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Robinson 44 "power check"

r44 performance power check

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#1 Narwhal

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Posted 25 August 2019 - 15:19

Hi,

 

I'm a pre-solo student working on an add-on commercial rating to my fixed wing ATP.

 

I have about 25 hours, all in the R44.  My question is about "power checks", spurred by other recent discussion topics on this forum.

 

The R44 performance charts are pretty meager, and just include IGE and OGE hover charts, but don't include the expected manifold pressure settings (charts are a function of altitude, temperature, and weight).  

 

My hypothetical oral question would be something as follows:

 

You takeoff from sea level at 15 degrees in an R44 loaded to 2300 lb. 

 

Your destination is a confined area mountain top helipad surround by 50 foot trees at 4000 ft elevation and 10 degrees.  It will take 30 minutes to arrive (landing weight 2250 lbs). How do you know you can land on the moutain then takeoff again safely at the destination?

 

My answer, I suppose would be a combination of:

 

1) Consulting the Robinson POH OGE hover chart and making sure I could hover OGE at 4000 MSL 10 degrees at a weight of 2250 (chart says I could, with only a 25 pound margin).

 

2)  High surveillance power check at 55 knots and 500 ft MSL. If I have 5" of mainfold pressure remaining to max takeoff power during high surveillance, power check is OK.

 

My other question is, once I had landed on the mountain, is there a rule of thumb for a power check during an IGE hover prior to subsequent max p. takeoff?  I.e. If I'm hovering IGE on the mountain and have 3"(?) remaining to max takofff power, should I be able to successfully perform a maximum performance takeoff?  I just made up the 3" because at a given weght, there seems to be about a 3000 foot difference between maximum IGE and OGE altitudes, and you lose roughly 1" of manifold pressure per thousand feet MSL in a normally aspirated engine.

 

Thanks for any help or guidance. I did ask my instructor and they just replied with the high surveillance 500 AGL 55 knot 5" margin power check and didn't elaborate.


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#2 r22butters

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Posted 26 August 2019 - 14:07

So why 55 kts for the power check? It will take more power to go slower, so how do you know 5" will be enough?

I always figured an in flight power check would be to bring it below ETL at 500', 'cause if you need to do a max takeoff to get out of that confined area, it would seem better if you could HOGE with 5" to spare?

...but then I haven't done off airport landings in years, so I'm a bit rusty on this stuff.
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#3 Wally

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Posted 26 August 2019 - 15:28

I'm kinda with Butters on this- do a HOGE to confirm your by-the-book calculations.

 

The method I was taught- calculate your by the book numbers.

Fly to the destination. 

High recon. 

While at altitude and wherever you are safe and comfortable decel to a zero airspeed, note power used to hold this hover and then resume safe airspeed.  Adjust from the zero airspeed hover readings for added load and power to climb.  Now you know, not just a swag, but a fair estimate of power available versus what you need.

 

Example, in a AS350 (it's been decades since I flew anything else and longer since a recip)-

Fly to the proposed LZ, say at 2000 MSL, I'm at 3000.  Another disclaimer here- pretty much anything below 4000 MSL and 90° F and I don't need to adjust for altitude in a B2 or B3.  Adjust for your aircraft and engine.

 

Circle a couple times on a high recon and plan the approach, landing and departure.

 

Approach leg into the wind over safest forced landing areas, I'll swing a little wide on the base leg, turn final and zero out airspeed, observe power readings and resume final approach leg.  I don't add climb power at a zero HOGE...

 

There are two critical numbers in an AS350- torque (TQ) and NG/N1- the rule of thumb is 70 lbs lift for 1% TQ (torque) available before the limit and/or 200 lbs for each 1% NG.  If TQ and NG at the zero airspeed hover allow for planned load (and a fudge factor- cut yourself some slack when you can) and I have sufficient power for planned climb-out (another whole topic)  then I'm good to go.


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Just a pilot (retired, so I have a LOT of time)...


#4 Narwhal

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Posted 26 August 2019 - 16:28

Thanks for the replies.  One CFI said 55 knots because that was point of least power required for the R44 (best rate of climb, minimum rate of descent, etc).  I don't know why you would use that speed but I will ask. Maybe it is analogous to an IGE hover power setting?  It makes more sense to me to do an OGE hover as well.

 

I really haven't gotten to do any off-airport landings yet but am just preparing in advance.  The last half dozen lessons have mostly been straight in, 180, and hover auto-rotations.  Interesting info about the AS350!  


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#5 Aluminium Mallard

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Posted 06 September 2019 - 00:20

I was taught the 55kt power check when I was doing my license... the problem with that method is if you have a slight descent rate, it gives a wildly optimistic power margin.

 

As previously mentioned try and HOGE whist into wind and note power margin. If you reach your power limit then its not going to work and fly away.

 

You need sufficient height and escape path to fly away if you run out of power (overpitch) before you reach your power limit. Can happen high enough or the machine is a bit tired. Can be a bit exciting for a student pilot which I suspect is why flight schools do not teach this method.


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#6 Eric Hunt

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Posted 06 September 2019 - 02:50

You are flying at 55kt because that is the spot on the power available line/power required curve chart that gives the greatest distance between the 2 lines, i.e. the biggest margin.

 

You note how much power you use at the landing height, and consult your performance chart (remember that as you go higher, the power available line comes down, and the power required curve goes up) and if you have 5" margin, go for it.

 

Student trying to come to a zero airspeed hover as suggested above might inadvertently end up going backwards, and it can all turn to scary worms.


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#7 r22butters

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Posted 06 September 2019 - 09:31

You are flying at 55kt because that is the spot on the power available line/power required curve chart that gives the greatest distance between the 2 lines, i.e. the biggest margin.
 
You note how much power you use at the landing height, and consult your performance chart (remember that as you go higher, the power available line comes down, and the power required curve goes up) and if you have 5" margin, go for it.
 
Student trying to come to a zero airspeed hover as suggested above might inadvertently end up going backwards, and it can all turn to scary worms.


Hmm, by the time I figure out what you just said here, I'd of pulled into a HOGE, made my decision, landed, shut down, taken a wiz, fired up, and taken off! :D
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#8 Eric Hunt

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Posted 09 September 2019 - 20:46

Butters, it shows me what a sad position you and your friends must be in, if you haven't the slightest grasp of basic aerodynamics and performance. 


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#9 r22butters

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Posted 09 September 2019 - 23:14

Butters, it shows me what a sad position you and your friends must be in, if you haven't the slightest grasp of basic aerodynamics and performance.


Well, I know that in an R44 55kts is the best rate of climb speed, which means it is also the speed at which drag is the least. Therefore going faster and/or slower requires more power!

What I don't get is why you would check to see if you have enough power to land off airport by using the speed which requires the least amount of power?
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#10 Eric Hunt

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Posted 10 September 2019 - 00:16

M - A - R - G - I - N

 

Of power.

 

Excess power. So you know before you try it.


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