Take-off to clear obstacle. Whats your method?

[WSHIII]

Helicopters tend to glide power off at a much higher rate than airplanes when the engine goes quiet, and one of the few things we can do is have energy stored in excess airspeed.

"Energy stored" translates over to airplanes as well. The greatest pilot of all time, IMO , Bob Hoover called it "putting money in the bank". He had it right too, it's ALL about "energy management".

I'll take missing the obstacle by less with more energy stored in airplane every time.

I think like Deakin, Hoover would agree!

[bigrenna]

delete

[bagarre]

Of course the engine came with a Skywagon....

There is a down side to everything, I guess

[GAHorn]

George,
...This is but one example where I think you've misrepresented what Deakin actually said and missed the point that he was making.

What Deakin actually said,

Deakin wrote: ...if you can't "pop" them at the last minute, they should be set before starting the takeoff run. We could get really fancy ... starting the extension at just the right time ...but that's hardly practical!

The airplane should be accelerated in a level (no lift) attitude to the minimum flying speed, then the airplane should be aggressively lifted off into ground effect, and no higher, because if any ground effect is lost, the airplane will stall. This is not good.

Again, respectfully, I don't see where he's contradicting himself at all. It seems crystal seems clear to me that he advocates, popping the flaps if you can,( to help reduce drag while you accelerate) and then using full flaps to give you the absolute shortest ground roll possible.

What's wrong with that?

I didn't "misrepresent" him. I know how to read. In this response, I shortened the verbage to the essential elements of the above sentences you quoted (again), in order to get to the "meat".

Deakin cannot have it BOTH ways! If the takeoff run is made with full flaps, then excess drag is the penalty during the run...a run which will necessarily be increased over a run made with lesser flaps.
If he "pops" them (in those airplanes that will allow that) then he cannot ignore the increased DRAG those full flaps present during the climb to the obstacle! (And I hope it's obvious what retracting the flaps will do to the climb-performance while still approaching an obstacle.)
The combination of techniques will not result in a reduction of the total distance required by the separate events.

And lastly, his article only makes suggestions that rely largely on the readers imagination. They have not been repeatedly demonstrated under controlled conditions so as to provide documentation...
Scientific method demands that each experiment be exactly held to the same process....something that only fills the readers imagination in an article that has not been supported/documented by strict, repeated flight-test. If it WERE possible to prove shorter takeoffs/obstacle-clearances....then why haven't the manufacturers of airplanes used those techniques in order to impress the flying public with their products' vastly-improved resultant performance? You'd think that would make one heck of a sales-tool, wouldn't you. But maybe mfr's engineering test-pilots don't possess the same skill-sets of aviation-writers.

[WSHIII]

George,

Gahorn wrote,

Deakin cannot have it BOTH ways! If the takeoff run is made with full flaps, then excess drag is the penalty during the run...a run which will necessarily be increased over a run made with lesser flaps.

I believe that is not always true and mostly wrong. Using your reasoning of "excess drag is the penalty", then using no flaps would produce a shorter ground roll than using 20 degrees of flaps. Right? Is that your position?

Of course every airplane is different but in general, the increase in lift gained and reduction in stall speed with full flaps has a greater effect on shortening the ground roll than the additional drag does increasing it. I've found to be true, at least in the airplanes I fly often. Deakin agrees.

Gahorn wrote,

If he "pops" them (in those airplanes that will allow that) then he cannot ignore the increased DRAG those full flaps present during the climb to the obstacle! (And I hope it's obvious what retracting the flaps will do to the climb-performance while still approaching an obstacle.)

Respectfully, he's not ignoring anything, your trying to create another straw man argument with something he never wrote. When Deakin writes about using "full flaps", it is ONLY with regard "to get the absolute minimum ground roll". Period! He has even underscored "ground roll" for emphasis. He addresses climbing out over nearby obstacles later, in the next paragraph, entitled "Obstacles". IMO, Your mixing and confusing two techniques with regard to minimum ground roll and high performance climb out, when Deakin is breaking them down seperately and explaining what works best individually.

