Manifold Pressure - Full Throttle

[HumblePilot]

Before taking off in a R22 or Schweizer 300 you are supposed to check and determine your limit manifold pressure on the placard. At low altitude and low temperatures you will read a value numerical value (e.g 25.9 in). However on a hot day or at high altitudes you will encounter the value FT (Full Throttle) or you might be depending on altitude and temperature between an numerical value and FT.

 

The questions is now how do you determine in the case of FT or between FT and a numerical value the limit manifold pressure ?

 

Looking forwards to good (qualified) answers.

[apiaguy]

If after you have calculated air temp and pressure altitude (density altitude) and cross reference that to the poh you find that the placard reads F.T. (full throttle) then there is no limit on manifold because you won't be able to reach a manifold pressure that would produce full power and they are telling you that full throttle will be the deciding factor if you can fly.

 

Available manifold pressure drops as density altitude increases...

If you reach the critical altitude of the engine (the point where the engine can no longer reach rated power)

You will be at the full throttle stop and you will not be able to pull additional collective (manifold) or you will drag the engine down in rpm and there is not sufficient power to recover without reducing collective (manifold) because you are already at the full throttle setting.

[HumblePilot]

OK... so If you are in the FT-range you will not be able to determine your available power reserve before take-off. This because FT can only be determined in flight respectively after during taken-off. Not knowing the power reserve before taking a take-off decision can be a risky thing (e.g. Getting out of a confined area or steep take-off).

 

How would you solve the following problem:

 

Assume that you are taking off from 2500 ft (Pressure altitude) and OAT is 80°F. On the placard you find the following values:

 

2000 ft / 80°F: 26.4

3000 ft / 80°F: FT

 

2500ft is between FT and 26.4. How much Manifold pressure do you pull ?

 

Looking forward to interesting answers.

[apiaguy]

you can know the power reserve at take off by knowing how much power it takes to hover...

 

Just because you are not able to develop full rated power... ie.. you're in the F.T. range... doesn't mean there is no power reserve...

 

Proper preflight planning...eg.. take-off weight, passenger loading... fuel load.. can help performance margins.

 

You have to do a little estrapolation with charts of any kind..

 

When you reach the last listed mp numerical value (26.4 in your example) any further increase in density altitude is going to occur linearly and even thou the next listed value is F.T. at 3000ft. that doesn't mean the F.T. range starts exactly at 3000 ft....

 

Frankly, I wouldn't worry about the exact value of mp available when comparing the final mp reading and F.T. operation....

Just use as much power as available above 2000 ft. as per your example....

I know that cuts short your question for an exact answer to a particular situation...

estrapolate!

[delorean]

Look at the MP gauge before you start the helicopter. Full throttle will be about an 1-2" below what it reads (on non-turbo charged helicopters of course).

 

Remember, when the engine is static, the MP gauge reads AMBIENT pressure........not barometric (or "altimeter" setting) which is adjusted to sea level. To double check the accuracy of your MP gauge and/or altimeter, twist the altimeter to 0 [zero] MSL and read the pressure in Kolsman windows. It should equal what the MP gauge reads at static.

[Wally]

OK... so If you are in the FT-range you will not be able to determine your available power reserve before take-off. This because FT can only be determined in flight respectively after during taken-off. Not knowing the power reserve before taking a take-off decision can be a risky thing (e.g. Getting out of a confined area or steep take-off).

 

How would you solve the following problem:

 

Assume that you are taking off from 2500 ft (Pressure altitude) and OAT is 80°F. On the placard you find the following values:

 

2000 ft / 80°F: 26.4

3000 ft / 80°F: FT

 

2500ft is between FT and 26.4. How much Manifold pressure do you pull ?

 

Looking forward to interesting answers.

