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Max. MAP Table Interpolation tips?


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Hey Guys!

 

I started doing my heli training last week here in Cape Town, SA. I'm enjoying every moment of it so far! :lol:

 

I was just wondering how to accurately work out, say the maximum manifold air pressure for an R22 Beta II at a given pressure altitude and OAT. I can roughly "see" what the value should be, but for test purposes I'd like to know what the proper way to interpolate for, say Max Continuous Power at 13C at 5500'. You know, values that don't fall exactly into the neat little box :)

 

Thank you so much!

DF

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It's been about 10 years since I flew the 22 but here's a rule of thumb I used to determine available manifold pressure while flying in the mountains. Observe you MP while engine is not running. Now just subtract 2 and that is your max available manifold pressure at that altitude you are flying, i.e if you ignore your mp limitations you can pull that much MP before power demand is more than engine can produce and you begin to droop the rotor. To find out how much MP is available at a given altitude just subtract 1inch MP for every thousand feet you are going to fly.

 

Heres a situation.

 

Altitude sea level.

MP reading 30

Now you can theoretically pull 28 inches of MP at that altitude before your power demand exceeds engine output and you begin to droop the rotor. (remember Robinson has its own limit which occurs much sooner)

 

Say you want to fly and land a 5000 ft. At 5000 ft you will have around 23-24 (28-5) inches MP available.

 

To see if you can hover at that altitude, observe MP while in a hover. Thats how much MP it will take to hover at 5000ft. If you are hovering at 25mp you might not be able to hover at 5000ft.

 

Its been ages since I flew the 22 but this worked when I flew it. You might also want to fly a 300. Much better and safer aircraft IMO.

Edited by 500pilot
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It's been about 10 years since I flew the 22 but here's a rule of thumb I used to determine available manifold pressure while flying in the mountains. Observe you MP while engine is not running. Now just subtract 2 and that is your max available manifold pressure at that altitude you are flying, i.e if you ignore your mp limitations you can pull that much MP before power demand is more than engine can produce and you begin to droop the rotor. To find out how much MP is available at a given altitude just subtract 1inch MP for every thousand feet you are going to fly.

 

Heres a situation.

 

Altitude sea level.

MP reading 30

Now you can theoretically pull 28 inches of MP at that altitude before your power demand exceeds engine output and you begin to droop the rotor. (remember Robinson has its own limit which occurs much sooner)

 

Say you want to fly and land a 5000 ft. At 5000 ft you will have around 23-24 (28-5) inches MP available.

 

To see if you can hover at that altitude, observe MP while in a hover. Thats how much MP it will take to hover at 5000ft. If you are hovering at 25mp you might not be able to hover at 5000ft.

 

Its been ages since I flew the 22 but this worked when I flew it. You might also want to fly a 300. Much better and safer aircraft IMO.

 

 

The Robinson has derated the engine of the R-22. So that rule of thumb won't work unless I don't understand something about what you said. It maybe 28" MAP at sea level before the engine produces it's max but Robinson has derated the engine so the limit will be lower than that. So acording to the chart if you are allowed 23" max continous that maybe the case but that won't be the at the engines limit. Robinson limits us to that for longtivity of parts. Now to interpolate numbers will come with practice(examiners I don't think really like rule of thumb applications too much). It dosen't have to be to the exact number for your test, just get it as close as you can. Remember your values may change during the flight. To show this to my students I will have them take off at sea level but we'll do an off airport landing up at 5,000'MSL. Now a lot of times they don't check their Vne or MAP limits prior to departing or enroute. To check enroute is as easy as taking the OAT temp ff the guage and setting your altimiter to 29.92 for pressure altitude (don't forget to set it back) and look at the chart. Hope this helps you some.

 

Almost forgot, the 300CB dosen't have a chart to read or any limit other than when your reach full throttle. You just have to be aware of being able to droop RPM. An example, I was flying one day in the pattern and went to increase my RPM with the throttle but it was full on and I was about 700' MSL. That didn't make any sense to me. Trying to determine what was wrong as everything was feeling fine except for the high MP and being at full throttle. My airspeed was reading 70knots which was typical. I landed and the mechanic found the pitot tube party blocked. I was going a lot faster than it was reading which led to the high MP and full throtle. This is a bit off topic but somewhat related. I would have caught the problem a lot sooner in the R-22 since I would have been using or limited to a specific MAP.

