Density Altitude

[Witch]

I got to wondering about density altitude the other day. I was wondering at what altitude will the power to drag ratio be equal?

 

Planes use a lot of power at lower altitudes because the air is more dense, and less power at higher altitudes because the air is less dense. SO, with the R-22-non-turbocharged engine, at what altitude will the engine have enough air to have adequate power and have enough air for the rotor to not lose lift or stall?

 

I know the max operating density altitude is like 14,000 feet, but what I seek is where the altitude is the highest and the drag is the lowest? Oh, the airspeed is 75kts.

 

Later

Edited August 11, 2008 by Witch

[PhotoFlyer]

I'm confused... Are you asking what altitude will have the highest lift/drag ratio, or what altitude will have the lowest power/drag ratio?

[Goldy]

dude, less peyote next time you write a post !

 

I'm still trying to figure out the question !

[Goldy]

Planes use a lot of power at lower altitudes because the air is more dense, and less power at higher altitudes because the air is less dense.

 

What kind of planes are you talking here? piston engines make less HP every foot they climb...turbine engines become more efficient at higher altitudes so they need less power to fly as fast, and of course there is less drag.

 

So as it relates to helo's, aren't you really asking at what altitude is the air too thin to maintain lift? In a piston engine, you will lose performance every foot that you climb, and of course, forgetting drag for a moment, your rotor will become less efficient in the thinner air. Drag will be less, but never enough to overcome the loss of rotor and engine efficiency.

[Gomer Pylot]

Turbines are more efficient at higher altitudes because the air temperature is lower. Drag has something to do with it, but the primary factor is temperature. High density altitude just means there is less lift available, thus more power is required to get the required lift. This mostly affects hover, and after you're in cruise the effects are minimal.

[Sparker]

I got to wondering about density altitude the other day. I was wondering at what altitude will the power to drag ratio be equal?

 

You mean where you have so little power the rotor blades can't spin anymore because what little power you have is equal to the drag?

 

40,820 ft in an Alouette SA 315-001. Really, I have no idea...

[Witch]

Well, I have no idea what I was thinking!

 

OK, so at what altitude will the total drag of the piston-engined helicopter be the lowest, and power consumption be lowest assuming a cruising speed of 75kts and standard day.

 

I realize the question is a little convoluded. I had no other way of phrasing the question until now. I think my brain has a delay.

 

Peyote?

 

:

Later

[PhotoFlyer]

Sea level? I'm still not sure I understand the question...

[Goldy]

Peyote?

:

Later

 

 

Yes, Witch, those are the little pink flowers in your drink !!!

 

Ok, minimum drag on the ship? That occurs in space, so lets say over 90,000 feet up. You have a trade off of power, you get a lower temperature (lapse rate) as you increase altitude, engines develop more power with cooler air, but your engine develops less horespower cause it also has less air to breathe....dont forget the cuurent POH doesnt allow you to lean the mixture on an R 22. The same friction that gets less with altitude also causes the rotor efficiency to fall. I have absolutely no way to figure this out...care to go up to 15000 feet and check it out?

 

Actually my old CFI once went to 12,000 in an R22 Beta, not a Beta II (this was in 1989) and did an autorotation back to the airport. Took about 10 minutes of free fall....gotta love it.

 

I'm adding more pink flowers to my ice tea. That might help.

 

Goldy

[Sparker]

Well, I have no idea what I was thinking!

 

OK, so at what altitude will the total drag of the piston-engined helicopter be the lowest, and power consumption be lowest assuming a cruising speed of 75kts and standard day.

 

I realize the question is a little convoluded. I had no other way of phrasing the question until now. I think my brain has a delay.

 

Peyote?

 

:

Later

 

The lowest 'total' drag is related to airspeed.. Although I don't know what speed is the lowest total drag in an R22.

[helihi]

Turbines are more efficient at higher altitudes because the air temperature is lower. Drag has something to do with it, but the primary factor is temperature. High density altitude just means there is less lift available, thus more power is required to get the required lift. This mostly affects hover, and after you're in cruise the effects are minimal.

 

 

efficient at what? Fuel burn.? no. If you lower the temperature at any altitude in a turbine that simply means more power and more fuel burn... A turbine LOSES power as the air density decreases... so when you ascend you always LOSE power. The lower air temperature allows you to "recapture" some density altitude... but you will always be burning more fuel at a specific altitude if air temp goes down. So what makes a turbine "efficient" at altitude? It is the thin air that reduces drag on the airframe for a given air speed. The engine simply burns less fuel at altitude because there is less air. So the engine is not actually the "efficient" one.... the airframe is.

[Gomer Pylot]

Helihi, I think you're confused. Specific fuel consumption definitely decreases with increased altitude. The higher you go, the better fuel consumption you'll get. That's why airliners fly in the upper flight levels - they burn much, much less fuel. The main reason for this is that the air is much cooler. You'll also get more power out of a turbine at lower altitudes if the air is cooler. In Alaska in the winter, turbines can produce far more power than in Death Valley in the summer. The turbine produces the same temperature regardless of the input air temperature, but the cooler the air at the inlet, the more power produced in the conversion of kerosene to noise.

[Witch]

Tell ya what...Forget I ever asked the question and go back to what ya'll were doing. I'll go change the litterbox for the cats. They can't seem to do it themselves.

 

Later

[helihi]

Helihi, I think you're confused. Specific fuel consumption definitely decreases with increased altitude. The higher you go, the better fuel consumption you'll get. That's why airliners fly in the upper flight levels - they burn much, much less fuel. The main reason for this is that the air is much cooler. You'll also get more power out of a turbine at lower altitudes if the air is cooler. In Alaska in the winter, turbines can produce far more power than in Death Valley in the summer. The turbine produces the same temperature regardless of the input air temperature, but the cooler the air at the inlet, the more power produced in the conversion of kerosene to noise.

 

No, I believe you are confused... you contradicted yourself in that statement. It is all about mass airflow... which is to say the air density at the inlet of the compressor. The turbine DOES NOT "produce the same temp regardless of the input air temp" The turbine is controlled by the amount of fuel introduced into the combustion chamber and the design airflow thru the engine. At higher altitude there is less airflow and less fuel... THAT is the prime reason for decreased fuel consumption.

[Gomer Pylot]

If you don't believe me, read this explanation. If you don't know who Nick Lappos is, then you need to read more and write less for awhile.

[helonorth]

If you don't believe me, read this explanation. If you don't know who Nick Lappos is, then you need to read more and write less for awhile.

Gomer, don't have such a thin skin. Why do you assume the poster doesn't know who Nick

Lappos is, and why would it matter?