Short Field Takeoffs

[dmccormack]

Wouldn't matter. As it was both Dans added/implied things I did not say. I teach it in person, not on a message board. If you want the alternate, come to 6Y9.

Which Dan? What did I "imply?"

 

[EdFred]

Which Dan? What did I "imply?"

You tell me.

It can't be.

The radius of a turn at 20 won't be the same at 3.

 

[dmccormack]

You tell me.

And that is incorrect how?

 

[EdFred]

And that is incorrect how?

Well if we look at it as a stand alone comment and remove all context, it could be wrong depending on the vehicle we are talking about. I haven't tested, but I am sure there are some sports cars that would have the same turn radius at 20 as they do at 3.

But, since it is not a stand alone comment, and taken in context it certainly looks to be implying that I am making turns at 20. Now, I may actaully be taking turns at 20, but I never said I did, and still am not saying I do.

 

[Dan Thomas]

Well if we look at it as a stand alone comment and remove all context, it could be wrong depending on the vehicle we are talking about. I haven't tested, but I am sure there are some sports cars that would have the same turn radius at 20 as they do at 3.

But, since it is not a stand alone comment, and taken in context it certainly looks to be implying that I am making turns at 20. Now, I may actaully be taking turns at 20, but I never said I did, and still am not saying I do.

Turns at 20 have nothing to do with gyroscopic propeller forces. Those are generated by a turn in any direction, not just by pulling Gs, and can be calculated by using RPM and rate of turn. Radius, speed and abrupt power changes have nothing to do with gyroscopic forces.

Dan

 

[Dan Thomas]

Do all you guys still drive your car with your hands always at 10 and 2?

Many of us fly taildraggers. Do you?

Dan

 

[EdFred]

Turns at 20 have nothing to do with gyroscopic propeller forces. Those are generated by a turn in any direction, not just by pulling Gs, and can be calculated by using RPM and rate of turn. Radius, speed and abrupt power changes have nothing to do with gyroscopic forces.

Dan

What extensive testing did you do without making abrupt power changes, shows that turns at high RPM affect the engine negatively?

 

[kevin7500]

Well, if you think about it, it's much easier to accellerate something that's already moving vs something that's already stopped.

Overlooking the fact that I disagree with this entire statement (A=F/M), I will comment within the boundaries of it. As soon as I release the brakes I am moving. Also, since the force is a result of the props motion through the air, the faster you are going the slower the rate of acceleration will be.

As a base for discussion, say that you are turning without acceleration at 10 knots. The only question to answer is how much distance is required to accelerate from 0 to 10 and at what point along the centerline of the runway are you able to accelerate after the turn.

There isn't any reason to debate the initial point of lift off. The point on the runway to debate is where you are established on centerline and accelerating compared to the velocity achieve by a standing start at that same point. That would be simple enough to accomplish with a speed trap.

 

[dmccormack]

There isn't any reason to debate the initial point of lift off. The point on the runway to debate is where you are established on centerline and accelerating compared to the velocity achieve by a standing start at that same point. That would be simple enough to accomplish with a speed trap.

Exactly.

Pardon the crudity of the drawing....

 

[EdFred]

I don't have a reversing prop to back my airplane up. Maybe I should get a Caravan. My turns are also not that wide.

 

[gkainz]

Dan - good illustration ... what's missing that would really help (me) is ground speed at the 80' mark in both examples.

No dog in this hunt - just curious.

 

[EdFred]

Overlooking the fact that I disagree with this entire statement (A=F/M), I will comment within the boundaries of it. As soon as I release the brakes I am moving. Also, since the force is a result of the props motion through the air, the faster you are going the slower the rate of acceleration will be.

As a base for discussion, say that you are turning without acceleration at 10 knots. The only question to answer is how much distance is required to accelerate from 0 to 10 and at what point along the centerline of the runway are you able to accelerate after the turn.

There isn't any reason to debate the initial point of lift off. The point on the runway to debate is where you are established on centerline and accelerating compared to the velocity achieve by a standing start at that same point. That would be simple enough to accomplish with a speed trap.

I will be at a higher speed 100' down the runway using a rolling start, than I would be from a dead stop. What does being on centerline have anything to do with using the least amount of runway length?