Gahorn wrote,

The combination of techniques will not result in a reduction of the total distance required by the separate events.

Well I agree, I think , however, as best as I can tell Deakin never made a claim to the contrary.

[GAHorn]

George,

Gahorn wrote,

Deakin cannot have it BOTH ways! If the takeoff run is made with full flaps, then excess drag is the penalty during the run...a run which will necessarily be increased over a run made with lesser flaps.

I believe that is not always true and mostly wrong. Using your reasoning of "excess drag is the penalty", then using no flaps would produce a shorter ground roll than using 20 degrees of flaps. Right? Is that your position? ....

That implication is insulting. If that were my "position".... well, never mind..

However, I've flown airplanes with flaps more than once or twice, and about a half-century ago I learned that the early/lesser application of flaps provides more lift-than-drag...and that larger/fuller application of flaps eventually goes beyond the benefit of lift-over-drag and provides more drag than is beneficial for takeoff performance.
But maybe they don't explain that adequately anymore in these days of "glass" cockpits and rocket-powered parachutes for those who need saving from themselves.

Anyway, I'm rapidly losing interest in playing "tit-for-tat" with you in what appears to be your admiration for Mr. Deakin and his hypotheses in this matter. The FACTORY recommends 20-degrees flaps for the shortest takeoff run (except at density-altitudes exceeding about four-thousand MSL, after which they state ground-runs will be greater with the use of flaps.) Guess why. (Hint: lift vs drag)
I agree with the factory for the reasons I've already stated. Why you seem to insist on interpreting Mr. Deakin's apparent disagreement with the factory test pilots documented results (following FAA standardized flight performance testing) ...requires the sort of imagination I spoke of previously.
I don't know your experience-level and don't wish to make this into something it's not, but if you don't easily see that a full-flaps takeoff run normally requires greater distance than a takeoff run using recommended takeoff flap settings, then we're going around in useless circles due to a basic lack of aerodynamic understanding.

I take it you've been listening-in on the latest national gun-debate issues and that's why you've mentioned "straw man"...a term which has nothing to do with this discussion, and certainly does not apply to anything I've mentioned in this discussion.
Mr. Deakins' opinon-article finds fault with the FAA "short/soft field takeoff" when combined with "obstacle clearance" takeoffs. There's some validity to what he says, i.e. that the best short-field technique will not likely result in the best obstacle-clearance technique...(because the drag of flaps..any flaps... hurt climb) ..and that's what I've also said in this discussion, except that I find it ridiculous to ignore obstacles when discussing short field performance. By definition, (it seems to me), an obstacle exists or it's not a short field. If that were not the case, then we'd only be talking about taking off from a flat, level mesa with the ground plummeting downward immediately after takeoff roll is completed...not a very realistic situation, not likely found at public airports. (Even Sedona, AZ has a perimeter fence that requires a four-foot climb!)
Like the by-line of one of our participant-members suggests: You cannot appreciate a short-field without a tree-line.

I agree with his (Deakins) observation that a "soft" field can be a different matter than a "short" field, but both may exist simultaneously....and still have an obstacle to clear after lift-off! It's the problem of combining techniques that face the real-world operator, and I don't think FAA has gone too far wrong in attempting to standardize the process of documenting aircraft performance using a standardized procedure. In fact, I don't see how one would accomplish it otherwise.

[akclimber]

George,
Well, there are the short gravel/sand bars that we operate out of that require ground run with 20 degree flaps and then popping full flaps to just get in the air and then slowly accelerate while milking the flaps off in ground effect over the water. I've tested this and it appears that I could always get in the air quicker popping 20 to 40 degree flaps than trying to rotate with just 20. I haven't tried starting the ground roll with 0 flaps as the johnson bar is too far down and it would require temporary loss of over the nose visibility and obstacle avoidance (logs, etc.).
I do have a question as to the increased ground roll with any flaps above 4k density altitude. Can you or someone explain to me why that is?
And not just because it's in the POH...