 

The real world answer is that charts and figures have their limits. You also have to remember that they are all SWAGs, just guesses with numbers. Your gaugeing has errors, your aircraft's performance will vary from book, as will your ability, and some of the data you're working with won't be accurate. Not being able to chart/calculate and numerically predict a proposed operation doesn't mean you can't do it. There are practical tests you can do before you commit to the operation, especially confined area/max performance maneuvers.

My technique is this, for a confined area operation: Do a HOGE check before landing. If that's within limits, the next step is to extrapolate a difference or apply a test of some sort and see if the engine/airframe is capable of the proposed operation. Land, load, and then do HIGE power check, then climb into another HOGE check. At this point, some operators require a second landing before the VERTICAL departure. If I'm confident of the plan, however, I'll continue the VERTICAL climb until I'm clear, and carefully accelerate into ETL and climbout.

There could be times, if my employer didn't prohibit it (EMS), when I'd translate in ground effect. That can get you in trouble if you're not assured of power available: you can end up without sufficient power to clear obstacles or stop; not to mention the risk of hitting something you didn't see.

[Marc D]

Look at the MP gauge before you start the helicopter. Full throttle will be about an 1-2" below what it reads (on non-turbo charged helicopters of course).

 

Remember, when the engine is static, the MP gauge reads AMBIENT pressure........not barometric (or "altimeter" setting) which is adjusted to sea level. To double check the accuracy of your MP gauge and/or altimeter, twist the altimeter to 0 [zero] MSL and read the pressure in Kolsman windows. It should equal what the MP gauge reads at static.

 

 

If you forget to look at your MP before startup, then use 28" as a base number and subtract 1 inch per 1000 feet. E.X. if you want to land at 6000' MSL and you forgot to check your MP before startup, then you can expect full throttle to be about 28 - 6 = 22. Full throttle will be about 22 inches MP. When you use 28 as a base number you will usually have a slight margin in your favor, but a slight margin in your favor is better than under estimating. Using 28 as a base allows for a quick(without the chart) reference in flight. If you figure it will take 21 inches to hover at your landing sight at 6000' and you figure you will be at full throttle at 22 inches, then you had better not have many obstacles to clear with just 1 inch MP to spare. Before landing at the sight at 6000' you can increase power until you reach full throttle and confirm it happens at 22" or more.

 

Marc D.

[HumblePilot]

Look at the MP gauge before you start the helicopter. Full throttle will be about an 1-2" below what it reads (on non-turbo charged helicopters of course).

 

Remember, when the engine is static, the MP gauge reads AMBIENT pressure........not barometric (or "altimeter" setting) which is adjusted to sea level. To double check the accuracy of your MP gauge and/or altimeter, twist the altimeter to 0 [zero] MSL and read the pressure in Kolsman windows. It should equal what the MP gauge reads at static.

 

Except for the fact that aircrafts I fly pressure in the Kolsman window is unfortunately quoted in hpa, I find your answer very straight forward, easy to understand and practicable.

Is this rule explained in serious papers or is it a old man's rule. Does it apply to R22/R44 and H269 ? What is your own experience?

[HumblePilot]

If you forget to look at your MP before startup, then use 28" as a base number and subtract 1 inch per 1000 feet. E.X. if you want to land at 6000' MSL and you forgot to check your MP before startup, then you can expect full throttle to be about 28 - 6 = 22.

Marc D.

 

Hallo Marc

 

I like this rule too... Can you also give me some more background information from where this rule comes (books, experience ...)?

 

looking forwards to your answer

Edited August 16, 2007 by HumblePilot

[lelebebbel]

that is simple, air pressure decreases by 1" of mercury every 1000ft of altitude. So, if the air pressure at sea level was 30", at 6000ft it will be 24". That means you intake manifold pressure will never get any higher than 24" at 6000ft, no matter how much you pull, unless you had a turbocharger.

Now due to friction effects, the effective intake manifold pressure is a bit lower than the ambient air pressure, which is why you'd calculate with 28" instead of 30".

 

At least that is how i was taught to do it in the R22.

