Drag reduction with altitude?

[smokyray]

Maule produced an M5-210TC for a short production run of 12 aircraft. The IO360TC was specifically built for that model on a by order basis. Since most people opted for the 0-360 or 0/IO-540 it didn't sell well but it's few customers touted some impressive numbers above 13K. I delivered a few new M5s back in the 80s and one of the factory test pilots told me he routinely saw 180KTAS at 18K. So the TC Maule is one of the few potential float planes that would go faster up high thanks to turbocharging.

Smokey

Airplane Review – Turbo Maule M5-210

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[speedracer]

But I don’t recall seeing pressurized flying boats or float planes.

Who says you need a pressurized plane at 20,000'? The current world altitude record for the 1,102 pound weight class is held by a 320 powered (normally aspirated) Long EZ (unpressurized) at over 35,000'

 

[Tiger Tim]

Who says you need a pressurized plane at 20,000'?

You don’t technically need a pressurized airplane but if the plan is to consistently go that high I would plan to climb and descend at 500-700fpm. That’s about a half an hour on the way up and another half hour to get back down, shortening the time spent in this advantageous cruise. It can still make sense on long legs, of course.

 

[wsimpso1]

Hmm. Most powered airplanes are designed to get off the ground or water at considerably lower speeds than they cruise, and have a fair amount of excess power when they get to the indicated airspeed where induced drag and parasite drag become equal, which also happens to be the indicated airspeed where minimum drag occurs. Most powered planes also happen to have excess power at this speed and and so they can climb best around this indicated airspeed. Take the airplane aloft until it won't go any higher, and it is flying highest at this indicated airspeed.

Parasitic drag - Wikipedia

en.wikipedia.org

So, most airplanes fly more efficiently as they go up, and they do this flying at lower indicated airspeeds than at lower altitudes. They may maintain true airspeed, but their indicated airspeed slowly decreases.

Now if you made a seaplane that could just barely get off the water and fly in ground effect, but as as the bird tries to leave ground effect, power required exceeds that required to climb, you have an Ekranoplane, which is an airplane with a ceiling about equal to the airplane's wingspan.

Our typical power plane with ceilings in the 12,000-18,000 foot range are more efficient as they go up NOT because drag at any given indicated airspeed does not decrease, but because they fly at lower indicated airspeed which is lower drag and thus lower power required. Why do we fly them at lower indicated airspeed? Because they won't fly as fast - the engine usually makes less power as we go up so we can not fly as fast.

Now an airframe that has more sea level drag also has more drag at all other altitudes. If you want more airspeed on a given amount of power, you gotta go lower drag.

Billski

 

[Pilot-34]

Who says you need a pressurized plane at 20,000'? The current world altitude record for the 1,102 pound weight class is held by a 320 powered (normally aspirated) Long EZ (unpressurized) at over 35,000'

Lol well I have to admit you’re right but it does seem to me with some pressurization up in the flight levels Might make things a wee bit more comfortable

 

[Pilot-34]

My thought is it would be handy have a turbo in a seaplane to get you off the water and above the obstacles.
Then again that turbo would come in handy later to help you maintain horsepower at altitude and thus a higher cruise speed at a more economical altitude?

 

[REVAN]

A draggy airframe and a slick airframe experience an identical % reduction in drag from decreasing air density.

Don't know if someone already brought this up, as I haven't read all the responses: Form drag (aka - parasite drag) will generally reduce proportional to the drop in air density. Mid to low Reynolds Number components like struts and wires can have non-linearities to consider. The big countervailing driver is that the induced drag will generally increase as you fly higher (especially true with aircraft powered by normally aspirated engines). At some altitude, you will eventually pass through best L/D and start creeping closer and closer to the aircraft's stall speed for that load and altitude. The increased induced drag, will at some point, negate the reduced form drag.

Even with large amounts of power, aircraft will hit limits. The U-2's operating ceiling is pinched between the aircraft's stall speed at altitude and the Mach limit for the airframe. Essentially it is flying at maximum Mach and just above stall speed, simultaneously.

