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Increase in Weight demands an increase in Power

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Increase in Weight demands an increase in Power

Old 11th Feb 2014, 16:25
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Increase in Weight demands an increase in Power

To maintain cruise speed an increase in all up weight demands an increase in power.


So, if you cruise along at 2350 rpm at your usual indicated airspeed for the cruise then add fat and heavy friends plus several pounds of baggage to your max all up weight then your 2350 which you may normally cruise around at will be insufficient to give you the same attitude or anywhere near the same airspeed. Although this seems obvious it is rarely stressed in most training documents.


An increase in weight demands an increase in power.
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Old 11th Feb 2014, 16:55
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Higher weight means more angle of attack means more drag means more power required. What is so difficult about it?

Moving weight aft (while remaining in CG limits) will also help since the tail will have less downforce which means less weight to be carried by the wing. Speaking of conventional tail here, not canards.
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Old 11th Feb 2014, 17:13
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I prefer to think of it as:

More weight = more lift.
More lift = more drag.
More drag = more thrust.
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Old 11th Feb 2014, 17:13
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"add fat and heavy friends plus several pounds of baggage to your max all up weight"

Doesn't adding to your maximum all up weight take you over the maximum all weight? More to worry about than power.
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Old 12th Feb 2014, 01:27
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Originally Posted by dirkdj
Moving weight aft (while remaining in CG limits) will also help since the tail will have less downforce which means less weight to be carried by the wing. Speaking of conventional tail here, not canards.
Ok, I am going to be the first to put my hand up and say I dont get this. I have only just started to study aircraft performance though.

I thought moving weight aft, moved CoG aft and therfore you need increased elevator input to keep the aircraft level. Just found this: Effect of Load Distribution but I need to go digest it a little longer.
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Old 12th Feb 2014, 01:32
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moved CoG aft and therfore you need increased elevator input to keep the aircraft level.
Just the opposite.
If CG moves aft (within allowed limits) there is less need for elevator 'input' (or force).
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Old 12th Feb 2014, 02:01
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Doesn't adding to your maximum all up weight take you over the maximum all weight?
If it takes you "over" it doesn't mean you will fall out of sky or be unable to take off. Your performance will be gradually degraded the more you go "over" on the other side but you may still be able to fly just fine. Professional ferry pilots routinely get permission to take off with aircraft weights significantly over the MTOW. I am not advocating operating outside of published book values just explaining basic realities and physics of flying.

Although this seems obvious it is rarely stressed in most training documents.
It was very clear in my training textbooks, I am surprised not in yours.
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Old 12th Feb 2014, 02:32
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Originally Posted by porterhouse
Just the opposite.
If CG moves aft (within allowed limits) there is less need for elevator 'input' (or force).
Ok, so clearly there is a gap here in my knowledge, and I guess I need to sort it out right about now!

This is how my brain is seeing it. I though rearward CoG means plane pitches up, requiring elevator input to pitch it down. What am I missing? Is the CoG always forward of the CoP when in the envelope?

Here I was thinking all confident, hoping to be able to do my BAK exam in 3-4 weeks time!
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Old 12th Feb 2014, 03:17
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I though rearward CoG means plane pitches up
So far OK.

requiring elevator input to pitch it down.
Yes, but precisely what KIND of input?
If the plane pitches up it means its nose goes up but its tail goes down. If the tail goes down what do we do to make the tail go back up? We lower the downward force on the elevator - which means less 'input. The force on the elevator is always down.
Yes, center of gravity is always forward of the center of lift in conventional (non-canard) airplane.
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Old 12th Feb 2014, 03:27
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Originally Posted by olasek
Yes, center of gravity is always forward of the center of lift in conventional (non-canard) airplane.
Ok, so that makes sense now.
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Old 12th Feb 2014, 03:43
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This is all on the first few pages of any aviation PPL textbook, really very elementary part of aeronautical knowledge.
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Old 12th Feb 2014, 03:54
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Andy,

