You can get carb icing if the degree difference between dew point and OAT is 20F/11C. Since you had a 15F/8C difference, it is likely that you experienced carb icing (it was already pretty cold that day).
As far as engine performance is concerned, the O-360-J2A (Lycoming engine for Beta II) was designed at 180 BHP; however, Lycoming only tested it at 145 BHP because they thinned-out the cylinder heads and used Aluminum pistons to save weight (and they wanted to cover their tail feathers). The engine is fully capable of producing 180 BHP, but the data plate says 145 BHP. So that's what we go by. If you stay within POH limits, you should not exceed 131 BHP. Why did RHC specify 131 BHP in their design?
In piston engines, you loose about 7 horsepower for every thousand feet (roughly). It's the pilot that derates the engine by staying within MP limits, which affects your angle of attack on the blades. There isn't really a mechanical device that "derates" the engine... just the pilot.
Therefore, at 7000' you have every bit of that 131 horsepower available to you...
180 HP
- 7 (thousand) x 7 HP = 49
--------------------------
131 HP
Schweizer's web site claims the 300 has a HIGE of 10,800 feet. This is a sneaky advertisment because this is at a weight of about 1400 pounds... one person and an hour of fuel. At gross weight, the Schweitzer can only handle 5,900 feet.
Piston helicopters that operate at their maximum horsepower at all times are not high altitude performers. Specifically, the performance power available to their engines is extremely limited when compared to the Beta II at the same altitude because you are subtracting from an available horsepower figure that was running at maximum at sea level! In other words, they don't have the same altitude "buffer" that the R22 Beta II does.
The Beta I and Beta II are almost the exact same aircraft. A few minor changes, but the biggest difference is the engine. So if you fly mostly at lower alititudes, you will NOT have ANY performance increase from a Beta II vs. Beta I. If you fly at higher altitudes, you will need the Beta II.
One more thing, most folks don't know how to properly use the chart that
Johe02 posted earlier. You are probably already aware of this, so forgive me if I'm speaking in infant terms...
There is a reason why the MAP lines extend past the "full throttle" line. The extension is used to calculate your max continuous power. Example...
Assume 6000 MSL @ 0 C
To calculate your 5-minute MAP rating, you simply follow the chart. Go to 6 line at bottom and follow it up to the 0 C line to find your 5-minute MAP rating. In this case, you run into the "full throttle" line BEFORE reaching the 0 C line. Therefore, your 5-minute MAP rating is about 23.5 inches.
Now, the chart says, "for MCP subtract 1 inch MAP." Without putting much thought into it, that's exactly what most people do... and that's exactly the wrong thing to do. To calcualte your max continuous power rating, you subtract 1 inch MAP from the
calculated 5-minute MAP rating!
Now, the reason for the extended MAP lines beyond the full throttle mark becomes clear. The calculated 5-minute MAP rating in this scenario is 24 inches (actual 5-minute MAP rating is 23.5 inches at full throttle). If you subtract 1 inch from the calculated 5-minute MAP rating, your max continuous power rating is 23 inches.
In summary the difference between your actual 5-minute MAP rating and your max continuous power rating is only 0.5 inches in this case (not the full 1 inch). If you use the "full throttle" line instead of the
calculated 5-minute MAP rating, you will cheat yourself a little power.
Hope this helps.
(edited for grammar & spelling)