"Principles of flight" by Jeremy Pratt
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Okay, we want to extend this thread to at least five pages so here's a serious question, concerning Va (manoeuvring speed).
Va is defined as the speed where the wings stall at the point where they reach the design limit (normal category that would be -1.8/+3.8g I think) when pulling up hard. As long as you stay below Va you can't break the wings by pulling the stick or moving the ailerons to maximum deflection. (This does NOT apply to the rudder, but that's another story.)
Common wisdom is that Va reduces as the aircraft gets lighter. The reasoning behind this is that with a lower weight (mass), the angle of incidence in level and unaccellerated flight is also lower (due to less lift needed). If you pull up hard, the angle you can pull up until the wings stall is greater and therefore you can pull more g's, assuming the same speed. So lower weight/mass requires a lower Va, in order not to exceed the design limit (measured in g) of the wings.
BUT... My gut feeling tells me that wings don't break due to accelleration. They break because a force is applied to them, and that force is mass x accelleration. As you reduce the mass, the wings are able to withstand a greater accelleration. That cancels out the mass from the equation, leaving Va to be constant.
In other words, if you have an aircraft of, let's say, 1000 kg MAUW with wings designed to withstand 4g (at that MAUW) and you fly it at 500 kg MAUW, at Va for 1000 kg, pull up hard, then the wings will encounter 8g before stalling, but this 8g will only need to accellerate 500 kg. In both cases the force on the wings is 4000 kgf. (About 40.000 N for the purists.)
Where does this reasoning go wrong? Am I assuming a linear relationship somewhere where it isn't?
Va is defined as the speed where the wings stall at the point where they reach the design limit (normal category that would be -1.8/+3.8g I think) when pulling up hard. As long as you stay below Va you can't break the wings by pulling the stick or moving the ailerons to maximum deflection. (This does NOT apply to the rudder, but that's another story.)
Common wisdom is that Va reduces as the aircraft gets lighter. The reasoning behind this is that with a lower weight (mass), the angle of incidence in level and unaccellerated flight is also lower (due to less lift needed). If you pull up hard, the angle you can pull up until the wings stall is greater and therefore you can pull more g's, assuming the same speed. So lower weight/mass requires a lower Va, in order not to exceed the design limit (measured in g) of the wings.
BUT... My gut feeling tells me that wings don't break due to accelleration. They break because a force is applied to them, and that force is mass x accelleration. As you reduce the mass, the wings are able to withstand a greater accelleration. That cancels out the mass from the equation, leaving Va to be constant.
In other words, if you have an aircraft of, let's say, 1000 kg MAUW with wings designed to withstand 4g (at that MAUW) and you fly it at 500 kg MAUW, at Va for 1000 kg, pull up hard, then the wings will encounter 8g before stalling, but this 8g will only need to accellerate 500 kg. In both cases the force on the wings is 4000 kgf. (About 40.000 N for the purists.)
Where does this reasoning go wrong? Am I assuming a linear relationship somewhere where it isn't?
Pompey till I die
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Unless you want a massdebate - now THAT is a pun 
It is indeed - and one I'm very keen on (boom boom). Ok, stand up comedian I am not.
Wait, nobody has said anything scientific for 3 posts and we've only got 6 to go until we get to page 5.
Given we've done Jeremy Pratt to death shall we move onto Trevor Thorn ?
His books suck and they are full of spelling mistakes. He can't spell colour correctly, he spells it color instead. What an idiot etc etc
Oh god, will I EVER finish this aircraft technical book ?
And on a related note: I'm still looking for an E-6B which has the density of Jet-A/Diesel on it, in addition to 100LL. Anybody knows where to get one?
Though given the scientific musings on this thread, it should technically be the range between two arrows pointing at .7739 and .8389 (and the reasoning behind those particular numbers could extent this thread by another few pages
)
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Dunno
Okay, we want to extend this thread to at least five pages so here's a serious question, concerning Va (manoeuvring speed).
Va is defined as the speed where the wings stall at the point where they reach the design limit (normal category that would be -1.8/+3.8g I think) when pulling up hard. As long as you stay below Va you can't break the wings by pulling the stick or moving the ailerons to maximum deflection. (This does NOT apply to the rudder, but that's another story.)