Edited August 16, 2007 by lelebebbel

[Marc D]

that is simple, air pressure decreases by 1" of mercury every 1000ft of altitude. So, if the air pressure at sea level was 30", at 6000ft it will be 24". That means you intake manifold pressure will never get any higher than 24" at 6000ft, no matter how much you pull, unless you had a turbocharger.

Now due to friction effects, the effective intake manifold pressure is a bit lower than the ambient air pressure, which is why you'd calculate with 28" instead of 30".

 

At least that is how i was taught to do it in the R22.

 

This would be my follow up answer. You won't find it in any books. It should work with any non-turbocharged piston helo. I've only used it in the R22/R44 up to 10,000' MSL.

Thanks Lelebebbel.

 

Marc D.

[captkirkyota]

If you forget to look at your MP before startup, then use 28" as a base number and subtract 1 inch per 1000 feet. E.X. if you want to land at 6000' MSL and you forgot to check your MP before startup, then you can expect full throttle to be about 28 - 6 = 22. Full throttle will be about 22 inches MP. When you use 28 as a base number you will usually have a slight margin in your favor, but a slight margin in your favor is better than under estimating. Using 28 as a base allows for a quick(without the chart) reference in flight. If you figure it will take 21 inches to hover at your landing sight at 6000' and you figure you will be at full throttle at 22 inches, then you had better not have many obstacles to clear with just 1 inch MP to spare. Before landing at the sight at 6000' you can increase power until you reach full throttle and confirm it happens at 22" or more.

 

Marc D.

I tried this today, and this way gave me a number WAY to high, unless I'm missing something here. When figured by the chart with the temp etc we had a 5 min max of 22.8 MP here at 3,400 ft elevation. By doing the way I understand yours to read I come up with 28 - 3.4 = 24.6 which is too high when compared to the map limit chart in the POH. Am I not seeing/understanding something correctly? Please show me the error if I am cuz I was all stoked about this very easy formula.

Edited August 17, 2007 by captkirkyota

[southernweyr]

The issue is the fact that you were flying a derated engine. On that helicopter, the POH limits the pilots to certain manifold pressure settings and thus the usuable power. In a 269c they do not derate the power so this rule works. In reality the R22 and R44 are capable of producing up to the 28" of manifold pressure but then you would be using more power than is allowed according to the POH.

[flyby_heli]

Remember, even though the engine is not derated in the 269C, you still have a max MP limit, in accordance with the MP limit chart in the POH/cockpit.

At sea level, on a standard temperature day, you are limited to 26" of MP. This is not because the engine can't produce more power, but because of a torque limit on the M/R transmission. Therefore at different temperatures your MP limit changes because the engine produces a different amount of torque at a given MP with different temperatures.

 

So the next question for you engine guys is: On a 269C, since the MP limit is on the M/R tranny and not engine, could you safely pull more then the MP limit for that day without overtorquing the transmission if your engine is close to TBO and/or has compression readings on the low side of the acceptable scale??

 

(I am not going to go pull the guts out of our C-model, just a question.)

 

Fly safe

Edited August 18, 2007 by flyby_heli

[Marc D]

I tried this today, and this way gave me a number WAY to high, unless I'm missing something here. When figured by the chart with the temp etc we had a 5 min max of 22.8 MP here at 3,400 ft elevation. By doing the way I understand yours to read I come up with 28 - 3.4 = 24.6 which is too high when compared to the map limit chart in the POH. Am I not seeing/understanding something correctly? Please show me the error if I am cuz I was all stoked about this very easy formula.

 

Your max usable power is either full throttle or the max MP per the chart. So to answer your situation, the chart is showing you at what MP you are going to be using 131 horsepower(MTOP). You are not allowed to use more than 131hp. Max MP per the chart simply equals 131hp, this is the "derating". A 180hp engine(r22) is voluntarily limited to 131hp. You are not permitted to operate the aircraft above 131hp except in an emergency. Once the engine will not produce 131hp anymore due to high DA, your are going to hit full throttle first. In your situation, the chart is showing you that you are still capable of getting 131hp.

