The International Cessna^® 170 Association

I'm at the end of a 7 day, 16 hour-a-day work week, but I'll lend my advices for what they are worth when I get some time. I fly out of a 650' strip with trees at the end and I'm still here...

Joe

integritywood wrote:My instructor taught me after lift off, level and accelerate to vx then climb at vx till clear. He said the old bush pilots around advocate lift off, accelerate in low ground effect as long as possible, then pull up just in time to clear obstacle. The theory being you waste less energy accelerating in ground effect so have more stored energy by the time you reach the obstacle? There's gotta be some opinions on this, maybe a good discussion.

If the question is what's the technique for maximum performance clearing an obstacle, then it's the Vx method. Anything else gets you to the obstacle with less altitude. The less you need max performance the more you can play with different techniques, like trading altitude for extra speed. Fine, but my first priority is to make damned sure I clear the obstacle! If I were a bush pilot, the advice "just pull up in time to clear the obstacle" would have more meaning, but it's actually kind of funny, it's like saying always take off with just enough fuel.

.......If the question is what's the technique for maximum performance clearing an obstacle, then it's the Vx method. Anything else gets you to the obstacle with less altitude.

Well, respectfully, doesn't that depend completely on where that obstacle is located?

Vx is only the best speed to use when the obstacle is so close, that the angle required to climb at Vy won't clear it.

In every other scenario, Vy will get you over the obstacle faster and clear it with more altitude than Vx. Guaranteed!

WSHIII wrote: In every other scenario, Vy will get you over the obstacle faster and clear it with more altitude than Vx. Guaranteed!

Not so.

Vx gives you the best height in the shortest forward distance however will require more time.
Vy gives you the best height in the shortest time however will require a longer distance.

So,
If the obstacle is 1000 feet in front of you, Vx will take more time to get over it but you will be higher over it.
If the obstacle is 1000 feet in front of you, Vy will take less time to get over it but you will be not as high over it.

http://www.flyingmag.com/technique/tip-week/vx-vs-vy

WSHIII wrote: In every other scenario, Vy will get you over the obstacle faster and clear it with more altitude than Vx. Guaranteed!

That is an incorrect statement. Vx is always going to give you the best ANGLE of climb (which is our concern with a fixed obstacle) Vy gives you the best RATE of climb over time - not distance.

David and Karl

I was in too big a hurry walking ut the door and I could have certainly worded it better. What I was trying to convey was, given the choice of either option, Vx or Vy, Vy will give your the better performance, higher altitude, over the same amount of time.

And of course I agree that Vx will eventually clear the obstacle at a higher altitude.

While you guys are sitting there looking at the air speed meter deciding whether you're going to use Vx or Vy you'll probably run into the tree. Have you considered rolling the airplane upside down so the landing gear will clear the obstacle?

WSHIII wrote:...The point Deakin is trying to make, and I'll grant you he doesn't provide many details on why in this article, if the circumstances will allow you, accellerating too and climbing out at Vy is a much safer, better way, than what the FAA advocates.

It's all about risk management, and climbing out at Vx, while hanging on the prop, has more risk involved than climbing out at Vy with excess energy stored in the airplane. If you lose the engine climbing out at Vx, the airplane is going to stall almost immediately and while climbing out at 100 feet, its game over. While climbing out at Vy, and excess energy stored in the airplane, maintaing postive control of the aircraft after an engine failure is less critical. I think we'd all agree, losing the engine in either case is not going to be fun but, given the choice, a controlled crash almost always has a better outcome than stalling out and going straight in from a 100 feet.

In broad general terms, it's better, safer, to clear the obstacle at 10 feet at Vy, then to clear it by 100 feet at Vx.

I think you might imagine yourself to be inside Deakin's mind a bit more than warranted.

