Newton, Runways and Rules

[slim_slag]

Back in the old days when grade A at A level was only given to the top 5%, Newtonian physics was taught at an early age. For those who didn't attend those classes, Mr Newton was also to be found on the back of the long missed £1 note. In those days if you handed Mr Newton over to your friendly non-theme-pub barman, you could get 3 pints of bitter and change. So the boys and girls who were down the pub instead of going to maths classes were still very familiar with Newton, even if for different reasons.

Victor Meldrew lives.

For those of you who were still twinkles in daddy's eyes back then and so will not have heard of the fellow, Isaac Newton was a famous Brit who developed calculus (ooops, another word no longer heard in the UK education system, and the French might disagree too) and also came up with some really cool laws. These are known as Newton's Laws of Motion.

Cutting to the chase, (and I did have to look this bit up :D) he said that
the acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its massi.e. F=ma

From the above, and using some other knowledge of the fundamental units, you can come up with the formula

v2 = u2 + 2as

Where
v = final velocity of a body
u = initial velocity of a body
a = acceleration applied to the body
s = distance travelled by the body

(and for those recent maths A-level grade A students, 2 is a number between 1 and 3) :)

If initial velocity is 0 (u=0)

v2 = 2as
or
s = v2/2a

Lets assume, for an airplane moving from 0 to Vr along a runway, the net force pulling the airplane forwards is constant and airplane mass is constant, so acceleration is constant. This is not going to be 100% correct, but given the speeds and environment it will be close enough for our purposes.

Lets say our Vr is 10 m/s, and a is 1 (so a drops out)

s = 100/2
= 50 m

So our runway has to be 50m long in reach to a Vr of 10m/s, given a constant acceleration which we have conveniently set to 1.

How fast will be be going half way down the runway, where distance = 25m???

v2 = 2as (remember a = 1, constant acceleration) s is now 25
v2 = 50
v = 7.1 m/s.

So we have to be at 7.1 m/s half way down the runway to be assured of reaching 10 m/s at the end.

That, my friends, is 71% of the final speed, half way down the runway.

What did I say? I said, lets start with 75%, because that is easier to work out, and it gives a little bit of a safety margin. That's why I use it, to start with. Actually I am far more cautious than that, because I don't want to use all available runway.

Put your own figures in, you will see that if you are not traveling at over 71% of Vr half way down the runway, you MUST abort. Otherwise, you WILL run off the end.

Where did I mention performance charts, altitude, temperature, grass, snow or anything else which has been brought up to argue against this rule. Those of you who think this is not a hard and fast, rigid rule should sit back and spend some time contemplating :) Mr Slag may not carry much weight round here, but Sir Isaac Newton should.

Cheers

[rustle]

slim_slag

Despite this being a splendid walk-through of Newtonian Physics, sadly it has limited real-world applicability...

Ignoring, for a moment, the non-linear acceleration of aircraft on the ground ;), the 71% Vr by 50% distance is really only relevant when the TORA is equal to distance required to achieve Vr - which in the majority of cases is not true...

If all continue/stop decisions were based on being at 75% Vr by 50% of theoretical "d", most take-offs would be abandoned despite miles of clear runway ahead - perhaps a little too conservative if we want to be pilots and not taxi-ers :D

But I can see where you're coming from and agree that if you aren't at 75% Vr by 50% of TORA you should abandon (which I think is what you set-out to prove ;))

Remembering that this whole discussion started with a comment from a student (with instructor present) who struggled to get airborne on a faltering engine, it would have been an excellent learning experience for said student to have calculated accelerate distances under those specific conditions and seen where his go/no-go point should have been. I doubt the instructor asked for this to be done though, either pre or post-flight.

[bookworm]

Interesting stuff. A bit of physics never hurt but...

Lets assume, for an airplane moving from 0 to Vr along a runway, the net force pulling the airplane forwards is constant and airplane mass is constant, so acceleration is constant. This is not going to be 100% correct, but given the speeds and environment it will be close enough for our purposes.

Unfortunately slim, it's nowhere near close enough. Reality is often much worse than that. There are two important features that change along the take-off roll.

