High Density Altitude take offs

Oldmanb777

Oldmanb777

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Oldmanb777
I'm opening this up for discussion, as its been bugging me. Hopefully a good civil discussion where we can all think about what we are doing and learn something.
I was recently at a fly in where density altitude is a factor. as well as mountainous terrain. I saw many pilots taking off using flaps. So I opened up discussion with a couple pilots standing around. Why would you use flaps on take off in this conditions? I would guess DA at about 9000 ft at the time. So what would be your performance limits? Simply engine power. Adding flaps creates lift, but lift creates drag and you need power to overcome that drag, and you don't have excess power. The runway is long here, 5000 ft I think. But suffice to say it's much longer than you need. No 50 ft " obstacle. No runway clutter, its clear and dry, good quality pavement. Lets say your average normally asperated SEL airplane.
So would you choose to use flaps for take off, or, use the available runway to make a no flaps take off? Hopefully I described the scenario accurately. Now open for discussion, please be civil.
 
Last week at Granby?

As the Piper POH, first setting of flaps...10 deg ... is recommended for takeoff, then retract once in the air. I need to go back and re-read my aerodynamics books but IIRC, the first setting of flaps provides more lift than the drag caused by the flaps.

I rarely use all 3 flap settings (45deg ?) on landing in the cherokee because trying to takeoff or go-around is not recommended nor fun...it really is a brick at times with that 3rd setting of flaps.
 
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Yes Granby. I don't have a copy of that POH handy, but does it differentiate above 5000ft DA?
 
Oldmanb777 said:
Yes Granby. I don't have a copy of that POH handy, but does it differentiate above 5000ft DA?
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“Take offs are usually made flaps up but under difficult conditions....flaps 25 deg”.

the difficult conitions include short or soft field. I don’t think Piper or many other GA manufacturers considerd high altitude 50+ years ago, most companies were at sea level....In fact the performance charts only go to 5000 ft on my 1969 model. Hence I’m illegal just sitting in the hangar.

Update: I prefer 10deg, the first setting of flaps on the cherokee.
 
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For the Bonanzas (from the BPPP program): "In general, at heavy weights, using flaps for takeoff results in a small, but noticeable improvement in both ground roll and obstacle clearance distances. At light weights, using flaps makes very little difference regardless of density altitude."
 
I’ll admit, aerodynamics is not my expertise.

Flaps reduce stall speed, so lift is also generated earlier in the roll.

Is that not applicable at 9k ft? Do the wings know what the mill up front is doing? It’s all about airspeed I think.
 
:') I would think you are right. I think i have disposed of most of my old POH's. My own thinking is that flaps are to get you off the ground sooner. So short or soft field, contaminated runways etc. Depending on the aircraft I want the most energy leaving the runway that I can reasonably get. No i don't need (or want) cruising speed on the runway, but just like gusty winds and such a bit of extra speed is extra energy and eliminates some pucker factor. I can see that in this case you may not have enough power to overcome the drag created by 25 degrees of flap.
 
Oldmanb777 said:
I'm opening this up for discussion, as its been bugging me. Hopefully a good civil discussion where we can all think about what we are doing and learn something.
I was recently at a fly in where density altitude is a factor. as well as mountainous terrain. I saw many pilots taking off using flaps. So I opened up discussion with a couple pilots standing around. Why would you use flaps on take off in this conditions? I would guess DA at about 9000 ft at the time. So what would be your performance limits? Simply engine power. Adding flaps creates lift, but lift creates drag and you need power to overcome that drag, and you don't have excess power. The runway is long here, 5000 ft I think. But suffice to say it's much longer than you need. No 50 ft " obstacle. No runway clutter, its clear and dry, good quality pavement. Lets say your average normally asperated SEL airplane.
So would you choose to use flaps for take off, or, use the available runway to make a no flaps take off? Hopefully I described the scenario accurately. Now open for discussion, please be civil.
Click to expand...

With high DA, you effectively have a short runway. I would use short field take off techniques. If that calls for flaps, then that's what I would do.
 
Reducing stall speed probably isn't a factor in this case, since you have plenty of runway.
Once you break friction with the ground you are adding drag through lift. Or trying to climb. I think climb will be hampered compared to a no flaps T.O.
 
