Drag reduction with altitude?

[Vigilant1]

Short answer: Yes, the decreased HP requirement (thus fuel burn) at altitude is due to lower drag.

Total drag on an airplane is parasite drag plus induced drag ("drag due to lift'). At low airspeeds, induced drag predominates, at high airspeeds parasite drag is more important.

With a reduction in air density (say, due to being at higher altitude) and a constant airspeed, parasite drag decreases, but induced drag increases. A look at the formulii for computing parasite drag and induced drag will reveal why this is the case.

Intuitively, it should make perfect sense that a plane traveling through a less dense fluid will experience less parasite drag. The situation regarding induced drag may be slightly less intuitive.

 

[Stolch]

You don’t need a pressurized airplane above 18,000 but the pilot needs pressurized (forced) supplemental breathing oxygen delivered through a face mask up there. Up there your lungs are not capable of drawing in and transferring oxygen into the bloodstream so a nasal cannula or similar won’t prevent hypoxia and it happens quickly up there.

 

[Stolch]

…also as altitude increases fuel burn is reduced because as air density decreases in the combustion chamber, less fuel is required to maintain the ideal air-fuel ratio.

 

[wsimpso1]

With a flat rated engine Would a aircraft have the ability to fly at faster indicated air speed as it climbed or would it simply hold the same indicated airspeed ?
It’s speed over the ground would be increasing if it did right ?

Somebody needs to do some basic physics.

Indicated airspeed measures dynamic pressure calibrated for sea level pressure and temperature, and tells us the forces of the airflow over the airplane. q = 1/2*rho*v^2, where rho is fluid density and v is true velocity. In slugs/ft^3 and ft/s, it comes out in lb/ft^2 or psf. If you take an airplane with q= 50psf and 3 sf of effective drag area (product of Cd and S), that is 150 pounds drag. It matters not at all how you get q=50 psd, you get the same drag if your airspeed indicator is reading the same.

Power is force applied through a distance in time we are not talking a theoretical distance, real distance swam through the air. Power = F*dx/dt = F*v.

Applying some numbers. At sea level, rho is 0.002378, and v to get q=50 psd is 205 fps or 122 knots true. Power is thus 150*205=30750 ft-lb/s or 56 hp.

At 10,000 feet, rho is 0.0017555, and v to get q =50 psf is 239 fps or 142 knots true. Same 150 pounds total drag. Power is 150*239=35850 ft-lb/s or 65 hp.

I put to you, through some really easy physics that it takes MORE power to fly same airplane to same indicated airspeed at higher altitude. Your flat rated engine will not take you to the same indicated airspeed high as it did at sea level - it does not have enough power...

Let's answer the "same power" question at two altitudes. First off we have to make some assumptions. Is the flat plate equivalent area the same as we slow down? Usually it drops some as we fly at lower q, but for the sake of keeping it simple here, let's say it stays the same.

Power = F*v = 1/2*rho*v^2*A*v = 1/2*rho*A*v^3

So for same power 1/2*rho1*v1^3*A = 1/2*rho2*v2^3*A, the 1/2's and A's are the same, so we reduce things a bit:

rho1*v1^3 = rho2*v2^3, and solving for v2

v2 = cube root(rho1/rho2*v1^3)

Given the sea level case, with 0.002378 spcf, 205 fps, 122 knots true, and 56 hp, while the 10,000 foot case of 0.0017555 spcf, you will go 227 fps, 135 knots true, and 110 knots indicated.

So, the flat rated engine will indicate lower as you go up, but will be going faster through the air as it goes up. Just not a huge amount and all airplanes get this same effect.

If you are wondering about how much a naturally aspirated engine loses with altitude, its power basically drops linearly with atmospheric density.

Please play with the inputs yourself and answer your own "what if" questions, here is the atmosphere A Table of the Standard Atmosphere to 65,000 Feet in US units, and a little Excel time will let you work out any question you want.

Billski

 

[Voidhawk9]

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[bmcj]

Since seaplane speeds increases at higher altitudes…
Sully preparing for takeoff:

 

[raytol]

Lower temperatures and lower humidities perhaps?

 

[terke]

With a flat rated engine Would a aircraft have the ability to fly at faster indicated air speed as it climbed or would it simply hold the same indicated airspeed ?
It’s speed over the ground would be increasing if it did right ?

It would hold the same IAS, but TAS would increase roughly 2% for every 1000 feet of altitude gain. GS is a resultant of TAS and wind conditions. Thus talking about GS has no bearing on your initial question.

 

[wsimpso1]

Lower temperatures and lower humidities perhaps?

What is this phrase referring to?

 

[ProfJ]

You don’t need a pressurized airplane above 18,000 but the pilot needs pressurized (forced) supplemental breathing oxygen delivered through a face mask up there. Up there your lungs are not capable of drawing in and transferring oxygen into the bloodstream so a nasal cannula or similar won’t prevent hypoxia and it happens quickly up there.

Lot of glider pilots go above 18k without forced pressure (in wave flight etc.). There's no real difference between a standard mask and a cannula if you breathe through your nose, and IIRC both work with diluter demand type regulators. FAA only suggests actual positive pressure O2 masks above 40k - see here https://www.faa.gov/pilots/safety/pilotsafetybrochures/media/oxygen_equipment.pdf . As a person who once walked to 19600ft without any O2 equipment, it would have come as a nasty surprise if my lungs had stopped working...