The first is the thrust developed by the propeller. The thrust developed by the propeller can easily fall to no more than 70% of its static thrust by the time you get close to rotation speed.

The second is the drag on the aircraft. The parasite drag, zero at rest, has often reached significant values by the time rotation speed approaches and it increases with the square of the speed.

These conspire together to mean that your acceleration falls sharply as you progress in the take-off roll. As a result, an aircraft that reaches 71% of its rotation speed half way down the runway may be nowhere near rotation speed by the time it reaches the end.

You also neglect the wind in your assessment. A headwind makes matters worse still. Imagine an aircraft that rotates at 60 knots taking off into a 25 knot headwind. It will reach an IAS of 30 knots almost immediately in the take-off roll, but has a very long way to go before it gets airborne.

The case you describe is the best possible case -- it's usually necessary to be much more conservative.

(BTW Leibnitz invented calculus first :))

[Keef]

Agree all of that, bookworm (you'll not be surprised to hear). There's another bit - the blade angle of the propeller.

Depending on the pitch of the prop, it may produce relatively little "pull" when stationary relative to the airflow, and a lot more as it reaches optimum. So you may find you get better acceleration from the prop as you approach Vr. Variable pitch props help a lot here!

However, aerodynamic drag being proportional to the square of the airspeed, it's not likely that the blade angle will help if you aren't past 75% Vr by 50% runway.

I did the sums for Chateau La Chassagne, and concluded we'd still be on the ground at the far end if I tried to take off at MTWA. So we were light: 2 up, not much baggage, and 2 hours' fuel (no reserves) and aimed for 80% Vr by 50% runway.

I didn't get to check - we were airborne by the 50% runway marker. Weight makes a big difference.

[bookworm]

Depending on the pitch of the prop, it may produce relatively little "pull" when stationary relative to the airflow, and a lot more as it reaches optimum. So you may find you get better acceleration from the prop as you approach Vr. Variable pitch props help a lot here!

I thought about that as was writing Keef, and even changed my mind and my text a few times! On reflection, I think I'm sticking to my story.

Though the efficiency of the prop gets vastly better with speed, the thrust does not. It's true that some very coarse fixed pitch blades may be stalled at low speed, but I don't think you have to be going very quickly for them to unstall themselves. From what I can see from the charts I have, you need to be getting into the 30+ degree pitch before there is any increase in thrust coefficient with advance ratio. Everything less than that shows a monotonic decrease in thrust coefficient.

So I think you're always going to be losing thrust as you accelerate.

[Kingy]

Book Worm

You also neglect the wind in your assessment. A headwind makes matters worse still. Imagine an aircraft that rotates at 60 knots taking off into a 25 knot headwind. It will reach an IAS of 30 knots almost immediately in the take-off roll, but has a very long way to go before it gets airborne.

Ok, if I understand this correctly you are saying a headwind on takeoff makes take off distance Longer!

You are in effect getting the first 25kts for free and the aircraft would only need a 35kt ground speed to become airbourne. Granted these 35 kts may not be gained as fast as the first 35kts in a nil wind situation, but remember the drag from the undercarriage also increases with the square of the speed so there is a drag saving to be had.

Once airbourne the aircraft will climb at a steeper angle into wind and this will help obstacle avoidance - but that's another thread..

Reading the air accident reports I can't recall one where a light aircraft ran off the end of the strip into a stiff headwind!

Kingy

[Negative 'G']

And there was me thinking that this thread was about the old RAF Newton being reopened for civie use !:rolleyes: :( :rolleyes:

Neg G:D

[Final, 3 Greens]

This is all well and good, but what about the practicalities?

Let's take a Bulldog for example, an above average performance trainer with a Vr of 45kias (POH), but an ASI bottom stop at 40kias, which is 89.8888% of VR.

So this aeroplane could never comply with Slim Slag's rule of being at 71% (or 75%) of Vr, because the ASI is not capable of reporting this.

If we apply Slim Slag's thinking, we would either take off because we got to 89.888% of Vr on a runway long enough to allow this before 1/2 way or we would would have to reject the take off, even though we might be between 71 and 89.7% of Vr.

This sounds a bit silly to me.