 

[dmccormack]

I don't have a reversing prop to back my airplane up. Maybe I should get a Caravan.

The drawing assumes the space is clear around the runway. When I do an "all available runway" takeoff I taxi slowly and swing the tail around using brake in a nosehweel.

In a taildragger I swing the tail around, just not over the edge of the runway.

 

[EdFred]

The drawing assumes the space is clear around the runway. When I do an "all available runway" takeoff I taxi slowly and swing the tail around using brake in a nosehweel.

In a taildragger I swing the tail around, just not over the edge of the runway.

When you use all availble runway, do you take off aligned with the centerline, or do you take off at an angle and gain an extra 3.8 feet on a 100 x 1300 strip?

I also run my wing over the lights/cones when rolling. But, also don't immediately get on centerline. I use the width of the runway to my advantage as well.

I also do not have differential braking available in my Comanche. Hand brake only, so I can not "lock" a wheel in place and spin around on it.

 

[mantakos]

I will be at a higher speed 100' down the runway using a rolling start, than I would be from a dead stop.

The pertinent questions are:

A ) at what speed are you taxiing onto the runway?
B ) how much distance does it take to accelerate to this speed at full power, from a dead stop?
C ) how much runway do you lose by rolling onto the runway instead of positioning yourself at the very start?
​

The distance you gain is roughly B minus C.

Unless you're seriously hauling ass around that turn onto the runway, then it's not likely that your savings are significant. It just doesn't require much runway to accelerate from a dead-stop to taxi speeds at full power.
-harry

 

[steingar]

Unless you're seriously hauling ass around that turn onto the runway, then it's not likely that your savings are significant. It just doesn't require much runway to accelerate from a dead-stop to taxi speeds at full power.
-harry

Depends on the runway.

 

[RMCN172RG]

In a true short field takeoff the issue is distance down the runway that the aircraft wheels leave the ground.

Using the same airplane standing or rolling start once at full power the acceleration and lift generated at any n airspeed, will all be the same.

The only questions are how close to the end of the runway do you complete the turn? (Relative starting point).

What is your actual forward airpeed at that point and percentage of power.

Where are the aircraft on the runway when the engine reaches full power at the same airspeed? (First coincident point from here performance is the same.)

I don't think you go to full power prior to or during the turn do you?

And we al know that turning wheels on the ground does not help or hinder a take off it is all about airspeed.

REMEMBER THE TREADMILL!!!!!

 

[EdFred]

The pertinent questions are:

A ) at what speed are you taxiing onto the runway?
B ) how much distance does it take to accelerate to this speed at full power, from a dead stop?
C ) how much runway do you lose by rolling onto the runway instead of positioning yourself at the very start?
​

The distance you gain is roughly B minus C.

Unless you're seriously hauling ass around that turn onto the runway, then it's not likely that your savings are significant. It just doesn't require much runway to accelerate from a dead-stop to taxi speeds at full power.
-harry

How often do you fly out of grass strips? B minus C can be huge sometimes.

 

[mantakos]

Depends on the runway.

A "slow" runway surface will extend the takeoff distance, of course, but it will extend all portions of the takeoff roll.

The point here is that as we accelerate, our progress down the runway as a function of time is exponential. We eat up a lot more distance accelerating from 50kts to 55kts than we do from 0kts to 5kts. So our rolling start attempts to "bank" some of that initial acceleration, but the initial acceleration from a dead stop to that taxi speed is a relatively small portion of the overall takeoff roll.
-harry

 

[el con]

Turns at 20 have nothing to do with gyroscopic propeller forces. Those are generated by a turn in any direction, not just by pulling Gs, and can be calculated by using RPM and rate of turn. Radius, speed and abrupt power changes have nothing to do with gyroscopic forces.

Dan

Dan if this is the case then the FAA is condoning excessive engine wear by instructing pilots to do soft field take off?
The method we are discussing does no more damage to the engine than taxi'ing to your starting point and you spin your tail dragger around and stop, on grass you may apply more force to your plane than we do with a smooth accelerating turn. I see many tail draggers "bliping" the throttle to make the tail go where you want it, are you doing damage?
Also one thing not brought up, we feel our method is safer as we are facing any incoming traffic just prior to our turn and departure, 99% of the planes today do not have a bubble canopy, so sitting there with your knee's knocking checking everything ,back facing potential traffic, and knowing the radio's are used less in remote area's where short fields are , well you create a hazard for yourself .