If you are taking off at 3400ft, plan a flight to 5000 ft. If you check the chart it will show full throttle. Where is full throttle going to occur? This is the point that the rule of thumb becomes useful. 28 - 5 = 23. If you are hovering with no wind at your departure point at 24 inches, then you will not be able to land at 5000'. This would be confirmed by referencing the performance chart, which is just not always practical if you do multiple take offs and landings at unplanned sites.

The best way to gain advantage of this rule of thumb is to try it multiple times in multiple senarios. When you see it work out you will see how it can be useful.

I hope I have been somewhat clear. If not I can try again. It is definitely confusing until you have seen it played out a few times and the "derating" factor just muddys the water.

 

Marc D.

[HumblePilot]

Many thanks to everybody for all these interesting contributions.

 

Without getting complicated please remember that the limitation placard always applies. The focus of this discussion is the FT (Full throttle) range.

 

After thinking twice about the rule of thumb 28'' - 1'' per 1000 ft I believe it is important to define what kind of "ft" that we are we talking about: is it elevation (TA), pressure altitude (PA) or density altitude (DA). In my opinion the only values that makes sense is density altitude.

 

I would also like to get the focus back to the following situation:

Admit that you are taking off from a confined landing site at 4000ft. According to OAT and PA you determine that FT is the limit manifold pressure. During the hover check before taking-off you read a MP of X inches. Now... In order to determine your take-off profile how do you determine the number of MP inches at which FT will be reached (or in other words ... how many inches will be available for take-off)?

 

The rule of 28-1 per thousand seems be suitable solutions and is easy to apply in a daily work. Does anybody have any other approach or solution to this question?

Edited August 18, 2007 by HumblePilot

[Marc D]

Many thanks to everybody for all these interesting contributions.

 

Without getting complicated please remember that the limitation placard always applies. The focus of this discussion is the FT (Full throttle) range.

 

After thinking twice about the rule of thumb 28'' - 1'' per 1000 ft I believe it is important to define what kind of "ft" that we are we talking about: is it elevation (TA), pressure altitude (PA) or density altitude (DA). In my opinion the only values that makes sense is density altitude.

 

I would also like to get the focus back to the following situation:

Admit that you are taking off from a confined landing site at 4000ft. According to OAT and PA you determine that FT is the limit manifold pressure. During the hover check before taking-off you read a MP of X inches. Now... In order to determine your take-off profile how do you determine the number of MP inches at which FT will be reached (or in other words ... how many inches will be available for take-off)?

 

The rule of 28-1 per thousand seems be suitable solutions and is easy to apply in a daily work. Does anybody have any other approach or solution to this question?

 

28-1 per thousand would work for takeoff and landing. I have another rule of thumb. Maybe this is what you are looking for.

4000' 28-4=24. Full throttle will be right about 24 inches. I will not expect to be able to make a vertical takeoff in an R22 without at least 2" of reserve power. If I only have 1" I will need room to gain ETL to accomplish a safe climb. Anything less than 1" reserve and you will need a normal takeoff profile. I have found that once you start getting up around 7-10000'msl you need to give yourself a little more margin.

So, you are sitting at 4000'msl. You pick it up to a hover and you are hovering at 23". You have 1inch to spare. You will need some room to run. Be careful before you decide to do this. Consider all your "outs" and "what ifs". Things can change quickly and once you start it is tough to stop. The wind can be calm behind the trees and be coming from the complete opposite direction you are expecting when you start to climb. I speak from experience on this one. Scared the crap out of me.

This formula doesn't work quit as well with the R44. I have found that if you have 2inches spare MP and you pull it straight up, it will just come to a dead hault at around 30 feet. You almost always need some forward movement in the 44. Or 3 inches or more.

 

Marc D.

 

Marc D.