Common wisdom is that Va reduces as the aircraft gets lighter. The reasoning behind this is that with a lower weight (mass), the angle of incidence in level and unaccellerated flight is also lower (due to less lift needed). If you pull up hard, the angle you can pull up until the wings stall is greater and therefore you can pull more g's, assuming the same speed. So lower weight/mass requires a lower Va, in order not to exceed the design limit (measured in g) of the wings.
BUT... My gut feeling tells me that wings don't break due to accelleration. They break because a force is applied to them, and that force is mass x accelleration. As you reduce the mass, the wings are able to withstand a greater accelleration. That cancels out the mass from the equation, leaving Va to be constant.
In other words, if you have an aircraft of, let's say, 1000 kg MAUW with wings designed to withstand 4g (at that MAUW) and you fly it at 500 kg MAUW, at Va for 1000 kg, pull up hard, then the wings will encounter 8g before stalling, but this 8g will only need to accellerate 500 kg. In both cases the force on the wings is 4000 kgf. (About 40.000 N for the purists.)
Where does this reasoning go wrong? Am I assuming a linear relationship somewhere where it isn't?
Va is defined as the speed where the wings stall at the point where they reach the design limit (normal category that would be -1.8/+3.8g I think) when pulling up hard. As long as you stay below Va you can't break the wings by pulling the stick or moving the ailerons to maximum deflection. (This does NOT apply to the rudder, but that's another story.)
Common wisdom is that Va reduces as the aircraft gets lighter. The reasoning behind this is that with a lower weight (mass), the angle of incidence in level and unaccellerated flight is also lower (due to less lift needed). If you pull up hard, the angle you can pull up until the wings stall is greater and therefore you can pull more g's, assuming the same speed. So lower weight/mass requires a lower Va, in order not to exceed the design limit (measured in g) of the wings.
BUT... My gut feeling tells me that wings don't break due to accelleration. They break because a force is applied to them, and that force is mass x accelleration. As you reduce the mass, the wings are able to withstand a greater accelleration. That cancels out the mass from the equation, leaving Va to be constant.
In other words, if you have an aircraft of, let's say, 1000 kg MAUW with wings designed to withstand 4g (at that MAUW) and you fly it at 500 kg MAUW, at Va for 1000 kg, pull up hard, then the wings will encounter 8g before stalling, but this 8g will only need to accellerate 500 kg. In both cases the force on the wings is 4000 kgf. (About 40.000 N for the purists.)
Where does this reasoning go wrong? Am I assuming a linear relationship somewhere where it isn't?
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Just scribble an arrow on the "8" and label it "Jet".
The DA-40 POH, by the way, defines the specific density of whatever you put in there as 0.84, regardless of whether its diesel or Jet-A or a blend of both. Safe side of caution I suppose. I've also seen the number 0.81 being used and WikiPedia claims that the density of Jet-A can get as low as .775.
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Backpacker - I can't answer the question due to your use of the word speed instead of velocity.. speed being the absolute value (scalar just for Paul) of velocity which is a vector blah blah....
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Va is defined as the speed where the wings stall at the point where they reach the design limit
)
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Well, the design rules for a "normal" category airplane require, as far as I know, +3.8 g limits (at MAUW). Based on that requirement the actual strength of spars etc. is designed and Va is calculated. I think.
In any case, the DA-40, the DR200-120/160 and the PA-28 all have +3.8g as the load limit in the normal category and +4.4g in the utility category. The Robin R2160 (aerobatic) does not do a normal category but has +4.4g in the utility category and +6g in the aerobatic category. All taken from the POHs I have lying around here. Coincidence? I think not.
In any case, the DA-40, the DR200-120/160 and the PA-28 all have +3.8g as the load limit in the normal category and +4.4g in the utility category. The Robin R2160 (aerobatic) does not do a normal category but has +4.4g in the utility category and +6g in the aerobatic category. All taken from the POHs I have lying around here. Coincidence? I think not.
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Ripped off from: http://www.auf.asn.au/groundschool/umodule2.html. Note the last sentence (in brackets) which should please PP immensely.