 

[mantakos]

How often do you fly out of grass strips? B minus C can be huge sometimes.

What we're trying to quantify is the proportion of the takeoff roll that's consumed in getting from a dead stop to our "rolling start" speed.

So propose a "rolling start" speed and a liftoff speed, then estimate, for us, what percentage of the total takeoff roll you think is consumed by accelerating from a dead stop to that "rolling start" speed.
-harry

 

[steingar]

A "slow" runway surface will extend the takeoff distance, of course, but it will extend all portions of the takeoff roll.

The point here is that as we accelerate, our progress down the runway as a function of time is exponential. We eat up a lot more distance accelerating from 50kts to 55kts than we do from 0kts to 5kts. So our rolling start attempts to "bank" some of that initial acceleration, but the initial acceleration from a dead stop to that taxi speed is a relatively small portion of the overall takeoff roll.
-harry

The equations become too complex for my tiny molecular geneticists brain, but at higher ground speeds where you're chewing up more turf your downward vector decreases due to lift off the wing. Hence the initial acceleration is a different beast from the acceleration is a different beat than that from 50 to 55 knots.

I genuinely don't get the argument. You've got to swing round to get on the runway. You can do it by stopping, holding the brakes, and spooling up, or you can just spool up, take the curve at your best speed and pour on the coals on the runway. Either way your aircraft is going to follow the same course, unless your aircraft has a reverse gear.

 

[N801BH]

Ok. Hopefully to reduce the what if's..Lets all analyze the Bo taking off and see who says what.

http://www.youtube.com/watch?v=wE6_4vW1fxQ


I hear a 'on the roll run up' before he makes his U turn so that should put to bed the ' is the engine ok' topic.

He swings wide to allow for greater speed coming out of the U turn, thus increasing the radius of the U turn,

He looks darn close to the end of the runway with his right wing so I don't think he could have taxied into position, align it straight, do a run up, go wide open throttle with locked brakes and then release and gain anything.

My opinion is he was far better off with a rolling take off then a dead start take off.

Comments guys ?

 

[el con]

Exactly.

Pardon the crudity of the drawing....

Dan, your picture is misleading as at 6Y9 your taildragger's tail would be in a drainage ditch in drawing 1, in drawing 2 you would be deadstopped in the same location we would already be going 5-10mph, no matter how you figure it, we will be off the ground sooner than you in identical aircraft.
Ask any drag racer if they would race identical cars one going through the start line at 10mph and one from a dead stop, both max throttle exactly at the same time and on the line. Who wins ? the guy thats rolling, besides the distance the guy rolling has already overcame the resistance that you have starting from 0-10mph
I'm no Einstein, but seriously, this makes since to you, Huh?
You're just taking advantage of the same physics and proven methods of yore, used in the recommended soft field and combining it with the short field, heck many short fields, are soft grass fields , why take the chance? Our only change is a little more or less back pressure on the yoke or stick for us nosedraggers
If you think your plane is going to change it's ability to produce power from the place where I run up to the place you run up,ya better get a different plane

 

[RMCN172RG]

How does his take off point compare with others using the stop method?

He seemed well down the runway on the wide turn before he was back to full power to me.

 

[dmccormack]

I'm no Einstein, but seriously, this makes since to you, Huh?
You're just taking advantage of the same physics and proven methods of yore, used in the recommended soft field and combining it with the short field, heck many short fields, are soft grass fields , why take the chance? Our only change is a little more or less back pressure on the yoke or stick for us nosedraggers
If you think your plane is going to change it's ability to produce power from the place where I run up to the place you run up,ya better get a different plane

I don't have a single idea what you're talking about...

I mentioned in my post that accompanied this drawing that it referenced a nosewheel airplane.

And -- we're talking about short field technique. Soft field technique is different.

A Short, Soft field is different yet again.

