The best rate-of-climb and angle-of-climb is always reached with flaps up.

[Mike Koerner]

I would like to address this from the standpoint of aerodynamic generalities instead of a specific aircraft's perceived performance.

For every airspeed and wing loading there is an optimum wing camber - a camber that gives the minimum drag. As the airspeed decreases or the wing loading increases, the optimum camber increases.

Flaps provide an effective means of adjusting a wing's camber, especially on our aircraft where the ailerons are interconnected with the flaps (like a sailplane) affecting the camber across the full span. Though there are some losses in any non-zero flap position, primarily due to wing root and tip effects (the LS has an added fence for at the root to reduce this loss in the negative flap position) these effects are relatively minor.

Our planes were designed with the intent that you use the first positive flap setting (15 degrees) at low speed, no flaps (0 degrees) at modest speed, and negative flaps (-6 degrees) at high speed. If you use too much or too little flaps for your airspeed and weight your L/D will decrease (drag will go up more than lift). Flap positions beyond 15 degrees are primarily to increase drag at low speed for steeper landing approaches.

It is desirable to get off the ground at low speed so you should use 15 degrees of flaps. If you want to climb steeply, such as to clear an obstacle, you will also need to fly slow (Vx for the best angle of climb) and again use 15 degree flaps. Low speed and 15 degrees of flaps should also provide the lowest rate of descent (the longest time to figure out what's wrong) in an engine out condition.

For the fastest rate of climb (Vy) you want the flaps at zero to minimize the minor losses associated with non-zero flap positions, so you should use zero flaps and moderate speed. This will also give the best glide angle (longest glide distance) in an engine out condition.

High altitudes will increase the true airspeeds but indicated airspeeds should remain the same.

For what it's worth, this is consistent with my perceptions of the performance of my CT2k.

Mike Koerner

[Ed Cesnalis]

Mike,

I always enjoy your posts and you spoke eloquently here addressing design intent.

I'm pretty sure result and intent differ, the easiest place to see an opposite result for me is looking for a better glide at zero than I already had at -6.  Dropping flaps to zero always reduces my glide by a big margin, at any speed I try.

Another thing to notice would be at what speed will you see an increase in speed or acceleration at neg six over zero flap setting?  My perception is that negative flaps offer the lowest drag even at lower speeds than intended.

-----------------------------

Edit: thinking more about why I don't see zero out performing negative six at slower speeds.  I think its because the wing isn't laminar flow and the flap adjustment is a drag adjustment mostly because topside flow has separated well forward of the flaps and the resulting camber change isn't meaningful.

 

[Ed Cesnalis]

The best rate-of-climb and angle-of-climb is always reached with flaps up.

This belief is obviously held by experimental builders as evidenced by the links I provided. It is also a long held belief by aviators evidenced by the practice of retracting once obstacles are cleared.

I used to wait for 600' to retract even to zero, even when no obstacles.  I'm now cleaning up in ground effect and getting better rate and angle as a result.  The departure end of our runway has a 500' high ridge adjacent to the numbers on the departure end and I can't help but compare the results, day after day.  

[FlyingMonkey]

1 hour ago, Ed Cesnalis said:

Mike,

I always enjoy your posts and you spoke eloquently here addressing design intent.

I'm pretty sure result and intent differ, the easiest place to see an opposite result for me is looking for a better glide at zero than I already had at -6.  Dropping flaps to zero always reduces my glide by a big margin, at any speed I try.

 

I almost posted something similar.  Upon re-reading, I see that Mike said we'll get the "lowest rate of descent" at lower speed and 15° flaps.  This is probably true, you'd get the most time aloft at those settings.  My testing mirrors yours, my best glide *range* occurs at -6° flaps and about 78kt.  Once I'm near my intended landing zone and am confident I have it made, I add more flaps in and start slowing down.

[FlyingMonkey]

28 minutes ago, Ed Cesnalis said:

I used to wait for 600' to retract even to zero, even when no obstacles.  I'm now cleaning up in ground effect and getting better rate and angle as a result.  The departure end of our runway has a 500' high ridge adjacent to the numbers on the departure end and I can't help but compare the results, day after day.  

I think I posted something similar, mentioning "splitting the difference" by using flaps to get off the ground quickly, then retracting ASAP to get to the lower-drag climb configuration.