You may not have the power to overcome your short field flap setting. In this case you are not runway limited. So I don't think short field technics would be appropriate. Your ground speed vs indicated airspeed is going to be very different. But again you are not runway limited. You are power limited. You need to improve the power to drag equation. Does that make sense?
 
This discussion is very airplane specific. What may be ‘correct’ for one, may not be the case with another.

There’s a fair bit of difference from flaps 10 to say flaps 25. I hate to get to absolutes, it depends. Then we get to all the performance & density altitude issues. That next plane may be lightly loaded too.

In my case, I’m mostly using flaps 10.
 
murphey said:
“…In fact the performance charts only go to 5000 ft on my 1969 model. Hence I’m illegal just sitting in the hangar.
Click to expand...

Perhaps meant partly tongue in cheek and I apologize for a bit of thread drift, but I was discussing this with someone the other day here in AZ.

Is there actually any regulation prohibiting extrapolating responsibly, like using a Koch chart, beyond the boundaries of the performance tables?

Clearly as PIC one must do so safely; however, I haven’t seen anything suggesting it is actually contrary to regulation.
 
It depends a lot on the airplane, type of flaps, and how much actual excess HP is available. For Cessna’s with Fowler flaps, the 1st 10 degrees of flaps actually increases the effective wing area. Granted for most 172’s Cessna says the advantage of using flaps will get you off the ground sooner but the advantage is lost by the time you reach 50 feet. However about 1979 Cessna changed this recommendation and recommends flaps for Max performance Takes-offs in 172’s. 182’, 206’s seem to benefit more from the flaps.

If you look at the aerodynamics you will see that Vx increases with Altitude and Vy decreases with altitude until they meet at a climb rate of about zero. Generally the penalty for going a bit slow is a bit more than the penalty for going a bit fast, so you are better to error on the fast side a bit(maybe 5kts).

For clearing an obstacle your best bet is do what the manual recommends. But better plan is start with the manual and do some experimentation on your own to see what you and your airplane can consistently achieve, Because for a lot of these airplanes the manufactures simply did test or produce data for 10,000ft DA take-offs. Even if they did, you and your airplane may get different results. This is why you ask 10 different pilots what the best technique is, you will get at least 5 if not 15 different answers, because it all depends on the equipment and experience.

As an example, this week we will be using a 180hp Scout with 1 pilot and 1/2 fuel to tow 900lb gliders off at a density altitude of probably 9000feet or higher. Try figuring out what the runway required or best technique for that is? As I recall I think the manual only shows a max gross take off distance at sea level.

Brian
CFIIG/ASEL
 
The mechanics of flight don’t change, just the density of the air.

Look at the problem from a different angle: At sea-level on a standard day but loaded to max gross weight, would you do anything in the cockpit differently on takeoff? I’d say no (other than sweat bullets as the mid-field marker crawled past and the end of the runway raced toward me). Flight dynamics haven’t changed, just the conditions.

I suppose you could make the case that if you had manual flaps, then popping a notch right before rotation would help you get up to speed & therefore airborne a little sooner. But electric flaps wouldn’t react quickly enough to give lift that “pop” and the extra distraction interferes with the loud praying in the cabin.
 
^^^ sounds like a real thriller. Yes certainly different planes will have different experiences. But for most normally asperated SEL's its similar enough.
 
Yes proper leaning is huge. But over coming the additional drag of flaps is also huge. I believe in most SEL airplanes the first notch or 10 degrees is probablymostly lift and not a lot fo drag, but still drag. And you leave the runway with less energy and need to overcome that on climb, when slow and generating the most drag.
 
Clip4 said:
The only thing I can think of is the flaps were allowing the pilot to break friction with the ground earlier and allowed more runway for acceleration to Vy in ground effect.
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Depends on the airplane if course but, exactly. One of the goals at very high density altitudes is to get the wheels off the ground. Many use and teach what is essentially a soft field takeoff configuration. Get into ground effect and accelerate to climb speed there.
 
sourdough44 said:
I’m thinking proper leaning for takeoff can be a larger factor.
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No one thing controls. Leaning properly is essential or you might not even get rolling. But even proper leaning does not affect the reduction of horsepower and the loss of airfoil efficiency on wings and prop in high D-Alt conditions. Appropriate technique for conditions is just as important.
 
murphey said:
“Take offs are usually made flaps up but under difficult conditions....flaps 25deg”.

the difficult conitions include short or soft field. I don’t think Piper or many other GA manufacturers considerd high altitude 50+ years ago, most companies were at sea level....In fact the performance charts only go to 5000 ft on my 1969 model. Hence I’m illegal just sitting in the hangar.
Click to expand...
You’re not illegal.
 
midlifeflyer said:
Many use and teach what is essentially a soft field takeoff configuration.
Click to expand...

