Altitude vs Performance
 

I appreciate that as you increase altitude the air gets thinner and the performance of a piston helicopter will go down. This being the reason that some piston helicopters have turbochargers fitted so that the actual hp is available at higher altitudes. How much power is actually lost in a piston helicopter at say 1000 feet above sea level and 2000 feet about sea level. If I had a 205hp engine and flew at 2000 ft would it be like 180hp or what?

Thanks

Why don't you do a little research yourself and get back to us?

Simply because I'm considering whether to go for the turbo or non turbo version of this particular helicopter and since I will only be operating between 0 and 2000ft for the majority of my use wondered if it would make a noticeable difference. The reason most people ask questions on this site is for some help and advice. Thank you for giving neither. Prat
:yuk:

More parameters than just altitude are required to do the calculation. See the link below. Multiply the engine power which is calculated for standard atmosphere by the air density ratio and you will get the actual power.

Cheers

ATN


density altitude calculator

Thank you for your kind and constructive reply.

A quick reply would be that the Turbo helps with altitude and that the difference between Ops at Seal Level to 2,000 feet is not very significant.

Step up to much higher heights and for sure the Turbo matters as it will allow you to maintain maximum rated power to a much higher altitude.

Best and quickest way is to take a look at the IGE charts for both models. Pick a OAT and run the line down to the altitude where you can hover at max. weight. Anything higher will need a weight reduction due to reduced power available of the normally aspirated engine. Now look at the chart for the turbo model, pick the same OAT line and see how much higher you are able to go at max. weight. This shows the increased performance due to the turbo's ability to hold rated power to a high density altitude.

Old Rules of Thumb
 

Rule of Thumb down in the lower reaches of the atmosphere is that you lose about 1 lb/sq. inch of atmosphere pressure for each thousand feet of climb, therefore if you could get 29" Manifold Pressure at Sea Level (for example), then at 2000' above SL you would have a max of 27" MP available.

This needs to take density altitude into account. How?

For each degree C above standard temp for the altitude, add 120 feet to the pressure altitude. This will give you Density Altitude.

Example: You are on an airfield, dial in 29.92 on your altimeter and it reads 1000 feet. This is Pressure altitude (close enough for your purposes). The Temperature is +21C. Standard Temp for 1000 ft PA is +13C. That means 8 degrees C above standard. 8 x 120 = 960...about 1000...so add 1000 feet to the pressure altitude of 1000 feet and you get a Density Altitude of 2000 feet.

Now using the Rule of Thumb above, if your mill would develop 29" MP at Sea Level, you can expect it won't do more than 27" MP where you are at in the example.

Now, Horsepower? If the book tells you the mill develops 150 HP pulling 29" of MP, then 27/29 = 139 HP, tops. In other words, about an 8% reduction in power...so you better get the ship about 8% below Max Gross, eh? For a hypothetical GW limit of 2000 lbs, this would mean offloading down to a TO weight of 1840 lbs. So there goes a lot of people or fuel...

But wait, there's more! Don't forget that Density Altitude of 2000 feet means less lift (less dense air...) being produced by the rotor blades (and less tail rotor thrust being produced, but we aren't concerned about that just this minute...). So less lift the blades can produce, means you better offload some more weight, eh? How much? Without actually doing a Lift Equation calculation, why not figure another 8%? So now your 2000 lb aircraft can only go with about a 1680 lb takeoff weight. A reduction of some 320 pounds...that's at least a pax and a half, or about 50 gallons of fuel...

Be advised, all the above is hypothetical and based on rules of thumb. The Rotorcraft Flight Manual is, in all cases, your best friend and will give you accurate information. But now you can understand why most folks opt for a turbocharged engine, if available.

A lot of manufacturer's data for these calculation is available online and you should visit the websites. In any and all cases, a call to a friendly sales rep will get you assistance and accurate planning figures.

Good luck.

applicable to piston engines onlly....