 

[N801BH]

On a paved runway with tires properly inflated my gut feeling is the outcome would be pretty close. On a grass/dirt runway the first 5 mph is the deciding factor. Just park a plane on a grass runway and try to push it to get it rolling.

 

[kevin7500]

I will be at a higher speed 100' down the runway using a rolling start, than I would be from a dead stop. What does being on centerline have anything to do with using the least amount of runway length?

Centerline could be replaced with established on a course that allows for full acceleration. As for rate of travel at 100', that is very easily verified at 9D9. Not today though....Brrrrrr

 

[jesse]

This argument is funny.

Look at it this way. You're taxing your airplane to depart. Assume the exact same turning radius / ground track (which is possible). At some point you turn the aircraft to align with the runway. While you're turning the aircraft you can either come to a stop or you can bring up the power. If you're flying an aircraft that can produce near full power almost instantly you're going to be better off producing that full power then you are stopping.

There are problems with the above though. Technique will seriously change the outcome and because of that you cannot build a performance chart that applies to all pilots. If you're teaching people to use their performance charts you need to teach them to takeoff in a manner that nearly any pilot can reproduce.

You can't build a performance chart telling people to roll onto the runway. How big is their turn? What angle did they turn? How long did they take to reach full power? Way too many variables.

The FAA / most instructors are teaching a method that will allow a person to reference their performance chart and get somewhat similar results. That is all.

Do I come to a complete stop? Depends how comfortable I am with the airplane. If the airplane has a lot of ponies and the nosewheel on the back I'm more likely to want to come to a complete stop to make sure I don't get myself in trouble when I'm bringing that power up while turning. If I'm flying a Cherokee or a 172 - I'm just going to roll onto the runway and instead of coming to a complete stop I just bring up full power. The faster I can roll onto the runway the better.

If you're flying an airplane that takes some time to spool up to full power then stopping is going to be an advantage.

 

[mantakos]

On a paved runway with tires properly inflated my gut feeling is the outcome would be pretty close. On a grass/dirt runway the first 5 mph is the deciding factor. Just park a plane on a grass runway and try to push it to get it rolling.

But I think this is a source of misunderstanding, because we don't care about the time it takes to get rolling, we only care about the distance we consume in doing it.

I doubt I'm having much success, but I'm trying to explain the notion that we eat up runway exponentially with time. Our acceleration from 0 to 5kts might seem to take up as much or more time as our acceleration from 50kt to 55kts, but it takes up only a tiny fraction of the distance, because we're going so much slower during that time.
-harry

 

[gismo]

Point 1) The plane will accelerate quicker (i.e. use less distance) when it is already rolling than it will accellerating it from a dead stop. Physics 101.

Ed, you're simplifying it too much. What you say is true if the starting point is the same but a turning start will consume some portion of the runway over which you could have been accelerating had you done the full power brake release from the very end. So the real issue is will you be going faster at the end of the turn than you would be by the time you reached the same point from a dead stop without the turn. I suspect that the optimum is somewhere between no turn and a 180 degree turn, and my best (untested) guess is that something a bit less than 90 degrees would be ideal and that the "perfect" starting angle would depend on several factors like the runway surface, tire size, airplane weight and power. Definitely not simple.

Point 2)
(applicable really only to manual flap airplanes that recommend more than 0º flaps) There is less drag produced at 0º flap setting than at say 25º flap settings as in say - a Cherokee. So you will accelerate faster to Vr quicker. Once there - click click 2 notches of flaps. I wouldn't try this in electric or hydraulic.

IMO this also depends on some external factors. If the ground is really soft, earlier application of flaps will decrease the ground resistance more than they increase the air resistance. Also the additional drag of partial flaps is pretty small in many light airplanes so the difference between starting with flaps extended vs popping them out partway through the takeoff roll might be so small as to not matter on anything but a dry paved surface where rolling friction is minimal.

Point 3) Leaning for max power. "But, but, but the manufacturer says..."
I call BS. As long as you don't spike your CHTs, you are fine. 375º is 375º whether you are taking off, or whether you are in cruise. Which you wont do in a 172, PA28, or 90% of the NA single fleet. The only reason the manufacturers say don't lean below 5k is because when they wrote the recommendation they weren't expecting 8, 12, and 16 channel analyzers to monitor that, so to err on safety, "don't lean below 5000 (or whatever the altitude). And if you are that worried about temps, slowly richen it up once clear of obstacles.