What is wrong with looking at the POH, calculating the performance, safety factoring by whichever method we choose and then picking a spot where we expect a certain performance, rather than starting out with a highly generalised premise, which as I have just proved, cannot work in a well respected light aircraft?

[slim_slag]

Guys,

All this fancy aerodynamics stuff is all well and good, but has nothing to do with a body moving along a surface under a constant force. This could equally be a sleek sports car, or something travelling in a vacuum.

rustle understands what I am getting at. Bringing practicalities into it, you have to be going FASTER then 0.71Vr at half way point. I find your discussion very interesting though, please continue.

Now then Flying3Greens. Sit down, give your PPL to mumsy so you can't hurt anybody, take your blinkers off and listen :)

The POH performance charts are the only legal means for you to know how much runway you need to get your plane off the ground given the conditions. It doesn't tell you how fast you will be going at any point on the runway between 0 and Vr.

Let me repeat. All it tells you is HOW MUCH RUNWAY DO YOU NEED.

Lets say the conditions are such that the POH says you need 3000 ft of runway and you rotate at 60knts. First decision is "is the runway 3000 ft long", that should be simple for even the most undereducated amongst us. (Then you have to consider the POH is best case, but I don't want to confuse).

Lets take the example of a 10,000ft long runway. You start rolling. If you are not at 45knts by 5,000 ft ABORT.

Lets take the example of a 5,000ft long runway. You start rolling. If you are not at 45knts by 2,500 ft ABORT.

Lets take the example of a 3,000ft long runway. You start rolling. If you are not at 45knts by 1,500 ft ABORT.

Simple stuff really. That's all I am saying. Hopefully in the case of the 10,000ft runway when the POH says you need 3,000 ft you will be airborne at 5,000ft. If you are not then something is wrong. So ABORT.

How anybody can be criticised for suggesting a rule which encourages and errs on the side of safety is beyong me. Safety is certainly not silly. Come to the Rockies and learn all about runways and performance. In the world where this really matters, people who think like you are a liability.

As for the bulldog. The only machine I know which can violate Newtons Laws is the Starship Enterprise. Just because your airspeed indicator says you are going 0knts doesn't mean you are going at 0knts. That is REAL basic stuff, and I give up. I think I will shut up too.

[bookworm]

All this fancy aerodynamics stuff is all well and good, but has nothing to do with a body moving along a surface under a constant force.

Well yes, that's the point really. A body moving along a surface under a constant [net] force (your analysis) is not very closely related to reality.

The problem is that it doesn't err on the side of safety. You could well be and 45 kt 2500 ft into your 5000 ft runway and not make it to 60 kts by 5000 ft. That's because your acceleration decreases as your speed increases. Your assumption of constant acceleration is optimistic.

I think the principle of assessing your speed at some point on the runway and aborting if it is below a certain speed is a good principle. But if someone takes your numbers literally, they could end up in the fence at the end.

I wrote:

You also neglect the wind in your assessment. A headwind makes matters worse still.

to which kingy replied:

Ok, if I understand this correctly you are saying a headwind on takeoff makes take off distance Longer!

I could have chosen my words better! -- that's not what I meant. I meant that in the case of a headwind, the proportion of distance at which 71% speed is reached is even further away from the 50% distance in slim_slag's constant acceleration case. Does that clarify?

[slim_slag]

bookworm

I think the principle of assessing your speed at some point on the runway and aborting if it is below a certain speed is a good principle.

OK, I have thrown out what I think makes sense, and people have disagreed.

How about you and others detail what speed and point you would choose, and lets see how that stands up to scrutiny.

This is the test I will apply

You could well be and xx kt yyyy ft into your zzzz ft runway and not make it to Vr kts by zzzz ft. That's because your acceleration decreases as your speed increases. Your assumption of constant acceleration is optimistic.

PS Bookworm. I'm not guaranteeing that you will get off the ground. I'm just guaranteeing that you will be able to stop.

The decision to GO is made in consultation with the POH and others. You decide to GO when you push the throttle in at the beginning of the takeoff roll. I am talking about the situation where the engine does not behave as advertised, a brake is sticking, or the wind changes. My rule is purely a decision relating whether to STOP.

Anyway, I look forward to your suggested figures.