I’d say this is wrong, at least as I understand soft field takeoffs.

Mine involved stick all the way back to get the nosewheel out of the muck as soon as possible, then hold a nose up attitude to take the weight off the mains. Both of these things add a lot of drag, and are not what you want or need at a paved, high DA runway.

My technique would be no flaps and to accelerate on the runway to Vx or Vy as appropriate, then briskly rotate so as to maintain those speeds. In general, I would not use any flaps, though there may be certain aircraft that call for flaps. In that case I would get rid of flap ASAP, since I think they can only hurt climb rate overall - though I’m not as certain about climb angle.
 
Nice discussion. thinking are caps on. Thanks. My own thinking is (obviously). I want as much energy as I can get (within reason) leaving the runway. Flaps are to correct for runway limitations. So short field, soft field, contamination , or the proverbial 50 ft obstacle. Drag is mostly produced as you create lift,very little drag accelerating parallel to the runway. So as you rotate to climb, drag increases thus making it harder for the powerplant to overcome that and accelerate you to comfortable climbing speeds. At high D.A's you don't have excess power to do that. Climb will be negatively effected, of course. Initial climb will be even worse with less speed and more drag. There will come a point where drag will be greater than the available power to overcome it, and you will "mush" right back into the ground. Maybe ground effect will slow the inevitable. So, using the available runway to accelerate to a better (more energy) flying speed makes sense to me.
 
A tip from the Old Guys when I was in the National guard. Before setting flaps: Ailerons full deflection. Run the flaps down to match the down aileron. The reasoning is that the engineers that designed the crate spent some time figuring just what the control surface up-down, left-right limits should be. The stops are set so that full down aileron is the best compromise between lift/drag.
 
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Another datapoint occurs to me. It turns out that glider polars, which basically represent the lift / drag ratio for a ship, do not depend on DA. So that suggests also that the DA should not affect the choice discussed here.
 
PeterNSteinmetz said:
...Is there actually any regulation prohibiting extrapolating responsibly, like using a Koch chart, beyond the boundaries of the performance tables?

Clearly as PIC one must do so safely; however, I haven’t seen anything suggesting it is actually contrary to regulation.
Click to expand...
91.9(a) says we have to comply with the operating limitations in the flight manual, but performance tables are generally not found in the Limitations section of the manual. Many years ago, I was in an online discussion with an assistant FAA administrator, and he agreed with me that information that is not in the Limitations section does not count as limitations (from a regulatory point of view).

§91.9 Civil aircraft flight manual, marking, and placard requirements.

(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.
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FastEddieB said:
I’d say this is wrong, at least as I understand soft field takeoffs.

Mine involved stick all the way back to get the nosewheel out of the muck as soon as possible, then hold a nose up attitude to take the weight off the mains. Both of these things add a lot of drag, and are not what you want or need at a paved, high DA runway.

My technique would be no flaps and to accelerate on the runway to Vx or Vy as appropriate, then briskly rotate so as to maintain those speeds. In general, I would not use any flaps, though there may be certain aircraft that call for flaps. In that case I would get rid of flap ASAP, since I think they can only hurt climb rate overall - though I’m not as certain about climb angle.
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You may think it wrong. I've done and taught it. It's modified - the goal is off the ground, not keeping the nosewheel out of the muck - but in many ways the same.
 
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midlifeflyer said:
You may think it wrong. I've done and taught it. It's modified - the goal is off the ground, not keeping the nosewheel out of the muck - but in many ways the same.
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Whatever works for you.

I would consider a takeoff from a paved, high DA airport to be analogous to a short-field takeoff. I did not pull my recommendation out of thin air (as it were)!