The only issue with this I could see is that you have a period after retracting the flaps where you are not yet at sufficient airspeed to get a better climb rate out of the wing with 0° flaps.  So you have to give up a couple of seconds of climb to let the airplane accelerate; if the additional climb performance doesn't offset that loss, you won't gain anything by using this technique.  That was illustrated in the calculations I did in my earlier post, though I don't know if that is the case here.

 

[Ed Cesnalis]

48 minutes ago, FlyingMonkey said:

I think I posted something similar, mentioning "splitting the difference" by using flaps to get off the ground quickly, then retracting ASAP to get to the lower-drag climb configuration.

The only issue with this I could see is that you have a period after retracting the flaps where you are not yet at sufficient airspeed to get a better climb rate out of the wing with 0° flaps.  So you have to give up a couple of seconds of climb to let the airplane accelerate; if the additional climb performance doesn't offset that loss, you won't gain anything by using this technique.  That was illustrated in the calculations I did in my earlier post, though I don't know if that is the case here.

 

I agree and see it as a feel issue.  If my acceleration is enhanced with each retraction then my timing isn't too soon.  If my acceleration is delayed by the retraction then I wasn't fast enough yet.

[Ed Cesnalis]

58 minutes ago, FlyingMonkey said:

I almost posted something similar.  Upon re-reading, I see that Mike said we'll get the "lowest rate of descent" at lower speed and 15° flaps.  This is probably true, you'd get the most time aloft at those settings.  My testing mirrors yours, my best glide *range* occurs at -6° flaps and about 78kt.  Once I'm near my intended landing zone and am confident I have it made, I add more flaps in and start slowing down.

Agreed

[FlyingMonkey]

You can definitely feel (or see on the VSI) when the climb rate changes with a flaps change.  When coming in the pattern I usually go from -6° to 0° at about 95-98kt a couple of miles out, and it feels like hitting a strong updraft.  You can certainly feel the change.  Not to mention the sink you feel when you go from 15° to 0° at under 65-ish knots on climb out.

[Ed Cesnalis]

2 hours ago, FlyingMonkey said:

You can definitely feel (or see on the VSI) when the climb rate changes with a flaps change.  When coming in the pattern I usually go from -6° to 0° at about 95-98kt a couple of miles out, and it feels like hitting a strong updraft.  You can certainly feel the change.  Not to mention the sink you feel when you go from 15° to 0° at under 65-ish knots on climb out.

Agreed.  As I approach the pattern I do everything I can do see 100 indicated and its the flap change that really gets me slowed down.  I often pitch up to get to 100 and then descend again.  Once I get to 100/zero slowing down becomes easy.

[FlyingMonkey]

Yeah, I have to throttle back to 4000rpm or so to get the speed to come down below 100kt to make the flaps change, then I can throttle back up a little.

[Tom Baker]

1 hour ago, FlyingMonkey said:

Yeah, I have to throttle back to 4000rpm or so to get the speed to come down below 100kt to make the flaps change, then I can throttle back up a little.

It is not throttling back that slows you down, it is the change in angle of attack.

[Ed Cesnalis]

51 minutes ago, Tom Baker said:

It is not throttling back that slows you down, it is the change in angle of attack.

True.  Pretty sure Andy didn't mean power only.  Its a pitch and power adjustment that maintains your descent angle while slowing you to flap speed.

A better way to say it is to throttle back to 4,000 while adding needed back pressure to maintain sight picture while descending and slowing to flap speed.

[Tom Baker]

9 hours ago, Ed Cesnalis said:

The best rate-of-climb and angle-of-climb is always reached with flaps up.

This belief is obviously held by experimental builders as evidenced by the links I provided. It is also a long held belief by aviators evidenced by the practice of retracting once obstacles are cleared.  

Only flight testing can determine which configuration will give the best angle. Rate of climb will always be lower with any amount of flaps extended. Angle is determine by forward speed vs rate of climb. If the reduction in climb has an even greater reduction in speed then the angle will be greater.

The reason flaps are retracted after the obstacles are cleared is because you are no longer worried about angle of climb and more concerned with rate of climb. Rate will always be better with zero flaps generally speaking.

[FlyingMonkey]

2 hours ago, Ed Cesnalis said:

True.  Pretty sure Andy didn't mean power only.  Its a pitch and power adjustment that maintains your descent angle while slowing you to flap speed.

A better way to say it is to throttle back to 4,000 while adding needed back pressure to maintain sight picture while descending and slowing to flap speed.

