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Load factors and % of wing span for ground effect

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Jase88 created the topic: Load factors and % of wing span for ground effect

Hi Bob and Co,

Just a quick one.

With the load factors will they ask for odd bank angles in the exam, eg. 23 degrees? Or will it just be the standard 30, 45, 60, 75, 80?

Also with the % of wing span for ground effect which ones do you need to memorise?

Thanks,

Jason
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John.Heddles replied the topic: Load factors and % of wing span for ground effect

With the load factors will they ask for odd bank angles in the exam, eg. 23 degrees? Or will it just be the standard 30, 45, 60, 75, 80?

It shouldn't matter. If you have memorised some points, that's fine (and that will be the likely story as there is not any real value to be had expecting the candidate to work out an answer).

If not (and you have a Jepp whizz wheel in your pocket) you can just run the sums for any oddball angle the examiner might throw at you. I don't bother memorising any values when I can very simply look it up on the Jepp.

Load factor = 1 / cosine (bank angle)

The Jepp wind calculations involve trig values and the scales give you sine and cosine to do just that. So, just look up the cosine on the Jepp and then plug it into the equation to get the load factor, n. How do you do that ? Pretty straightforward.

You'll notice, on the wind side, (line up the TAS index on the inner scale with the 10 on the outer scale and put this up to make it easier to read) that, if you read to the left from the TAS index (usually a black background scale) near the end of the black, but inside it, there is the term COS , which is short for cosine. Beyond the end of the black, ie outside the black bit, there will be another term SIN, which is short for sine. The inner scale, marked in angles, is exactly identical to the outer scale, marked in numbers except that the numbers on the inner scale have been replaced by the angles whose sine and cosine equal the removed numbers.

Several other bits of information you need to know.

(a) Sine of an angle is the same as the cosine of (90-the angle). Or vice versa.

(b) the whizz wheel scale leaves off some angles for reading clarity in the range of the black cosine scale. So while the sine scale (reading to the right of the TAS index) appears to stop at around 45 degrees, it actually keeps going, although it is not marked. You can read it easily by looking at the cosine scale and subtracting the cosine angle from 90. So, where the cosine scale angle says, say, 30 degrees, that is exactly the same as the sine scale reading

(90-30) = 60 degrees.

Similarly, you can continue the cosine scale around to the left past the black bit using the (90 - the angle) trick.

For example,

sin (40) = cos (90-40) = cos (50)

cos (80) = sin (90-80) = sin (10)

What this all means is that, if you set the TAS index against 10, then if you

(i) read around to the right until you get to whatever angle, the outer scale will give you the sine value of the angle.

(ii) read around to the left ... the outer scale will give you the cosine value of the angle.

(c) you will notice that the angle scale goes around twice. For instance, if you look at the black section at, say, 30 degrees, you will notice another angle value outside the black at a bit under 5 degrees. All this means is that, for larger angles greater than about 5.73 degrees, the sine values are greater than 0.1 while, for angles less than about 5.73 degrees, the sine values are less than 0.1. So, for example,

sin(7) = 0.122
sin((4) = 0.070

Generally, we aren't going to be working with such small angles so you shouldn't have to worry too much about this point.

The Jepp, unfortunately, isn't set up to do chain calculations like a normal scientific slide rule, so to work out the load factor for, say, 60 degrees,

(a) you set the TAS index against 10

(b) read around to the left until you get to cos (60) which we recall is where the sine scale is (90-60), or 30 and we read off 0.5 which is the value for cos(60) and, also, sin(30).

(c) turn over to the calculator side, and set 10 on the outer scale against 50 on the inner scale and read off 20 on the outer scale against 10 on the inner scale. Load factor is 2.0. Notice that you have to figure out where the decimal point goes - the whizz wheel can't do that for you.

The story may have appeared a bit complicated but it is really simple if you just follow the last three steps.

You can run a bunch of calculations with the whizz wheel against your scientific electronic calculator to make sure that you have it all under control.

Easy peasy for the exams, don't you think ?

Also with the % of wing span for ground effect which ones do you need to memorise?

Again, it is a bit pointless memorising a bunch of numbers although the usual story is that we can detect ground effect things quite readily once we are around half wingspan above a flat surface. Conversely, once we are more than a wingspan above the surface, we aren't going to observe anything much at all. Just where the pilot can see the effects easily will vary a bit with the aircraft but, certainly, by 50% span it should be pretty obvious. For ultra low flyers, like WIG aircraft, some more extreme effects are noticeable but, for aeroplanes we don't have to worry much about that.

If you are interested in a bit of brain strain, you can look up various papers on the net which will give you lots of information on ground effect research and equations to delight you over a coffee or ten.

Engineering specialist in aircraft performance and weight control.
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Jase88 replied the topic: Load factors and % of wing span for ground effect

Hi John,

Unfortunately the permitted materials for the CPL aerodynamics exam are just a basic calculator (non scientific).

It seems in this case that i would only have to memorise the main load factors then?

Thanks
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John.Heddles replied the topic: Load factors and % of wing span for ground effect

Unfortunately the permitted materials for the CPL aerodynamics exam are just a basic calculator (non scientific). It seems in this case that i would only have to memorise the main load factors then?

Afraid you might need to read my post again. The whole idea is that you ONLY need a Jeppesen whizz wheel and that is fine for the exam. You don't need an electronic calculator.

Have another read and run the exercises on your Jepp. If you are using a Dalton, then you have a problem as they don't run the wind triangle with trig. I prefer to use the Dalton but, really, you probably are better off for CPL/ATPL using the Jepp - only due to its having a couple of extra goodies incorporated.

Engineering specialist in aircraft performance and weight control.
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Jase88 replied the topic: Load factors and % of wing span for ground effect

Hi John,

Sorry for the misunderstanding.

Could you please direct me to where it says that you're allowed a whizz wheel for the CADA exam? I"ve looked on the CASA page and i cant find it anywhere www.casa.gov.au/licences-and-certificate...a-clwa-cfpa-and-csya

Thanks
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John.Heddles replied the topic: Load factors and % of wing span for ground effect

I may be out of order on the exam specifics - will follow that up and come back in due course. I had presumed (perhaps incorrectly now that you have highlighted it) that the slide rule would be considered equivalent to a calculator. I've run the question to the examiner for clarification.

If the whizz wheel is out, you could use a series expansion but it probably would be easier just to learn a few points. Either way, I'll come back next week when I get a response.

However, the discussion is general and it should be known that you can use the Jepp whizz wheel for a range of the usual trig calculations.

Engineering specialist in aircraft performance and weight control.
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Jase88 replied the topic: Load factors and % of wing span for ground effect

Hi John,

That's all good.

Thanks for the explanation on how to use the whizz wheel, it will probably come in handy at some stage. I'll wait to hear from you regarding the exam allowances.

Cheers
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John.Heddles replied the topic: Load factors and % of wing span for ground effect

Looks like I was out of order. Gavin has come back and confirmed that the whizz wheel is not permitted in CADA. I can't really agree with his reasoning but that is irrelevant as he gets to make the call.

So, it's an electronic gadget to do the sums.

Engineering specialist in aircraft performance and weight control.
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