 

JD

Edited by JDHelicopterPilot
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I was just wondering how to accurately work out, say the maximum manifold air pressure for an R22 Beta II at a given pressure altitude and OAT. I can roughly "see" what the value should be, but for test purposes I'd like to know what the proper way to interpolate for, say Max Continuous Power at 13C at 5500'. You know, values that don't fall exactly into the neat little box :)

Well, for test purposes I would just take the more conservative number and go with that rather than interpolate. From a practical standpoint it's kind of difficult to hit 23.3 inches MAP as opposed to 23.5 inches on the gauge so I tend to think in terms of 23.0 - 23.5 - 24.0.

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The Robinson has derated the engine of the R-22. So that rule of thumb won't work unless I don't understand something about what you said. It maybe 28" MAP at sea level before the engine produces it's max but Robinson has derated the engine so the limit will be lower than that. So acording to the chart if you are allowed 23" max continous that maybe the case but that won't be the at the engines limit. Robinson limits us to that for longtivity of parts. Now to interpolate numbers will come with practice(examiners I don't think really like rule of thumb applications too much). It dosen't have to be to the exact number for your test, just get it as close as you can. Remember your values may change during the flight. To show this to my students I will have them take off at sea level but we'll do an off airport landing up at 5,000'MSL. Now a lot of times they don't check their Vne or MAP limits prior to departing or enroute. To check enroute is as easy as taking the OAT temp ff the guage and setting your altimiter to 29.92 for pressure altitude (don't forget to set it back) and look at the chart. Hope this helps you some.

 

Almost forgot, the 300CB dosen't have a chart to read or any limit other than when your reach full throttle. You just have to be aware of being able to droop RPM. An example, I was flying one day in the pattern and went to increase my RPM with the throttle but it was full on and I was about 700' MSL. That didn't make any sense to me. Trying to determine what was wrong as everything was feeling fine except for the high MP and being at full throttle. My airspeed was reading 70knots which was typical. I landed and the mechanic found the pitot tube party blocked. I was going a lot faster than it was reading which led to the high MP and full throtle. This is a bit off topic but somewhat related. I would have caught the problem a lot sooner in the R-22 since I would have been using or limited to a specific MAP.

 

JD

 

 

 

Actually Robinson did not derate the engine, they derated the transmission. I believe the R-22 has the same engine as the 300 or a very similar one and it is still able to produce the same amount of HP. The only thing stopping you from using that HP is you. Robinson cannot simply wave his magic wand and say this engine can only produce x horsepower. What limits you is the transmission. It can only withstand x amount of horsepower before it starts blowing pieces. In this regards you can use whatever HP that engine is capable of producing. However you risk becoming a test pilot if you do. What happens when you pull more than the max allowed MP in the 22? Nothing. The helicopter will still fly. Unfortunatelly you have no idea what damage that overtorque has caused.

 

Now if you found yourself in a sticky situation and you accidently pulled more hp than allowed but you saved your ass doing so you did right so long as you reported the overtorque and let mx check it out. However in the same situation you refused to use what you had available to you and only used the MP that was max and now crashed the helicopter you can very well have killed yourself. (disclaimer, you shouldnt let yourself get into one of these situations to begin with but after all we are human and s*&t happens.)

 

My point is the engine will produce whatever amount of HP it was designed to produce at a given altitude. MP is HP and you limit yourself to how much you use. Bell does something similar with thier products. The 206 engine is capable of producing 420hp yet the transmission is only capable of handling 318 with up to 348 for ten seconds. (unintentional overtorque of 110% for up to 10 sec according to limitations) Do you think that Bell said the engine can only produce 318 hp? No, its just when the gauge reads 100% torque the engine produces 318hp. You can keep pulling collective all the way up to 420hp but in the meanwhile you will be destroying the transmission. Similarly when you reach your max allowed mp at sea level it is only producing the max hp the transmission is designed to take. If you pull more mp the engine produces more hp.