Right.  I'm usually already at pattern altitude well before getting to the airport.  The only way I can keep my altitude constant *and* slow down is to reduce power and increase pitch angle.  At least until I'm slow enough to change flap setting.

[FlyingMonkey]

3 hours ago, Tom Baker said:

It is not throttling back that slows you down, it is the change in angle of attack.

If you want to pick nits, it's drag that slows the airplane, which goes up with AoA.

[Ed Cesnalis]

2 hours ago, Tom Baker said:

Only flight testing can determine which configuration will give the best angle. Rate of climb will always be lower with any amount of flaps extended. Angle is determine by forward speed vs rate of climb. If the reduction in climb has an even greater reduction in speed then the angle will be greater.

The reason flaps are retracted after the obstacles are cleared is because you are no longer worried about angle of climb and more concerned with rate of climb. Rate will always be better with zero flaps generally speaking.

You might have said that backwards but:  your saying if climbing with flaps deployed the angle is only steeper if the reduction in speed is greater than the reduction in climb.

Okay, I'm not sure there is any conflict.

When saying that rate and angle will always increase with flaps up I am saying that retracting flaps increases rate over speed satisfying your requirement but notice rate goes up too so they both go up without flaps.

To make testing more valid the zero flaps plane starts at the numbers and the 15* plane starts farther down the runway so that their rotation points on the runway are the same. That's when the zero config will become more capable of clearing obstacles.  

[Roger Lee]

At the end of my 6500' runway I want to be at 600' at the end or close to it so I could turn 180 degrees and land if when I do change flaps or change throttle something happens. It also gives me good chute deployment altitude. 15 flaps puts me there. Zero flaps gets me to the end of the runway faster, but only about 400' - 450'.

15 flaps gets me off the ground sooner and in 15 flaps at 60 knots if the engine quits I'm already in a landing configuration and all I have to do is lower the nose.

I had 8 CTSW's at my field and we always take off in twos. We split the runway. The guy in 15 always leaves the runway first and is higher by the end of the runway. The zero guy got to the end faster, but less altitude. This was real time flights for 10 years this way. If you haven't done flights in twos then it would be tough to debate otherwise because perception is never as good as the real deal..

 

Toss in aircraft gross weight for a flight and things change quick. Two vs one occupant. 1/2 tank vs full and baggage and density altitude.

When I'm at 9500'+ DA I would never use zero flaps.

[Ed Cesnalis]

50 minutes ago, Roger Lee said:

If you haven't done flights in twos then it would be tough to debate otherwise because perception is never as good as the real deal..

No-one is debating otherwise.  The fact that your comparison includes ground rolls of different lengths means you are not comparing the climb angles. There may be an inclination to climb at zero closer to Vy while at 15 closer to Vx and this too would cloud your comparison.

In general your results are expected and like your advice that 15 is already set up for landing you are talking about which selection to use for take-off not addressing the concept that we get both best rate and angle from a clean configuration.  

The concept being discussed probably requires taking off at 15 and cleaning up for the initial climb if and when free of obstacles to take advantage of.

[FlyingMonkey]

I don't think that anybody would argue the idea that, flying in cruise, the best climb will be at the cleanest configuration.  The reason this ended up in a takeoff configuration discussion is that is the only time where there might be some question as to which flap setting to use in a climb.

Now here is another question:  at a relatively high DA, say 8000ft or more, will the best cruise climb config be at 0° or at -6° flaps?  I know that that -6° should have the lowest drag.   But I don't know how much lift it sacrifices to get there, and whether at the higher DAs where drag is already lower and lift at a premium, if you might get better climb at 0°...

This discussion has talked mostly about 15° vs 0°, and I am curious how the reflex setting affects the equation.  I usually climb at -6° in cruise and it works fine, but I'm wondering if I might be leaving some climb on the table in cruise.

[Ed Cesnalis]

37 minutes ago, FlyingMonkey said:

I don't think that anybody would argue the idea that, flying in cruise, the best climb will be at the cleanest configuration.  The reason this ended up in a takeoff configuration discussion is that is the only time where there might be some question as to which flap setting to use in a climb.

Now here is another question:  at a relatively high DA, say 8000ft or more, will the best cruise climb config be at 0° or at -6° flaps?  I know that that -6° should have the lowest drag.   But I don't know how much lift it sacrifices to get there, and whether at the higher DAs where drag is already lower and lift at a premium, if you might get better climb at 0°...