The FAA take is pretty darn close to mine, so I think I’m on firm ground:


“Takeoff Roll

Taking off from a short field requires the takeoff to be started from the very beginning of the takeoff area. At this point, the airplane is aligned with the intended takeoff path. If the airplane manufacturer recommends the use of flaps, they are extended the proper amount before beginning the takeoff roll. This allows the pilot to devote full attention to the proper technique and the airplane’s performance throughout the takeoff.

The pilot should apply takeoff power smoothly and continuously, without hesitation, to accelerate the airplane as rapidly as possible. Some pilots prefer to hold the brakes until the maximum obtainable engine revolutions per minute (rpm) are achieved before allowing the airplane to begin its takeoff run. However, it has not been established that this procedure results in a shorter takeoff run in all light, single- engine airplanes. The airplane is allowed to roll with its full weight on the main wheels and accelerate to the lift-off speed. As the takeoff roll progresses, the pilot must adjust the airplane’s pitch attitude and AOA to attain minimum drag and maximum acceleration. In nose-wheel type airplanes, this involves little use of the elevator control since the airplane is already in a low drag attitude.”

(Bolded mine)

Source: Airplane Flying Handbook
 
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FastEddieB said:
Whatever works for you.

I would consider a takeoff from a paved, high DA airport to be analogous to a short-field takeoff. I did not pull my recommendation out of thin air (as it were)!

The FAA take is pretty darn close to mine, so I think I’m on firm ground:


“Takeoff Roll

Taking off from a short field requires the takeoff to be started from the very beginning of the takeoff area. At this point, the airplane is aligned with the intended takeoff path. If the airplane manufacturer recommends the use of flaps, they are extended the proper amount before beginning the takeoff roll. This allows the pilot to devote full attention to the proper technique and the airplane’s performance throughout the takeoff.

The pilot should apply takeoff power smoothly and continuously, without hesitation, to accelerate the airplane as rapidly as possible. Some pilots prefer to hold the brakes until the maximum obtainable engine revolutions per minute (rpm) are achieved before allowing the airplane to begin its takeoff run. However, it has not been established that this procedure results in a shorter takeoff run in all light, single- engine airplanes. The airplane is allowed to roll with its full weight on the main wheels and accelerate to the lift-off speed. As the takeoff roll progresses, the pilot must adjust the airplane’s pitch attitude and AOA to attain minimum drag and maximum acceleration. In nose-wheel type airplanes, this involves little use of the elevator control since the airplane is already in a low drag attitude.”

(Bolded mine)

Source: Airplane Flying Handbook
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I agree with this. For the particular planes I fly, the PoH recommends 10° flaps for short field.
 
Even in planes that have solid DA charts in the poh, there is still value in the old Koch Chart. For older planes with no good DA info, Koch is indispensable. Personally, I do a lot more sober reflecting when I have to stop to calculate “TO run increases 300% & Rate of Climb is reduced by 70%,” than when I use a look up table.

a side benefit Of Koch is that you also get a feeling for how squishy the hard numbers found in some poh really are. “Maybe I should plan for 400% increase in TO run & 80% decrease in ROC, just in case.”
 

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I flew with a guy that told me to keep 10 degrees from sea level up to cruise at 5K. I said ok sure. I did find that I got to cruise alt quicker and a shorter distance. But I have an 182 so I can keep those flaps out with a higher airspeed.

I see know harm in keeping the 10 degrees out. Remember the basics, flaps to crate more lift.
 
I found that using 10 degrees of flaps for almost all take offs in my Commander worked out well. I had a turbocharger so high altitude was less of an issue - but noted that the engine handbook called for a bit of leaning at high DA... the biggest issue there was keeping the carb inlet temp within limits.

The one thing to be aware of (regardless of power available or flaps) is the additional runway length required for takeoff - the first few times will seem unusual if you're use to low altitude airports.
 
PeterNSteinmetz said:
Another datapoint occurs to me. It turns out that glider polars, which basically represent the lift / drag ratio for a ship, do not depend on DA. So that suggests also that the DA should not affect the choice discussed here.
Click to expand...

Lift over drag maybe the same, but you are not power limited in a sailplane, you already have the energy you need to fly. You are not trying to leave a runway with enough performance to climb away from the ground.
 
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