There's a well-established and respected company (APG) who calculates alternative takeoff/obstacle clearance SIDs based upon specific airport/aircraft performance numbers. Their primary thesis revolves around the standardized FAR 25 formula of the end-of-takeoff-distance being calculated at 35' above the surface, and projecting aircraft climb-performance from that point towards the obstacles. (For this discussion, it makes no difference whether we begin calculations at 35', or whether we begin calculations at 50', or at the point of lift-off such as commonly used operationally with light planes....the after takeoff flight-path climb to clear obstacles is what we are talking about.)
The chief advantage of using their SIDs is found in the fact that ...rarely... does the actual aircraft departure-path begin at a point 35' (or 50', etc.) above the end of the runway.... but, in-fact it begins considerably before the end of the takeoff-run-available, which is that point where the aircraft actually achieves 35' above the surface. In other words, the climb-path of the aircraft is actually parallel to, but begins significantly prior to the end of the runway/takeoff-distance-available....and provides much greater obstacle-clearance performance than the published procedures. Their product-performance is well established and accepted.
Those procedures are based upon the mfr's recommended climb speeds and procedures which, if correlated to Cessna 170 aircraft, will be found to be Vx. In other words, "zoom/climb" will be less safe and provide less obstacle clearance than accellerating to Vy (excess speed/equating to excess-drag and wasted climb-opportunity) then aiming to barely clear the obstacle.
Put another way: If you have "room-to-zoom", I.E., to accelerate beyond best-angle to Vy, and then use your calibrated eyeball to convert excess-speed-to-altitude in order to just-clear-the-trees....then you weren't really obstacle-clearance-challenged to begin with. You threw away your advantage to trade for unnecessary excess-speed.

Any hypothetical engine falure imagined will carry a risk-of-technique as to how quickly the pilot can convert to a landing attitude. If you imagine yourself to be so shocked/amazed at the unanticipated failure that you will be paralyzed-into-inactivity sufficient to require excess speed to allow yourself time to mentally recover from the shock and regain aircraft control....then your entire excersize has been futile indeed,if not fatally-flawed from the moment of brake-release!
(After-all...what were you thinking? Were you not performing this entire task with the ready-knowlege there was a short takeoff/obstacle distance involved? Are you finding yourself with a windshield full of trees and windmilling-prop, suddenly-and-completely unaware of what this was all about? )

The comparison to the techniques used in helicopters may not apply. Last time I saw one.... helicopters seemed to be able to land in pretty small spots, and with low forward-speed ...compared to airplanes.

Mr. Deakin's article was intended to sell prop-wash and headsets over at AvWeb, and wow readers with stories about 747s and old warbirds and he's good at that. I like to read his stuff and I agree with many of his engine operating articles.
But as for flying little Cessnas out of soft-short fields when it really counts, I plan to keep to the mfr's test pilots techniques and data.

gahorn wrote:The comparison to the techniques used in helicopters may not apply. Last time I saw one.... helicopters seemed to be able to land in pretty small spots, and with low forward-speed ...compared to airplanes.

As George has astutely observed, helicopters can indeed land and depart in a much shorter distance than most airplanes, certainly the 170 model. I can think of no technique used in a helicopter that directly applies to an airplane except one and it is an important one. I always plan to miss an object.

The difference is that I, and helicopter pilots in general, don't really care by how much we miss the object. And helicopters operating at the edge of the power available for more departures than not, as use to be the case when lots of us were trained, we are accustom to just missing objects. At least much more than airplane only pilots in my experience. 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. So I am much more comfortable with the zoom and miss technique rather than the climbing struggle to miss technique.

BTW an interesting fact. Most helicopter pilots I've meet, myself included, are scared of heights. Most of us would rather be below 1500ft AGL than above.

George,

gahorn wrote:

He denounces takeoff configurations which begin the roll with unnecessary drag...then recommends full flaps application for the shortest possible takeoff rolls.

In another contradiction, he seems to endorse the idea of "popping" flaps immediately prior to the end of the takeoff roll and/or obstacle. How does that compare with the full flaps takeoff he recommended earlier? And what about the excess speed (which equates to unobtained/lost altitude) using that technique?

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: Real Short Fields,

What's the best technique for really getting off short? As usual, "it all depends." To get the absolute minimum ground roll, you need to get it airborne at the slowest speed possible, just above the stall, with the flaps set to the position that gives the greatest lift, but without so much drag the airplane won't accelerate in level flight, in ground effect. This can vary from airplane to airplane.

For many of the common GA airplanes, at sea level, that implies full flaps, and if you can't "pop" them at the last minute, they should be set before starting the takeoff run. We could get really fancy and time them, starting the extension at just the right time so that they reach full down precisely at the point of liftoff, 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?

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!

delete

bigrenna wrote: Of course the engine came with a Skywagon....

There is a down side to everything, I guess

WSHIII wrote: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.

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.