[QDMQDMQDM]

Now I remember why I fly a Super Cub. If I flew one of those aircraft with more seats and baggage space than it deserves this thread could make me REALLY anxious:

Airspeed creeps up to 45 kts...
QDM thinks: "Damn, what was it the Slag said?! 50% of speed by 75% of the runway? Yes, that was it. Phew. Looks like we'll manage to get..."
SLAM!

[Aussie Andy]

What an interesting discussion - I appreciate Slim's theoretical approach as (although I am supposed to be an engineer, albeit of the electronic kind) I hadn't thought it through from these first principles.

The criticism that there are some variables not taken into account (e.g. that the acceleration is not constant due to the way say fixed pitch props perform and due to increase of parasitic draq etc.) are all fair, but don't detract too much from the value of the theoretical illustration for me.

I was never taught, nor have I seen in any publication, a good rule of thumb for speed halfway down the runway. The only thing I have read is that one should pace out a halfway point at home airfield and then on subsequent take-off runs, note the airspeed speed at halfway point in order to build up a practical knowledge of the normal numbers, so that one's brain might notice when this isn't achieved. But I have not made a habit of doing this (perhaps I should!)

So the value of this discussion for me is provocation of thought!

Thanks guys ;)

Andy

[Chimbu chuckles]

I cast my mind back to the (approx) 3500 takeoffs a year for 8 years of bushflying I did and wonder how I managed to succeed > 26000 times when SS said I should have aborted.

I cast my mind back to those many occassions when I trundled off the end of very short runways perched on cliffs at something in the vicinity of 5+Kts under flying speed and converted suddenly available altitude into much needed IAS:D

My rule of thumb, which served me well through quite some considerable amount of trial, was 'when in doubt go all the way to the end of the cut grass and then aim one inch higher than the highest thing in front of you'.

I destinctly remember a peer deciding that he wasn't going to get airborne by the end of the strip, aborting, and putting a perfectly good Islander into a rocky creek. When asked how fast he was going at the instant of his decision to abort he answered "39kts"...for the uninitiated an Islander will fly at 39kts, ya just gotta be motivated:D

SS my way meant there was a small chance I'd crash...I never did...your way (and my peers) gauranteed a crash.

SS I very much doubt Newton himself would be so arrogant as you appear to be...In fact I rather think his intellect would be greatly stimulated by flight but he would be far to astute to try and make the claims you are for his science.

Chuck.

[Flyin'Dutch']

Hi Slag

I actually think that your attempt in trying to work out a rule of thumb is very laudable, and that you get some undeserved flak from people making assertions that they can not substantiate.

Though the efficiency of the prop gets vastly better with speed, the thrust does not.

How is that then?

Certainly most fixed pitch props are not efficient at the low speed end as they will be optimised for cruise conditions. And how about the power output of the engine. Again fixed pitched props do not allow the engine to put out the rated power and as you accelerate and the prop becomes unstalled and more efficient you will see that the rpm increases thereby allowing the engine to put out more power.

It is unlikely that the acceleration of the typical GA mount is constant. But never had the impression that it decreases during the take off run either as bookworm states. :eek:

Of course you need to consult POH and factor in the various parameters before you attempt your t/o. But to do a real life check whilst trundling along seems a clever plan.

As always, MHO

FD

[Final, 3 Greens]

Slag

I was never taught, nor have I seen in any publication, a good rule of thumb for speed halfway down the runway. The only thing I have read is that one should pace out a halfway point at home airfield and then on subsequent take-off runs, note the airspeed speed at halfway point in order to build up a practical knowledge of the normal numbers, so that one's brain might notice when this isn't achieved.

If I may quote Aussie Andy, this is my point exactly, although whether that point is halfway or some other convenient landmark
such as a taxi way at a known point doesn't conern me greatly. I would also extend this principle to 'away' airfields, accepting that familiarity will not be as great and thus more conservatism required in the decisions.

Chimbu Chuckles also makes very telling points based on a considerable amount of real world professional experience gained in very challenging conditions, certainly well beyond my personal experiences.

As for the bulldog. The only machine I know which can violate Newtons Laws is the Starship Enterprise. Just because your airspeed indicator says you are going 0knts doesn't mean you are going at 0knts. That is REAL basic stuff, and I give up. I think I will shut up too.

