Newton, Runways and Rules
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Newton, Runways and Rules
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
) he said that
i.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
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
) he said that
the acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass
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
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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
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.
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
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.
Interesting stuff. A bit of physics never hurt but...
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)
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.
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
)
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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.
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.
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!
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.
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Book Worm
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
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.
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!
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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?
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?
Last edited by Final, 3 Greens; 25th Jan 2003 at 15:53.
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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.
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.
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.
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.
Ok, if I understand this correctly you are saying a headwind on takeoff makes take off distance Longer!
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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.
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.
Last edited by slim_slag; 25th Jan 2003 at 19:17.
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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!
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!
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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
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
Grandpa Aerotart
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
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
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.
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
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
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.
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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.
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.
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
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.
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.
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
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Slag
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.
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.
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.
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.
How about you and others detail what speed and point you would choose, and lets see how that stands up to scrutiny.
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Anyway, I look forward to your suggested figures.
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Anyway, I look forward to your suggested figures.
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.
I wrote:
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.
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.
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 at AvWeb that describes thrust vs airspeed for a typical aircraft with fixed-pitch prop.
Does that help to "substantiate" it for you?
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I missed this point before....
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
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.
Come to the Rockies and learn all about runways and performance
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
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.
Avoid imitations
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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.
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.