Ed, if you'd seen the engine test cell at GAMI in action you'd probably feel different, at least WRT running a big bore high compression engine at full power. Detonation at best power mixture in those engines is a real issue which can transition to pre-ignition and seriously damage an engine in as little as 20-30 seconds. Low CHTs are certainly a factor in preventing this but I believe that on a hot day the CHTs in many planes can get hot enough for pre-ignition before you get around to increasing the fuel flow after clearing obstacles. That said, I completely agree that the admonition to never lean below 5000 is nonsense and I regularly lean to achieve around 1300°F EGTs during any takeoff where the DA is much above 1000 ft as well as every 1000 ft or so when climbing. From what I've seen and read, this will allow the engine to develop at least 99% of it's rated power available for the altitude while eliminating the risk of detonation and/or pre-ignition.

I've done this at near gross on a grass field. I've done this at minimum fuel no pax or baggage on a grass field. I've done this at gross on a paved field, and I've done this at minimum fuel no pax or baggage on paved field. Every single time, I have beaten the takeoff and 50' obstacle clearance compared to following the manufacturer's and FAA "perfect" procedure. After all the FAA and manufacturer is never wrong, oh yeah, except in this case.

It's good that you tested this and I'm not surprised that your turning takeoff is "better" than a full power brake release from the end of the runway (you did start those from the absolute furthest point from the far end, right?). I suspect that no airplane manufacture is ever going to recommend a turning takeoff simply because a lot more can go wrong if it's not properly executed and they don't want to be on the hook when some idiot ends up catching a wing in the trees on the side of the runway. Now when are you going to try this with different starting angles to find the optimum?

I supposed demonstrated crosswind is a limitation too. After all the super awesome test pilots never flew it beyond that.

Come on, you know full well that the tail will fall off if you attempt to land in conditions that exceed the max demonstrated crosswind, just like the wings fall off of any plane if you slip with flaps extended.

 

[RMCN172RG]

But I think this is a source of misunderstanding, because we don't care about the time it takes to get rolling, we only care about the distance we consume in doing it.

BINGO!

 

[EdFred]

What we're trying to quantify is the proportion of the takeoff roll that's consumed in getting from a dead stop to our "rolling start" speed.

So propose a "rolling start" speed and a liftoff speed, then estimate, for us, what percentage of the total takeoff roll you think is consumed by accelerating from a dead stop to that "rolling start" speed.
-harry

At 6Y9, we have 100' wide runway, and what I do is use the runway width to my advantage. I'm going to simplify it, but I'll basically use that 80' radius to my advantage. I roll in full power just past the apex and use that arc length to my advantage. Yeah, I don't take off right on the centerline...

Ok, using out of the book physics and assuming a constant rate of acceleration, and a perfectly driven arc I come up with the following for my Comanche, and we'll use Dan's 20kt taxi speed.

From a full stop:

Takeoff distance = 800ft
Take off velocity = 117ft/s (70kts)

Using the formula of
Vf2 - V02 = 2ad
we can determine an acceleration rate of 8.55f/s2 and a take off time of 13.68s
Seems about right when I count in my head, and if the time is less I'm probably accellerating slower at the beginning than I am at the end.

Assuming the same acceleration rate (same engine, same plane, same weight) It will take me 2.78seconds to drive the arc - 80ft radius, and I will have a velocity of 56.8fps at the end of the arc, and I'll even give you 20 foot of wasted runway and call it 100' from the end.

Back to
Vf2 - V02 = 2ad
and determining our velocity at that same 100' mark we get
Vf2 = 2(8.55)(100)
Vf = 41.3f/s

From here on this is what the splits are (f/s) rolling listed first
200ft: 70.3
200ft: 58.4

300ft: 81.5
300ft: 71.6

400ft: 91.4
400ft: 82.7

500ft: 100.3
500ft: 92.4

600ft: 108.5
600ft: 101.3 ("The Fox Rocks!" - sorry local radio station)

700ft: 116.1
700ft: 109.5

712ft: 117 - takeoff
712ft: 110.3

800ft: xxxxx
800ft: 117.0 - takeoff

I just beat you by 11%.