I think you have missed the point here, the 'dog's ASI CANNOT DISPLAY 75% of Vr, because it doesn't read anywhere near that low - what is your problem in understanding that your rule of thumb is unusable in a Bulldog, Period.

[WorkingHard]

By the time you work it out as you travel the r/w you may forget the other essential priciples of piloting an aircraft!!!

[bookworm]

How about you and others detail what speed and point you would choose, and lets see how that stands up to scrutiny.
...
Anyway, I look forward to your suggested figures.

It's not possible to have universal figures for all aircraft types. They would have to be calculated and tested for specific aircraft types, just as other take-off performance figures are.

PS Bookworm. I'm not guaranteeing that you will get off the ground. I'm just guaranteeing that you will be able to stop.

You're guaranteeing nothing of the sort! You've made no attempt to calculate how far it will take to bring the aircraft to a halt from this decision speed. That's a whole section of performance theory that we haven't touched on, and it's very important for Performance Category A aircraft where there's a decision to be made at V1.

The decision to GO is made in consultation with the POH and others. You decide to GO when you push the throttle in at the beginning of the takeoff roll. I am talking about the situation where the engine does not behave as advertised, a brake is sticking, or the wind changes. My rule is purely a decision relating whether to STOP.

If the engine doesn't behave as advertised, a brake sticks or the wind changes, stop the aircraft. But the assessment of whether the aircraft is behaving normally on the take-off roll often needs to be more subtle than just 75% speed by 50% distance.

I wrote:

Though the efficiency of the prop gets vastly better with speed, the thrust does not.

to which Flyin'Dutch' responded

How is that then?

Certainly most fixed pitch props are not efficient at the low speed end as they will be optimised for cruise conditions. And how about the power output of the engine. Again fixed pitched props do not allow the engine to put out the rated power and as you accelerate and the prop becomes unstalled and more efficient you will see that the rpm increases thereby allowing the engine to put out more power.

You're correct that there are competing factors, particularly where the prop pitch is fixed and particularly in the early stages of the take-off roll. Nevertheless, I stick to my assertion that the general trend is for thrust to fall during the take-off roll.

I can only direct you to books on propeller theory. My source was Barnes McCormick's Aerodynamics, Aeronautics and Flight Mechanics, Chpater 6.

If you want a source on the WWW, John Lowry has an article (http://www.avweb.com/news/airman/182410-1.html) at AvWeb that describes thrust vs airspeed for a typical aircraft with fixed-pitch prop.

Does that help to "substantiate" it for you?

[Final, 3 Greens]

I missed this point before....

Come to the Rockies and learn all about runways and performance

Well, I never made it to the Rockies, but I have done a bit around the Sierras and the high desert.

And yes, performance is very different.

So what is the effect on braking distances, of operating to IAS at altitude, compared to near sea level operation?

Mind you, I can only judge from my experiences gained at an airport elevation of 6782' amsl, which is probably much lower than Slim Slag has flown from :D

But, I would say that stopping distances were significantly longer as the higher groundspeed has to be lost, whether rejecting a take off or on landing.

IMHO, mountain flying requires a modified approach to near sea level flying, for many reasons.

[ShyTorque]

With regard to the efficiency / thrust of fixed-pitch props, I don't agree that the average prop is optimised for the cruise.

The average light aircraft propellor, which must be matched to the aircraft type, is a compromise for all-round performance, i.e. acceleration to take-off, climb and cruise.

If you wish to look at props optimised for the cruise, take a look at the old Schneider Trophy seaplanes. They were set to an extremely coarse pitch for high speed cruise, which gave the aircraft a very long take-off run and relatively poor climb performance due to poor efficiency at low airspeeds. This wasn't too much of a problem for them, bearing in mind they were seaplanes, but this situation would be unsuitable for the average club aeroplane.

[ajsh]

I have read this thread with great interest, both for the entertainment value and for the more important purpose of understanding the mathematics and physics involved in flying.

However as a PPL(A) I line up, apply full throttle and then check that the static rpm is at the correct level and constant, the airspeed in building in line with expectation, apply slight back pressure and finally check the “feel” of the a/c. At say 65-70knts depending on type I rotate.