 

[Voidhawk9]

...the induced drag will generally increase as you fly higher (especially true with aircraft powered by normally aspirated engines).

This is not because of the altitude directly. It's only if you fly at reducing IAS.

 

[pfarber]

Turbocharging, outside racing, is for altitude, not power,generically

You are confusing turbonormalizing vs flat out turbocharging.

The former makes high altitude seem like sea level, the latter simply pressurizes the cylinder to whatever psi you want.

 

[Pilot-34]

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 ?

 

[wsimpso1]

Dynamic pressure is 1/2*rho*v^2. That is the stagnation pressure of a fluid with density rho and speed v. Our airspeed indicators measure this dynamic pressure, called q, and have the display calibrated in airspeed for standard density of 0.002378 slugs/ft^3. This density of air at 29.92" Hg and 59 F. Forces of lift, drag, pitching moments all start with q. The airframe "feels" q.

No matter what the density, pressure, and temperature, if your airspeed indicator and g meter both read the same as they did under some other condition, you have the same forces from air on the airframe, well until you get significantly into compressibility effects, but I doubt we are talking v > 0.75 Mach.

So if you hold indicated airspeed and g over a variety of conditions, the airframe drag and power required are the same. Now if one condition has lower density air while you hold indicated airspeed and g, the true airspeed will be higher than at higher densities.

But at no point short of spaceflight can we make a draggy airplane behave like a clean one simply by cruising higher. Everybody else gets the same bonus...

Billski

 

[Pilot-34]

OK let’s ask the question in a simpler way as possible
If I have a draggy amphibian that cruises at 120 miles an hour at 1000 feet will the same hourspower get me 140 at 15000 feet ?

 

[Vigilant1]

OK let’s ask the question in a simpler way as possible
If I have a draggy amphibian that cruises at 120 miles an hour at 1000 feet will the same hourspower get me 140 at 15000 feet ?

Are those indicated airspeeds, calibrated airspeeds, true airspeeds, or ground speeds?

 

[Pilot-34]

Ground

 

[Voidhawk9]

Ground

Then you will need to define more variables, such as wind at 15,000ft, and aircraft heading.

 

[Pilot-34]

So you guys really don’t know the answer do ya ??

 

[Marc Zeitlin]

OK let’s ask the question in a simpler way as possible
If I have a draggy amphibian that cruises at 120 miles an hour at 1000 feet will the same hourspower get me 140 at 15000 feet ?

I'm not sure why I'm bothering here, because if you're going to be a dick, folks aren't going to want to help you out. You not understanding the responses is not the same thing as people not knowing "the answer".

The rule of thumb is that for a given HP output, you want to fly at the highest altitude at which you're still able to maintain that HP output in order to maximize both speed and efficiency.

IOW, if you want to use 75% power, then you want to fly at ~8k ft DA, as this will maximize the speed/efficiency at that power output. Fly lower, and you'll be slower and less efficient, and fly higher, and you can't get to 75% power.

 

[Vigilant1]

For someone who genuinely wants to gain knowledge and insight, I think the best approach would be to look at actual data. A possible approach:
- Refer to the Cessna 180K performance data. Assume fuel burn is proportionate to HP used. Pg 5-17 gives data for cruise at 2000 ft msl, pg 5-22 gives data for 12k msl.
https://washingtonseaplanepilots.org/resources/Seaplane POHs/Cessna180KFloatplanePOH.pdf

- Convert KTAS speeds to groundspeed MPH or IAS, etc or any other desired units.
- Note trends

Okay, there's the fishing pole, tackle, and book.

 

[Pilot-34]

Great guys! Thanks for the help ! I finally got it.
The difference is about 10%.

(edited to clarify what I meant ( I hope ))

 

[Pilot-34]

When I look at the chart that vigilant supplied me with its pretty easy to see the difference in speeds for a given horsepower between the lowest 2000 and the highest 12,000 feet.
Nothing else is changed between them except for the effect of dragieness
In other words what could account for the extra speed except for the reduction in drag?