 

[tls258]

Here's a poke at logic. "Dragieness" is explained here: What is Drag?
So, taking a gander at "air", we know its density varies with altitude, humidity, and temperature. You/we are pushing stuff (forms) through this air. At a given horsepower and shape (form drag), you can push it just so fast until the drag uses up all of the horsepower. Looking at the velocimeter gives you how fast you are pushing through the fluid (air). Altitude and air density will not change the shape of your airplane (usually). Air density will affect the ability of the power plant to produce power, especially normally aspirated piston engines. So, going up in altitude results in less dense air so the form drag at a higher altitude and given speed is less. However, as Marc pointed out above, the engine will start gasping for breath as the air gets thinner. The intersection of those two lines (altitude and horsepower) gives the maximum airspeed at a given altitude. Which is why light sport rules certify the aircraft as its top speed at sea level. For instance, a Sonerai can't exceed the 130 at sea level as designed, but can easily exceed that airspeed at 7,500. Start up the mountain, and speed will degrade as horsepower tapers off. You can try this yourself if you like hiking.

 

[PMD]

I refrain from commenting on personal experience as regulators, enforcers and insurers may well be following this thread. One of the airplanes able to exploit clean airframe and flat rated engines is Beech 18 - often put on floats. I can no longer remember the exact (critical) altitude where full throttle results in 65% desired cruise power but can comment that those airplanes cross the Rockies at much nicer true airspeeds than normally aspirated airplanes that also do the same trip at the same altitude. Reality is, though: once you put one on floats the mission profile usually changes from long range to very short range trips, so those who fly their D18S or for that matter DHC2 off of water are not so much concerned with efficiency as access to a paying load.

The biggest difference is that these used to be 17 and 18k crossings but due to the 500 nonsense are now forced down to 17.5 and 16.5 screwing up those who might pursue the ultimate in efficiencies (and available Westerly components) without violating middle altitude enforcement limits exposed by this new fangled "Mode C" stuff...so I have heard.

 

[Pilot-34]

Great guys thanks 10%.

I need to clarify that statement then I will edit it.
Vigilant 1 Recommended I take a look at the specifications for a Cessna on floats.
It turns out it was perfectly and exactly what I was looking for.
Sadly the difference was only about 10%.
What I meant was
“Great guys! Thanks for the help ! I finally got it.
The difference is about 10%.”

I appreciate the help

 

[speedracer]

You don’t need a pressurized airplane above 18,000 but the pilot needs pressurized (forced) supplemental breathing oxygen delivered through a face mask up there. Up there your lungs are not capable of drawing in and transferring oxygen into the bloodstream so a nasal cannula or similar won’t prevent hypoxia and it happens quickly up there.

I have over 600 hours cruising between 16,500' and 17.500' in my Long EZ using a canula. In summer the DA is as high as 20,000+'. I carry an oximeter. Near sea level I'm usually around 96%. At altitude it will read in the low 90's. That tells me that I'm only a little bit dumber up there, A glider guy told me he uses a canula and sometimes gets up to around 30,000' working waves. He uses a proprietary breathing method. He breaths in as far as he can then blows as hard as he can without letting any air out. He said this forces oxygen into the lungs. Works for him, I guess.

 

[Pilot-34]

A lot of this thread was way over my head and beyond anything I meant to ask.
And while I understand you’re trying to help , and do it with accuracy . Please remember that for a lot of us that quit paying attention to math in fourth grade the stuff is like trying to drink from a fire hose.
I appreciate your responses honestly I do but I do feel frustrated at times.
I feel like I’m on the interstate driving a hooptie 45mph while rocket scientist go by me at rocket speed!
I suppose that’s because this is one of the few places in the world Where fourth grade educated mechanics rub shoulders with the rocket scientists And world-class engineers of our world.
You don’t know how many times I have grabbed one of my buddies and shown him a particular post in this forum and said “look this is the guy that did that”
So let me once again say thanks for the help I really appreciate it even if I do get frustrated when I need help to understand the help that’s helping me understand the help that I need.

 

[wktaylor]

P-34.

Elements of Your questions were addressed/discussed in parallel within a thread that was discussing adapting available float planes to operate on 'remote, high-altitude small mountain lakes'. There are huge technical challenges and safety issues operating these type aircraft at 'O2-high altitudes'.

Any aircraft equipped to operate on water has significant disadvantages, not the least of which are higher drag due to the 'water hulls', empty-weight increases due to the hulls, power[thrust] available [engine-aspiration and propeller combos], fuel/payload trade-offs, pilot training, etc...

Marc Zeitlin's exasperation with Your persistence RE: open-ended/expansive question... is very real... I feel it too.

 

[Pilot-34]

I can feel that but at the same time I was exasperated by the continuing expansion of my question.
We need a keep it simple flag!

 

[Pilot-34]

I meant to ask a fairly simple question but there are so many here that know so much about things I’ve never heard that they see it from that veiw.
I didn’t mean for it to go into those areas so I try to narrow it down. Then others want to throw unheard of variables into the situation and I just want to scream does it really matter?
And of course each time something like that happens I think of a new way I should’ve asked the question the point being to keep it simple.
Lol And now I can see that I was being drawn into a more complicated question in a different thread

 

[Voidhawk9]

Then others want to throw unheard of variables into the situation and I just want to scream does it really matter?

Yes, that is why they were mentioned. If ignored, the answers become meaningless, or at worst, misleading.

 

[Pilot-34]

How so ?
I went to the POH. That Vilagent1 recommended and got exactly the answer I was looking for and they didn’t discuss any of that.
Should I be suspect of the results?