[pdb]

George,
Well, there are the short gravel/sand bars that we operate out of that require ground run with 20 degree flaps and then popping full flaps to just get in the air and then slowly accelerate while milking the flaps off in ground effect over the water. I've tested this and it appears that I could always get in the air quicker popping 20 to 40 degree flaps than trying to rotate with just 20.

I agree but I think this is a special situation that is separate from the original question regarding best climb over an obstacle which I think presumes, for POH purposes, that we are taking off from a paved strip.

The problem of how to get airborne the quickest from a viscous surface, water on floats, snow on skies, or mud, gravel, or tall grass on land, presents a special circumstance wherein I believe popping the flaps does help us get airborne sooner where we might otherwise stay stuck longer.

Once we are airborne, the normal rules apply once we are above ground effect.

[WSHIII]

gahorn wrote,

I don't know your experience-level and don't wish to make this into something it's not, but if you don't easily see that a full-flaps takeoff run normally requires greater distance than a takeoff run using recommended takeoff flap settings, then we're going around in useless circles due to a basic lack of aerodynamic understanding.

My experience level? Hey George, here's a hint for you, John Deakin wrote that article! He's the one who made the original claim that using full flaps gives you the shortest possible ground roll. And while I agree with him and have found that to be true, its his article, his statement.

You want to question someone's " basic lack of aerodynamic understanding" , then why don't you question his! He's only got 35,000 hours in everything from Bearcats to 747's, along with being a DPE and able to give check rides in 43 different aircraft. How about you George? Lets hear about all your vast experience and what makes your opinion more insightful than Deakin.

[GAHorn]

George,
Well, there are the short gravel/sand bars that we operate out of that require ground run with 20 degree flaps and then popping full flaps to just get in the air and then slowly accelerate while milking the flaps off in ground effect over the water. I've tested this and it appears that I could always get in the air quicker popping 20 to 40 degree flaps than trying to rotate with just 20. I haven't tried starting the ground roll with 0 flaps as the johnson bar is too far down and it would require temporary loss of over the nose visibility and obstacle avoidance (logs, etc.).
I do have a question as to the increased ground roll with any flaps above 4k density altitude. Can you or someone explain to me why that is?
And not just because it's in the POH...

The technique of "popping flaps" at just the moment of reaching flaps-deployed-flying-speed has validity for reducing takeoff run by eliminating the effect of the drag produced by deployed flaps while accelerating. In other words, the ability to get airborne with the additonal lift of flaps can be used without exposure to the combined-drag of increased ground-run in soft surfaces AND the drag of the flaps. The penalty of such technique is that climb-performance to clear an obstacle suffers. Of course, if there is no obtacle in the area being used as ground-effect (for the purposes of accelerating to a flap-retraction-speed, and further climb), then that's the beauty of that technique. It's drawback is the added complexity of the manuever and the risk of a less stabilized initial climb path.
It might be helpful to define a few terms.

Takeoff RUN is the distance the wheels are actually on the ground during the take-off, i.e., from the start of the takeoff roll until the tread leaves the earth's surface.
Takeoff DISTANCE is the distance from the beginning of takeoff roll until the aircraft is above the takeoff surface a specified altitude, i.e, in most light aircraft performance charts...the accepted altitude is 50 ft. Therefore the total takeoff distance includes the takeoff RUN plus the climb to 50 ft AGL.
Note that an obstacle may be higher than 50' AGL, in which case the takeoff distance plus climb to obstacle-clearance may be longer than that minimum distance specified in the takeoff performance charts (which in the case of the C-170 charts available, only goes as far as 50' AGL.)

As for the second-part of your question: This is a function of the same problem of taking off with flaps already deployed... It should be obvious that increased density altitude will degrade aircraft climb-performance, partly thru engine performance losses but also thru lifting losses from "thinner air" if-you-will.
Since deployed-flaps introduces induced-drag...and since the aircraft acceleration-performance suffers ever-increasingly from higher density altitudes... it was found that the advantage of additional lift produced by flaps was completely negated by the drag-penalty of deployed flaps at 4K and higher density altitudes.
(An extreme illustration/example of this might be to deploy flaps at the aircraf'ts' service ceiling, and observe the impossibility of maintaining altitude due to the induced-drag of flaps. Some low-powered aircraft may not be capable of maintaining altitude in that configuration even at sea-level.)
In other words, the takeoff distance was not shortened by the use of flaps due reduced aircraft acceleration capability combined with increased drag of flaps at higher altitudes.
The total distance to 50' AGL was better by eliminating the drag of the flaps, i.e. zero-flap takeoffs.