Competition glider pilots will often use ballast to put the CG at the aft limit in order to reduce trim drag and thereby improve performance.
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Old 12th Feb 2014, 04:02
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This is all on the first few pages of any aviation PPL textbook, really very elementary part of aeronautical knowledge.
Yup, I am still in the elementary stage, hence the reason I put my hand up and asked the question. FWIW, I just looked up my text book and can confirm that is says most conventional aircraft have the CoG foward of the CoP. I have not studied in 20 years, so not am I only learning about flying, I am also learning to study again!
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Old 12th Feb 2014, 04:20
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OK, good luck with your study.

Your previous post confused me when you said your were 'all confident' hence my thinking you already spent a great deal of time studying the material.
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Old 12th Feb 2014, 05:17
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Although this seems obvious it is rarely stressed in most training documents.
It is mentioned (indirectly) in places but you are right - it is not stressed. It is not stressed because it should be intuitive - if you could carry more by using the same amount of energy you would be violating basic laws of physics, last I checked all other forms of transportation require more energy to carry more cargo, it would be naive to think we could pull some sort of miracle in aviation. Wouldn't it be nice if 747 used less fuel than Cessna 172?
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Old 12th Feb 2014, 05:54
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Originally Posted by olasek
OK, good luck with your study.

Your previous post confused me when you said your were 'all confident' hence my thinking you already spent a great deal of time studying the material.
Thanks.

I am confident I can do it in 3-4 weeks!! Don't worry, still think I can. I am going to do a practice BAK test tomorrow, that should isolate those area where I need to put in more effort. I did read my text book cover to cover a couple of months back so have the basics covered, but I have only really started to knuckle down into the study this week.
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Old 12th Feb 2014, 06:00
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Originally Posted by Andy_P
Yup, I am still in the elementary stage, hence the reason I put my hand up and asked the question. FWIW, I just looked up my text book and can confirm that is says most conventional aircraft have the CoG foward of the CoP. ...
And the primary reason for that is longitudinal stability.
Pitch excursion (due turbulence, say) down, speed increases. Downward "lift" on tailplane/elevator increases. Mainplane lift increases. Plane pitches up.

Some types, like the PA28, are very stable longitudinally. You can see it easily by moving the tailplane through its full range on a preflight. So in stable trimmed flight there's quite a lot of downward force produced by the tail.

Friend and I flew a 181, 2 up, on a quick 30 mile trip. When I climbed into the back seat, and he re-trimmed to compensate, it was good for another 10kts IAS.
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Old 12th Feb 2014, 07:45
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And the primary reason for that is longitudinal stability.
Pitch excursion (due turbulence, say) down, speed increases. Downward "lift" on tailplane/elevator increases. Mainplane lift increases. Plane pitches up.
Hah, your post made me go off and do a little more research in aircraft stability, which in turn led me to another pprune post that I learned 2 new things from! Maybe I should post that in the "can we learn anything new here" thread! See, us noobs take a lot from this place, even though we may not be able to contribute...

Anyway, I shall go away and study now, I have hijacked this thread enough. Thanks folks for the pointers.
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Old 12th Feb 2014, 08:43
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Friend and I flew a 181, 2 up, on a quick 30 mile trip. When I climbed into the back seat, and he re-trimmed to compensate, it was good for another 10kts IAS.
That's very interesting. We have two 28's at our club, both essentially the same although one is heavier than the other due to a lot more avionic stuff being on board. I always assumed that the heavier one flew slower because it was heavier (bit of a no brainer!) but the CG on the heavier one is way forward of the lighter one. The lighter one is a good 15kts faster for the same power setting, I wonder if it's because of the reason you mention. I'm intrigued now and shall get fat blokes to sit in the back next time.
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Old 12th Feb 2014, 09:39
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See How It Flies

This is an excellent on-line free book if you want to dig a bit deeper into Stability, Balance, etc. Will give you a much better understanding without excessive math.
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