I was not taught not do I feel the need to necessarily worry about what speed I should have achieved at various points along the runway. I am more concerned with the weather conditions, the take-off weight and take-off configuration.

May I just say again however, that notwithstanding the above, this thread of very interesting and informative and while it may not change the way I fly, one of the fundamental joys of learning to fly in the first place was the learning curve.

Please, before anybody shoots me down, I am incredibly safety conscious, aware of my limitations and certainly not arrogant enough to accept that I have learnt something from this thread.

[slim_slag]

Chimbu,

I apologise for offending you, and I certainly defer to your vast experience of bush-flying which is more than I will ever log. However, even if you dislike and dismiss my approach, you cannot do the same with physics. Physics works the same in PNG as in SE England, and therefore so do airplanes.

You give an example where you do not climb from the runway, but you push down! Hair-raising and an entertaining tale, but not relevant to what most of us do, or this example either.

Why didn't you crash? I suspect you used the benefit of experience so your Vr was actually a lot lower than what the books said (39knts example). It was still a Vr, and if you weren't at 71% of it by half way down the runway, and if you were accelerating at a constant rate, you would not make that Vr by the end. That's not a rule I made up, and that's the rule I am pushing.

More interestingly. I also suspect you hopped off the ground well prior to published Vr, used ground effect to increase lift and reduce drag, and accelerated faster than you would have done on the ground. This is a very effective way of getting off a field which is shorter than you like. Of course, you have to get out of ground effect at some stage. You obviously do this with great effect by diving off a cliff. It doesn't always work that way, most runways don't end like that, and sometimes you cannot climb out and you hit something. Better to stop :)

Hey, just trying to throw out some numbers here as a safety guide. Take it or leave it.

bookworm,

Show me a light aircraft where the stopping distance is greater than the starting distance. POH figures will show that you need less runway to stop than go. So if start at zero, and you brake properly at 50% of the runway length, you will always reach zero before the end.

If somebody wants to think about ground effect then that's cool. I do not recommend you plan to use ground effect anywhere except above a runway, unless you know what you are doing and are bush-flying in PNG, where you might have no choice.

F3G,

Strap a decent airspeed indicator on it, but remember it will stop your bulldog going into warp drive. You just have one less piece of data to assist you in safe decision making. You increase your risk of running off the end of the runway if you fly a plane with an ASI not calibrated at takeoff roll speeds.

[Flyin'Dutch']

Hi Bookworm

Indeedy it shows that trust decreases with airspeed. But how about the assertion that you stop accelerating.

If I just may take you back to your AvWeb quote (interesting reading)

http://www.avweb.com/news/airman/182410-1.html

in figure 6 Thrust vs drag you can see that the Thrust vector is greater than the drag vector until 69 KTS so until that point for this particular mount (a C172) the resulting factor will be acceleration.

You will agree that at 69 KTS a 172 will be flying.

Over to you.

FD

[slim_slag]

Aussie,

Yep, I think people should consider the theory. There are many old wives tales out there with no basis in fact, and individual experiences mean nothing.

What you are doing in your pacing out is calibrating a specific airplane for specific conditions. This can be useful because you can determine true braking distance for that runway. So start by brakeing at half way. You WILL stop, measure distance to end, divide by two, and add to half way point of runway. Perform roll again, brake at position determined by previous run. Continue until you calibrate that plane for that runway.

That will give you a better figure of speed and distance along a runway you can brake and still stop. JUST.

Using this method, you will take off when I would not. I will never kill myself, but I will have to face the ridicule of fellow pilots at the airport restaurant for turning back because I want to stay unkilled. I think I can handle that (being the arrogant git I am). I also have NEVER been ridiculed by any pilot in the high country for erring on the side of caution.

If you cannot think of a reason not to teach my method, why not consider it?

It might save somebody when their engine is not performing as it should. We should use ALL means at our disposal to evaluate engine performance at such a crucial phase of flight.