[GAHorn]

...My experience level? Hey George, here's a hint for you, John Deakin wrote that article! He's the one who made the original claim that using full flaps gives you the shortest possible ground roll. And while I agree with him and have found that to be true, its his article, his statement.

You want to question someone's " basic lack of aerodynamic understanding" , then why don't you question his! He's only got 35,000 hours in everything from Bearcats to 747's, along with being a DPE and able to give check rides in 43 different aircraft. How about you George? Lets hear about all your vast experience and what makes your opinion more insightful than Deakin.

He's not the one flinging junk around here. I suggest you ask him how many times he's actually made a full-flaps takeoff in a 747 or Bearcat, and if those mfr's recalled their performance charts for re-printing since he's clarified matters for them.
As far as I know, FAA hasn't followed those leads either.

Perhaps you should enlighten them.

[akclimber]

As for the second-part of your question: This is a function of the same problem of taking off with flaps already deployed... It should be obvious that increased density altitude will degrade aircraft climb-performance, partly thru engine performance losses but also thru lifting losses from "thinner air" if-you-will.
Since deployed-flaps introduces induced-drag...and since the aircraft acceleration-performance suffers ever-increasingly from higher density altitudes... it was found that the advantage of additional lift produced by flaps was completely negated by the drag-penalty of deployed flaps at 4K and higher density altitudes.
In other words, the takeoff distance was not shortened by the use of flaps due reduced aircraft acceleration capability combined with increased drag of flaps at higher altitudes.
The total distance to 50' AGL was better by eliminating the drag of the flaps, i.e. zero-flap takeoffs.

I understand what you are saying above, but it's really just restating the POH.

The lift and drag equations show that thinner air acts equally on reducing lift and drag.
Lift = CL× ½ pV^2 × a
Drag = CD × ½ pV^2 × a
where,
CL = lift coefficient (angle of attack)
CD = drag coefficient
a = wing area
V= TAS
p = density of air.

From the above equation if we change the air density (p) the Lift and Drag will be affected equally (V will also change since it is TAS).

The question is how is the change in the air density affecting drag at a different rate than lift.

What am I missing?

[GAHorn]

The major difference (as implied previously) is found in engine/thrust losses.
While the lift/drag are equally affected (illustrated similarly with IAS vs TAS comparisons) it's the loss of engine performance that affects the configuration recommendation. (Higher density altitudes place the deployed-flaps on the back-side of the power-curve.)
And there is no POH for this airplane.

[WSHIII]

Aryana,

That's the most sensible thing I heard here yet.

Deleted

[bagarre]

We now all know what Deakin's experience is, seems pretty impressive, at least to me. George what is your experience? Why should I, or more importantly, why should anyone believe what you have to say over Deakin?

Just because one hasn't crashed yet doesn't mean they're doing it right. And I know airline pilots with that much time that I woudln't let taxi my airplane.

Furthermore, this is a true statement only if the actual field is the absolute minimum-length field from the beginning of the runway to the assumed 50-foot obstacle. Frankly, it's very unlikely that any pilot in all aviation history has ever needed this very peculiar combination.

So, if WE are talking about the best technique to clear a 50 foot obstacle in the minimum forward distance, what is this guy talking about?

Whether or not a pilot in all of aviation history has ever needed this peculiar combination is beside the point. They could have chosen the minimum distance to clear a 100 foot obstacle, the technique would be the same because Vx is (all safety margins aside ) the speed where the maximum height is achieved in the least forward distance. So, if you want to know the MINIMUM distance to clear ANYTHING, the answer will be Vx.

I don't fly that way because I like safety margins and clean under shorts but that's what the conversation is about.