This discussion has talked mostly about 15° vs 0°, and I am curious how the reflex setting affects the equation.  I usually climb at -6° in cruise and it works fine, but I'm wondering if I might be leaving some climb on the table in cruise.

Your assumption that drag is lower and lift is at a premium at altitude is probably wrong.  There is less drag and lift but only at the same true air speed.

We operate at faster true air speeds when taking off at 8,000'.  I think neg six outperforms nex six all the way up above 14,000'.

[FlyingMonkey]

30 minutes ago, Ed Cesnalis said:

Your assumption that drag is lower and lift is at a premium at altitude is probably wrong.  There is less drag and lift but only at the same true air speed.

We operate at faster true air speeds when taking off at 8,000'.  I think neg six outperforms nex six all the way up above 14,000'.

So you say I'm probably wrong, but then in the next sentence say that I'm right...  

Lift and drag being lower is what allows us a higher TAS, and also what requires us to fly at a higher TAS to maintain flight, but whatever, we all know what the situation is with increasing DA.  

My point was only that there might be a crossover point where the 0° setting might yield better performance, whether it's at 8000, 14000, or 18000.  There is probably a crossover point where 0° is required just to maintain altitude at WOT.

[Tom Baker]

10 hours ago, Ed Cesnalis said:

You might have said that backwards but:  your saying if climbing with flaps deployed the angle is only steeper if the reduction in speed is greater than the reduction in climb.

Okay, I'm not sure there is any conflict.

When saying that rate and angle will always increase with flaps up I am saying that retracting flaps increases rate over speed satisfying your requirement but notice rate goes up too so they both go up without flaps.

To make testing more valid the zero flaps plane starts at the numbers and the 15* plane starts farther down the runway so that their rotation points on the runway are the same. That's when the zero config will become more capable of clearing obstacles.  

Ed, according to the CTLS AOI the CTLS has a rate of climb of 800 feet per minute with 0° flaps at 0 density altitude, with a climb speed of 73kts CAS. With 15° flaps the rate is 740 feet per minute at 67kts CAS. With 0° flaps the first airplane will travel 7392.6 feet forward in 1 minute reaching 800 feet. For 15° flaps the airplane travels 6785 feet to reach 740 feet in one minute. To continue climbing to the 800 foot mark using the same values the airplane will travel an additional 484.6 feet. reaching 800 feet in 123 feet shorter distance than the airplane with 0° flaps.

I don't know, but to me if you are reaching the same altitude in a shorter distance you are climbing at a steeper angle.

As a disclaimer I only ran the numbers for on altitude at one weight, and didn't go any farther.

[Ed Cesnalis]

10 minutes ago, Tom Baker said:

Ed, according to the CTLS AOI the CTLS has a rate of climb of 800 feet per minute with 0° flaps at 0 density altitude, with a climb speed of 73kts CAS. With 15° flaps the rate is 740 feet per minute at 67kts CAS. With 0° flaps the first airplane will travel 7392.6 feet forward in 1 minute reaching 800 feet. For 15° flaps the airplane travels 6785 feet to reach 740 feet in one minute. To continue climbing to the 800 foot mark using the same values the airplane will travel an additional 484.6 feet. reaching 800 feet in 123 feet shorter distance than the airplane with 0° flaps.

I don't know, but to me if you are reaching the same altitude in a shorter distance you are climbing at a steeper angle.

As a disclaimer I only ran the numbers for on altitude at one weight, and didn't go any farther.

Clearly FD numbers favor zero over reflex by a small amount on rate while reflex wins on angle.

I question FD's numbers, they certainly do no reflect my plane's performance trends accurately.

FD's numbers seem more in agreement with a design intent that produces better performance at zero over neg six at slower speeds.  I don't believe that actually happens probably because the wing isn't fast enough and the flap changes are far more drag changes than camber changes.

[Tom Baker]

13 hours ago, FlyingMonkey said:

If you want to pick nits, it's drag that slows the airplane, which goes up with AoA.

I thought we were talking about actions. I agree that it is drag that slows the airplane down, but to change the amount of drag you must change the AoA.

[Ed Cesnalis]

12 minutes ago, FlyingMonkey said:

So you say I'm probably wrong, but then in the next sentence say that I'm right...  

No, here I don't agree.  Your statement is wrong in that lift and drag are only lower when measured at the same true air speed but we don't fly or measure that way we use indicated airspeed and at that constant L&D don't change either quantity or ratio, why would they?

The wing 'doesn't care' about the true speed but only the amount of force that results from the air impact/flow.