accelerate. Obviously, while we are slowly accelerating, we are doing three things that hurt our climb:
The first is, we have the flaps down (until we get to retraction speed). Beating a dead horse, but flaps hurt the climb. Nothing we can do about that, though. We chose our flaps for take-off performance (and hopefully for the right reasons, and we can't raise them below no flap stall speed.
The second is, we are using some of the excess power we have to do accelerate, rather than climb. So we aren't climbing as much as we could be, because we aren't using all of our power for the climb. This would be a similar effect to trying to climb out at a reduced power setting (except that here, we are getting acceleration out of the engine, whereas with a reduced power setting we would have reduced climb, AND no acceleration).
Third: we have begun to accelerate, so we are no longer at Vx for flaps-extended. Because we are at a non-optimum speed for our configuration, we don't climb as well as we would at the optimum speed, because our ratio of excess thrust to drag is not as good as it was. In a jet, we kept thrust the same but we increased our drag by moving away from L/D max. In a prop, we were probably below L/D max (Vx in a prop normally is a little less than L/D max... see previous post), so accelerating probably reduced our drag. But because props develop the most thrust at the lowest speed, we reduced our thrust by accelerating. The reduction in thrust hurts us more than the reduction in drag.
So this whole time we are accelerating while climbing, we aren't climbing as much. Unfortunately, it's impossible to tell how bad it's hurting us. Are we accelerating one knot ever 5 seconds, or one knot every 30 seconds? Or some other of the infinite possible ways we could do that?
As it turns out, just like the accelerate in ground effect case that we looked at earlier, there are situations where using this slow acceleration method can improve our overall climb performance. But unlike the accelerate in ground effect case, this one is much harder to predict whether it will help or hurt, because you have to know what technique the pilot is going to use for his acceleration-climb, and the performance varies widely based on the acceleration rate you use. If you accelerate too slowly, you spend more time in this sub-optimal condition. If you accelerate too quickly, you use too much of your power for acceleration, and not enough for climb.
The aircraft I fly (KC-135, a large 4-engine jet), actually has a computer and flight director system that allows us to compute and fly an acceleration-climb. Basically, by following the computer we can ensure we get the advantages from the ACCEL-CLIMB profile. We don't accelerate too slowly and we don't accelerate too quickly, because the computer figures it all out and directs the perfect pitch to balance all of these factors. And we still don't use that mode if the obstacle is close.
Ok, so we've come a long way to finally get to the answer to your question: There are times when any of these methods has their advantages, but most POMs don't recommend them, and especially not for a low, close in obstacle, like a tree at the end of a short runway. The acceleration-climb really requires perfect technique to get any advantage out of it at all (so probably a fancy-schmancy computer system like in some of the big jets). The ground-effect acceleration technique eats up too much distance towards an obstacle that's close. Not to mention the workload, sink, reduced stall margin, etc. that you were already thinking about.
In short (yeah, right, as if I were capable of posting anything SHORT)... the POM recommends flaps until clear of the obstacle (especially the low, close in obstacles) because that's usually the best method of clearing the obstacle, and the other methods are difficult/impossible to predict the outcome. The transition from flaps to no flaps costs you some climb performance, and it takes a while to make that loss back up with your better climb performance after the flap reduction. For a distant obstacle, it may be worth it... but the POM usually doesn't worry about distant obstacles, especially for GA airplanes (some large jet airplanes do consider that scenario, though). After all, if the obstacle were distant, you could just take-off and climb in a spiraling turn until you cleared it. Usually, it's the close-in obstacle we worry about, especially in a GA airplane.