 

So, if your max MP allowed at sea level is 25, (I believe thats the max the 22 allows) then thats when the engine produces its max allowable HP. Now, remember since you dont have a super or turbocharger the engine produces less hp max as you climb. So in my example if at sea level you had a max avaible of 28mp but were limited by Robinson to 25mp at around 3000-4000 ft the max hp your engine can produce is the max the R-22 is rated for. (I cannot even come close to remembering whtat it is) Now if it takes 23mp to hover at a given weight at sea level you might not be able to hover when you get to 5000-6000 ft since it will always take the same mp to hover given the same weight. Now considering you will most likely have burned off fuel and lost some weight by the time you got that high you still might be able to hover but I would not advise doing many off airport landings. I used to fly in the mountains all the time in the 22 and would routinely go up above 10000 ft in Cali. It all comes down to how you calculate performance before you fly. Err on the high side with the charts since those are based on brand new aircraft and rarely can your aircraft perform to those standards.

 

Actually the rule of thumb you say was taught to me by one of the most respected examiners in the country. I wont say his name but his checkrides are legendary in Northern California.

Edited by 500pilot
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Actually Robinson did not derate the engine, they derated the transmission. I believe the R-22 has the same engine as the 300 or a very similar one and it is still able to produce the same amount of HP. The only thing stopping you from using that HP is you. Robinson cannot simply wave his magic wand and say this engine can only produce x horsepower. What limits you is the transmission. It can only withstand x amount of horsepower before it starts blowing pieces. In this regards you can use whatever HP that engine is capable of producing. However you risk becoming a test pilot if you do. What happens when you pull more than the max allowed MP in the 22? Nothing. The helicopter will still fly. Unfortunatelly you have no idea what damage that overtorque has caused.

 

Now if you found yourself in a sticky situation and you accidently pulled more hp than allowed but you saved your ass doing so you did right so long as you reported the overtorque and let mx check it out. However in the same situation you refused to use what you had available to you and only used the MP that was max and now crashed the helicopter you can very well have killed yourself. (disclaimer, you shouldnt let yourself get into one of these situations to begin with but after all we are human and s*&t happens.)

 

My point is the engine will produce whatever amount of HP it was designed to produce at a given altitude. MP is HP and you limit yourself to how much you use. Bell does something similar with thier products. The 206 engine is capable of producing 420hp yet the transmission is only capable of handling 318 with up to 348 for ten seconds. (unintentional overtorque of 110% for up to 10 sec according to limitations) Do you think that Bell said the engine can only produce 318 hp? No, its just when the gauge reads 100% torque the engine produces 318hp. You can keep pulling collective all the way up to 420hp but in the meanwhile you will be destroying the transmission. Similarly when you reach your max allowed mp at sea level it is only producing the max hp the transmission is designed to take. If you pull more mp the engine produces more hp.

 

So, if your max MP allowed at sea level is 25, (I believe thats the max the 22 allows) then thats when the engine produces its max allowable HP. Now, remember since you dont have a super or turbocharger the engine produces less hp max as you climb. So in my example if at sea level you had a max avaible of 28mp but were limited by Robinson to 25mp at around 3000-4000 ft the max hp your engine can produce is the max the R-22 is rated for. (I cannot even come close to remembering whtat it is) Now if it takes 23mp to hover at a given weight at sea level you might not be able to hover when you get to 5000-6000 ft since it will always take the same mp to hover given the same weight. Now considering you will most likely have burned off fuel and lost some weight by the time you got that high you still might be able to hover but I would not advise doing many off airport landings. I used to fly in the mountains all the time in the 22 and would routinely go up above 10000 ft in Cali. It all comes down to how you calculate performance before you fly. Err on the high side with the charts since those are based on brand new aircraft and rarely can your aircraft perform to those standards.

 

Actually the rule of thumb you say was taught to me by one of the most respected examiners in the country. I wont say his name but his checkrides are legendary in Northern California.

 

 

It was also my understanding from Robinson that they "derated the engine" not only by limiting the MP but also using a smaller carb on it since the cylinder walls were designed thiner to save weight.

 

JD

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