QDM,

You mentioned you flew on backcountry strips in Idaho. Never notice the markings on the side of the runway on some of them, half way along. What do you think they are for? Quick look down, takes fraction of a second, stop or continue with butt clenched :) On high altitude strips round here I ALWAYS know where half way down is. This can either be an intersection, or sometimes they have numbers on the side of the runway with distances to the end. White on black background.

Down low where the runways are long, I don't make such an effort. That is wrong of me, I should.

Flying Dutch

Cheers!

[Final, 3 Greens]

Strap a decent airspeed indicator on it ...... You increase your risk of running off the end of the runway if you fly a plane with an ASI not calibrated at takeoff roll speeds.


Hmmm. So Slim Slag knows better than Scottish Aviation who designed the thing and the RAF who used a fleet of of them over nearly 30 years, to say nothing of the CAA who have certified it for general use in this 'risky' condition.

Anyone out there know of a Bulldog which has run off the end of the runway???? I've just done a search and can't find any, but would be interested in anyone else's experience.

Lets assume, for the sake of argument, that the fleet of about 300 'dogs has flown 50 hrs each per year for 25 years (average lifespan over the fleet) - which makes 375,000 hours, which I would argue would involve a landing on average at least every hour (given that she is a trainer.)

So that would also mean 375,000 take offs .... so how many Bulldogs have run off the end of the runway out of an estimated 375,000 attempts? (Having flown a few hours in 'em, I reckon not too many, since they take off like rockets :D )

In fact, the very reason that the Bulldog does not meet Slim Slag's 'rule'is that it has extremely good take off performance and requires a Vr of 45kias at about the same weight as a 4 seat PA28-181 requiring 65kias.

The examples of both types that I have flown have ASIs with a bottom stop of 40kias, so the latter can meet Slim Slag's 'rule' as 75% of Vr is 48kias, but the former cannot, because it's Vr is 20kias LESS meaning that 75% of Vr is an unmeasurable (if very healthy) 34kias, but this ASI calibration 'defect' apparently increases the risk of running off the end????

So, I think there's a bit of a logic problem here somewhere.

Now for another beauty.....

So if start at zero, and you brake properly at 50% of the runway length, you will always reach zero before the end.

ALWAYS ?????? - what if there is the mother and father of downslopes on the second half, what if it changes from tarmac to wet, long grass, what of the friction layer is considerably poorer on the second half than the first etc etc etc - oh and by the way, which POH for a light SEP did you read that quoted accelerate/stop distances?

Please let me know where I can get these for the PA28/PA32/B121/SA120 as I would gladly invest in them tomorrow.

I have to say, Slag, that your posts have given me much more entertainment that Coronation Street going to 5 days a week, please keep it up as I can't wait for the next episode :D

[distaff_beancounter]

Only just got brave enough to stick my nose into this thread, as I am still trying to understand the opening post :confused:

Anyhow, I am with Final, 3 Greens on the matter of accelerate/stop distances.

For instance, what about an SEP with a magnificent power/weight ratio, but only the bog standard brakes, that are fitted to most light aircraft. If it accelerates like s@#t down the runway, why is it guaranteed to stop in the same distance?

[rustle]

distaff_beancounter

For instance, what about an SEP with a magnificent power/weight ratio, but only the bog standard brakes, that are fitted to most light aircraft. If it accelerates like s@#t down the runway, why is it guaranteed to stop in the same distance?

Where can I find one of these? :D

I think you can take it as read that on a dry, paved runway the brakes will out-perform the thrust. Were that not the case your power-checks and short-field takeoff (full power before brake release) would be "interesting" to say the least :)

Where this balance shifts is where the friction is reduced such as on ice or wet grass - here the thrust (unaffected by such things) may be greater than the braking ability.

These circumstances may lead to a situation where you need less distance to accelerate than you do to stop (NB remember we are talking about speeds less than "flying speed") - but that would be the exception rather than the rule.

=========================================

I found this an interesting thread, but can we play the ball, and not the man?

[Final, 3 Greens]

Rustle

I think you can take it as read that on a dry, paved runway the brakes will out-perform the thrust. Were that not the case your power-checks and short-field takeoff (full power before brake release) would be "interesting" to say the least

I see where you are coming from, but what about the kinetic energy already in the airframe?

AFAIK, there aren't any accelerate/stop distances published fror SEPs, which could be factored to account for the variables you mention.