 

[dmccormack]

Assuming the same acceleration rate (same engine, same plane, same weight) It will take me 2.78seconds to drive the arc - 80ft radius, and I will have a velocity of 56.8fps at the end of the arc, and I'll even give you 20 foot of wasted runway and call it 100' from the end.

You're doing 40 MPH at the end of the arc?

 

[EdFred]

You're doing 40 MPH at the end of the arc?

This was paper physics. I'll have to bring a pax and check the GPS for me when we hit it. Yeah, 12mph advantage sounds about right. Comanche gear strong like bull!

 

[dmccormack]

This was paper physics. I'll have to bring a pax and check the GPS for me when we hit it. Yeah, 12mph advantage sounds about right. Comanche gear strong like bull!

Gear may be strong -- how's the bead on your tires??

Airplane wheels don't cant the way car front wheels do...

 

[N801BH]

But I think this is a source of misunderstanding, because we don't care about the time it takes to get rolling, we only care about the distance we consume in doing it.

I doubt I'm having much success, but I'm trying to explain the notion that we eat up runway exponentially with time. Our acceleration from 0 to 5kts might seem to take up as much or more time as our acceleration from 50kt to 55kts, but it takes up only a tiny fraction of the distance, because we're going so much slower during that time.
-harry

I respectfully disagree with you on the "tiny fraction" of distance.

Taking off from a grass runway with alot of resistance holding back the wheels from getting rolling might be in the 5-10% range of the entire sequence. Yeah, it seems like forever in the actual time of it but getting up to 5 kts could take a few plane lengths. Not only does it seem to take forever, it looks like it takes forever. ymmv.

 

[mantakos]

... It will take me 2.78seconds to drive the arc - 80ft radius, and I will have a velocity of 56.8fps at the end of the arc...

Let me see if I understand you correctly. At the beginning of the "experiment", you're facing perpendicularly to the runway centerline. At "time zero", you put in full power and immediately begin tracing out an arc'ed path onto the runway. This arc is a quarter of a circle with a radius of 80', and ends with you parallel to the centerline (albeit offset laterally by about 30').

Is that correct?
-harry

 

[EdFred]

Let me see if I understand you correctly. At the beginning of the "experiment", you're facing perpendicularly to the runway centerline. At "time zero", you put in full power and immediately begin tracing out an arc'ed path onto the runway. This arc is a quarter of a circle with a radius of 80', and ends with you parallel to the centerline (albeit offset laterally by about 30').

Is that correct?
-harry

Correct, but I do enter the runway with speed - Dan made the 20kt to 3kt comment so I used that as my entering speed since he used that number earlier. Even if I enter at 15kts, I am off 6% shorter distance.

 

[SteveR]

The biggest mistake I see most pilots make is rotating too early, and sometimes dropping the flaps too early (depends on the plane)...typically tricycle gear pilots. I tend to think that what you do once you are rolling down the runway will have a bigger effect than a rolling or stopped start.

Rotating too early results in substantially increased drag because the wing has a high AOA at a fairly high airspeed. When the flaps are down, the problem is even worse. You can actually feel the reduction in acceleration when people do this. Also when you rotate too early you also move the thrustline so it is not parallel to your direction of travel, which reduces your thrust by some amount (5%? 10%?). I've even seen guys go down the runway with the tail dragging in the dirt, again extending the roll, potentially even by hundreds of feet.

Proper short field technique requires that you keep the wings level or close to it until the plane is ready to fly. It will not fly until you've reached that critical airspeed, so stop trying, you are just adding drag and reducing thrust. I've seen guys rotate a good 5 to 15 MPH before the plane will fly. This is terrible technique and substantially increases the ground roll. You need to know the airspeed at which your plane will fly given the current conditions, and rotate only once you are at that airspeed, or within 1-2MPH.

Also, in planes with Johnson Bar flaps, pull the flaps at about the time you rotate, or immediately before. Even at slow speeds pulling the flaps adds a lot of drag as they are in the high velocity propwash.