OC619
OC619
Va – design manoeuvring speed. Sometimes referred to as the 'speed for maximum control deflection' or perhaps the 'rough air airspeed' though the latter is usually designated as Vra. It is unwise to make full and abrupt applications of any one primary flight control if you are flying at a speed greater than Va because the force applied could exceed the aircraft's structural limitations and particularly so if you apply more than one control e.g. apply lots of elevator and aileron together. Also when flying above this speed gust-induced loads can exceed the structural design limit, and gust loads in Australian arid region high temperature conditions can be very high. Va is the recommended indicated cruising speed (CAS) when flying in moderate turbulence – strong intermittent jolts. At this compromise speed the aircraft will produce an accelerated stall, and thus reduce the aerodynamic force on the wings, if it encounters a vertical current imparting enough energy to exceed the design wing or tailplane loading.
Va is a fixed theoretical calculation relative to Vs1 for all aircraft within the same category; for a normal category light aircraft (whose certificated vertical load limit factor is +3.8g) Va = Ö3.8 Vs1 or nearly twice Vs1. For a utility category light aircraft (whose certificated vertical load limit factor is +4.4g) Va = Ö4.4 Vs1 or just over twice Vs1. Va is not marked on the ASI but for non-aerobatic aircraft you can assume it's twice Vs1.
If you look at the V-n diagram for a particular aircraft type below you will note that Va is 90 knots whereas the stall speed at a 4.4g load is 94 knots.You can also see from the accelerated stall curve in the diagram that flying at speeds much below Va in turbulent conditions also enhances the possibility of stalls induced by vertical gusts – and also may reduce aileron and rudder effectiveness.
Va decreases as the aircraft's weight decreases from MTOW because it is directly related to Vs1, which decreases as weight decreases – refer rule of thumb 3 above. The aircraft's flight manual may specify lower design manoeuvring speeds for weights below MTOW. (Actually Va decreases with mass rather than weight, but that is splitting hairs a bit.)
Va is a fixed theoretical calculation relative to Vs1 for all aircraft within the same category; for a normal category light aircraft (whose certificated vertical load limit factor is +3.8g) Va = Ö3.8 Vs1 or nearly twice Vs1. For a utility category light aircraft (whose certificated vertical load limit factor is +4.4g) Va = Ö4.4 Vs1 or just over twice Vs1. Va is not marked on the ASI but for non-aerobatic aircraft you can assume it's twice Vs1.
If you look at the V-n diagram for a particular aircraft type below you will note that Va is 90 knots whereas the stall speed at a 4.4g load is 94 knots.You can also see from the accelerated stall curve in the diagram that flying at speeds much below Va in turbulent conditions also enhances the possibility of stalls induced by vertical gusts – and also may reduce aileron and rudder effectiveness.
Va decreases as the aircraft's weight decreases from MTOW because it is directly related to Vs1, which decreases as weight decreases – refer rule of thumb 3 above. The aircraft's flight manual may specify lower design manoeuvring speeds for weights below MTOW. (Actually Va decreases with mass rather than weight, but that is splitting hairs a bit.)
Back to the thread within the thread...
Does the POH actually use the words "Specific Density", "Density", or "Specific Gravity", coz technically, they're all different 
Assuming they meant density, then the DEFSTAN 91-91 specs for Jet A-1 specify a density @ 15C (in vacuum) in the range 0.775 to 0.840
For diesel, the EN590 specs are for a density @15C in vac 0.820 to 0.845
If you wanted to do the job properly you'd then need to convert the density for both temperature and the buoyancy effect of air.
Cutting through the BS, if fuel supply is in litres, multiply by .8 for jet and .83 for diesel to get the weight in Kilos
If your fuel supply is in US galls, complain! (or to get lbs from US galls multiply by 6.7 for jet and 6.9 for Diesel)
The DA-40 POH, by the way, defines the specific density of whatever you put in there as 0.84, regardless of whether its diesel or Jet-A or a blend of both. Safe side of caution I suppose. I've also seen the number 0.81 being used and WikiPedia claims that the density of Jet-A can get as low as .775.
Assuming they meant density, then the DEFSTAN 91-91 specs for Jet A-1 specify a density @ 15C (in vacuum) in the range 0.775 to 0.840
For diesel, the EN590 specs are for a density @15C in vac 0.820 to 0.845
If you wanted to do the job properly you'd then need to convert the density for both temperature and the buoyancy effect of air.
Cutting through the BS, if fuel supply is in litres, multiply by .8 for jet and .83 for diesel to get the weight in Kilos
If your fuel supply is in US galls, complain!
(or to get lbs from US galls multiply by 6.7 for jet and 6.9 for Diesel)
