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Effect of Weight on TAS

  • Aviatordan
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Aviatordan created the topic: Effect of Weight on TAS

G'day!

On page 4.12 of the performance text book, there is a sentence that reads (regarding the cruise TAS charts for the Echo):
"Note also that while at 10 000' in ISA conditions at 2500 kg and 65% power, the TAS is 185kts. If the weight is increased to 2950 kg, the TAS only decreases by 3kts. This is true in real aircraft where a fairly large increase in gross weight produces only a small decrease in TAS. It does however, produce a much greater decrease in ROC."

Why does the TAS decrease if weight increases? I have an explanation but I'm not certain as to its accuracy.

The increase in weight will require an increase in lift to maintain level flight. Since the aircraft is being operated at constant power, the increase in lift must come from an increase in AoA. An increase in AoA will cause an increase in parasie drag and the resulting increase in lift will cause an increase in induced drag. These increases in drag will reduce the IAS and thus TAS at constant power.

So is it fair to say that if the aircraft wasn't being operated at constant power and the extra lift was created by an increase in IAS, the TAS would no longer decrease, but actually increase? Therefore the only way TAS would decrease in level flight with an increase in weight is if the aircraft was being operated at a constant power setting? I mean, I know in this scenario the fuel flow would increase, but all I'm worried about it the effect of weight on TAS.

Cheers guys!!

Dan :)
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bobtait replied the topic: Re: Effect of Weight on TAS

Dan

It's great to see that you are putting some thought into your study and not yielding to the temptation to just 'learn the questions'. Your reasoning is sound. More weight requires more lift. It is impossible to increase lift without increasing drag since lift and drag are simply components of the total reaction. So one option would be to increase the angle of attack to get the extra lift.

On page 6.2 of the Aerodynamics book, I have shown a mathematical proof that:

Power = Drag x Speed.

It follows therefore, that if power is constant, any increase in drag must be accompanied by a decrease in speed. So if you fly with a constant power setting [which is what we do in GA], any increase in weight will result in an decrease in speed.

However if you chose to get the extra lift by increasing IAS instead of angle of attack, you would have an increased TAS [assuming height is maintained]. You would also have an increase in fuel consumption of course. But it would not be true to say that the increase in TAS was due to the increase in weight. The in this case, the increase in TAS is due to the pilot's decision to increase power and therefore IAS.

Bob
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  • Aviatordan
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Aviatordan replied the topic: Re: Effect of Weight on TAS

Hey Bob,

Thanks for that reply! So is that the reason the airliners tend to stick to the higher flight levels? I mean, aside from the fact that the engines are more efficient at that height. I now have another for you, surprised? :P


With regards again to the cruise TAS charts.

You give two methods for solving the example on page 4.11.

I tried to work this one out by myself, without looking at the solution first (which is what I try to do with all the examples) and got it 2kts off target. The method I used was:
- Find the density height;
- Enter the ISA table and locate 11200' density height under 65% power.
- Since 11200' is close enough to the mid-way point between 10 000 & 15 000 I took the speeds for those two heights (185 & 192 kts respectively) and averaged them to get 188.5 kts.

The reason I went with that method was becauce it was the method used in interpolating the the max rate of climb tables for the echo. My question is, is that still a valid way of interpolating between the two? Would it be fine to not only use that method in the exam but also in practice?

Cheers!

Dan :) :side: :side:
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bobtait replied the topic: Re: Effect of Weight on TAS

Firstly Dan remember there is no need in practice to get too excited about perfect accuracy in extracting this kind of information. After all there are so many variables. Such as:

Have you got exactly 65% power? This depends to some extent on having the mixture exactly set for economy. You are using a forecast wind to establish a ground speed. If you manage to extract the TAS figure exactly to a fraction of a knot, you are then going to find a ground speed by applying the Met man's guess for wind which is to the nearest 10° and 5 knots. Anyway, how accurate is you dip-stick? Do you know how much fuel you have on board to a fraction of a gallon? - that's one reason why we carry reserves.

The good news is that these types of questions will not appear in the CASA exam because they don't give you a TAS table in the first place. The exam questions will simply give you a TAS to work with.

As for your calculation, if you want to work to half a knot, 11200 feet is not exactly half way between 10000 and 15000. The TAS is 185 at 10000 and 192 at 15000 so there is a 7 knot increase in 5000 feet that's about 7 ÷ 5 = 1.4 knots per thousand. So the TAS at 11200 would be about 1.5 knots higher than the 10000 foot figure. That makes it about 186.5 knots.

Having said all that, it's not the way professional pilots plan. Most pilots of GA type twins simply 'block the TAS'. They fly at the same power every day so they pick one value for TAS [maybe with a slight adjustment for weight and height in a turbo charged aircraft]. It is certainly not necessary to go to extremes interpolating. These examples were included in the book to help you become familiar with the performance of a turbocharged twin like the Echo. I was not suggesting that you need to spend a lot of time flight planning to a fraction of a knot.

The TAS tables are useful when you have just been endorsed on a new aircraft type. Sit down on your patio one Sunday afternoon with a stubby and have a play with the performance tables. It helps you get a feel for the performance you can expect from this aircraft.

Keep having fun

Bob
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Aviatordan replied the topic: Re: Effect of Weight on TAS

Hey Bob,

I guess you're right re. the half a knot working out. That would be very hard to work to in practice! I guess the fact that for the last 6 or so months I've only been doing theory tends to put me in the 'half a knot' frame of mind. I'm sure once I get out there an flying again I'll have a different view of things :)

Thanks for that, Bob!!

Cheers,

Dan =].
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