So there must be an element of uncertainty about stopping performance, although practically speaking I've rejected three take offs and never had a problem stopping - with one being a decision made at 50knots in a PA28 on a 700m dry grass runway with a slight downslope.

[Sensible]

Blimey :confused: :confused: :confused:

[rustle]

Final, 3 Greens

"...but what about the kinetic energy already in the airframe"

Yikes, haven't thought about stuff like this for far too long :D

Whilst I concede that (kinetic energy) would be additional if our intrepid pilot decided to leave full power on whilst aborting :confused:, if we assume he/she doesn't ;) the loss of thrust should at least compensate for the inertia/kinetic energy of movement.

Without, hopefully, confusing things here's an example of accelerate-stop distances - albeit for a twin where such things are in the P.O.H.

Turbo Cessna 310R, 5500lbs (MTOW), pressure alt. 2000', temp +10C, nil wind, Vr = 92KIAS.

Accelerate distance (ground roll) = 1790'
Accelerate-stop distance = 3460'

So stopping, under these conditions, requires 120' less than accelerating. No tricks ;) - at this speed (92KIAS) the wings are producing sufficient lift to fly. (Stall speed, this config, is 80KIAS)

So does this illustrate that you need less space to brake than accelerate? (Okay, in this aircraft, under these conditions)

So if by 50% TORA you abort there should be sufficient space remaining ahead to brake and stop, within the TORA. (In the context of this discussion, which was that if you are not at 75% Vr by 50% distance)

If you now add-in the other variables - ice, wet grass, - which we agree (I think :)) don't affect thrust but do affect braking, we end up where Bookworm started:

The "50% rule" isn't conservative enough.

slim_slag said the same thing too: "Actually I am far more cautious than that, because I don't want to use all available runway"

[Final, 3 Greens]

Rustle

Whilst I concede that (kinetic energy) would be additional if our intrepid pilot decided to leave full power on whilst aborting

I wasn't thinking of such a horror scenario :) Just uncertain about how well SEP brakes deal with the non linear nature of kinetic energy in extreme cases.

As I said I had no problem stopping from 50kias (1 above VS1) on a 700m runway, dry grass, very slight downslope, with a PA28 that was ready to start flying.


So does this illustrate that you need less space to brake than accelerate? (Okay, in this aircraft, under these conditions)

Absolutely and I would accept that the aircraft could stop under different conditions with the appropriate factors being applied to the distances you quote.

My problem was with the original statement "I'm just guaranteeing that you will be able to stop", which I don't think can be true as an absolute assertion.

The lack of accelerate/stop performance tables for SEPs does leave a bit of uncertainty in my opinion though, because what do you apply safety factors to? Say short wet grass, 2% downslope, no headwind, ISA +15.... I can calculate the TORA and distance to 50' and apply the CAA safety factors, but have to hope for the best that I could stop in the event of a rejection being needed.

[Stosser]

This all sounds a bit technical guys.

Just aim at the fence at the other end and make sure you miss by an inch :rolleyes: :rolleyes:

[slim_slag]

Sorry for dragging this to the top kiddies, but aussie andy said he had never seen this in print, and last night while perusing the latest FAR/AIM (which everybody should do every now and again, even though it is a nasty US rag) I found this little beauty.

7-5-6. Use of Runway Half-way Signs at Unimproved Airports (http://www1.faa.gov/ATpubs/AIM/Chap7/aim0705.html#7-5-6)

Don't expect anymore from me on this, as I actually think my presence is clouding the issue and I don't want to be responsible for your blood pressure blowing your head off :D. Some on this thread understand, but unfortunately not most. I refer to my original post bemoaning the standard of education nowadays :p :p

[Flyin'Dutch']

Phew,

Just as well that I said you'd made some valuable pointers.

Joking apart I think you can feel vindicated.

FD

[rustle]

Excellent reference, S_S. Did you write it? ;)

[slim_slag]

Yes rustle, it does seem a bit of a suspicious coincidence, but I promise that if I had thought about it earlier, I would have cited it then. Honest, guv! I would have missed out some fun though.

Thanks dutch, but was there ever a doubt ;) :D