Jump to content

Minimum rate of descent


Recommended Posts

The Robinson POH is very specific compared to others I've seen. So not having the figures for minimum rate of descent in an auto in the R22 POH, like they do in the R44 one, does that mean that they don't want you doing it in the R22?

Link to post
Share on other sites

It has something to do with the type certification and the requirements at the time the aircraft was developed. That information is in the R-44 POH because it has to be there and not because robinson felt that you needed it. It should also be in any other newish helicopter POH and not in something like a jet ranger POH.

 

I believe the R-22 certification is under some old rule and the R-44 is under Part 27. I don't really remember the specifics but someone else will...

 

Robinson is ok with you doing anything you want in an auto as long as you don't crash.

Link to post
Share on other sites

I don't know much about the R22, but in all the rotorcraft I have flown min sink rate was a couple of knots slower than max glide.

Link to post
Share on other sites

With my limited knowledge I think it relates to pitch attitude. The slower you go, the more the air is coming from underneath, increasing your angle of attack making you use more up collective. Once you go slower than best climb the induced flow increases, and reduces the driving region.

Link to post
Share on other sites

Eagle5-

Best rate of climb, best endurance (longest flight time), and minimum descent auto airspeed are usually all the same airspeed or very close: L/D Max, the speed at which Lift produced relative to Drag incurred is at a maximum.

I know what it is just wondering why they left it out. Airhead is probably right in that at the time the 22 was certified it just wasn't required to be in the POH?

Link to post
Share on other sites

Best Rate (Vy) and Max Range (greatest distance per fuel consumed) will be achieved at L/D max. The speed at which the least total drag (induced and parasite) are created. Any increase in speed will correspond to an increase in parasite drag and any decrease in speed will create an increase in induced drag. Because of this, at this speed you have the most "excess thrust available" or energy in the case of sailplanes or engine out situations, relative to drag created.

 

Min Sink and Best endurance (max loiter time) as pointed out will typically be achieved at a speed slightly slower. I've never heard of an aircraft that wasn't, and will be based on performance calculations. Quite simply by graphing sink rate at corresponding speeds. As the angle of attack is increased beyond best L/D lift continues to increase and although induced drag increases slightly at first the sink rate will decrease initially. As you continue to slow at some point the increase in lift will not offset the increase in induced drag and the sink rate will then increase. The speed which corresponds to the lowest sink rate is it. At this speed you can now lose the least altitude (or maintain the same altitude) utilizing the least amount of power (or "energy" in the case of sailplanes or engine out situations. Now the original question as to why some aircraft manufacturers present this information and others don't... no idea. To my knowledge there is still no requirement to put min sink/best endurance information in the RFM. But that certainly doesn't mean that I am not supposed to understand the concept and apply knowledge to various flight situations to my advantage.

 

Now back to max glide for a second. The part that most people totally disregard is the effect that wind has on this. We all know that a glide into the wind will be shortened and a tailwind will stretch the glide but simply flying best glide speed isn't maximizing your performance. To take an extreme, say that your ship has a best glide of 70 knots and you are flying into a 70 knot wind do you still think that 70 knots will give you the best forward distance for altitude lost? Obviously I need to fly at some faster speed to achieve any penetration. You can draw tangential lines on your performance charts (sorry, I don't have the skill set to upload graphics) and calculate exactly what sailplane pilots call "speed to fly" but as a general rule of thumb adding 1/2 the headwind or subtracting 1/4 the tailwind to/from your Best L/D speed will put you really close to truly covering the most distance possible.

Link to post
Share on other sites

HighCounty,

Are you sure about best climb V(y) and best range V(BR) speeds being the same? In my experience, V(BR) is significantly higher. In piston Enstroms, for example, V(y) = 57 MPH indicated decreasing to 42 MPH @ 10,000ft and V(BR) is 85 MPH.

Link to post
Share on other sites

You are absolutely correct. I brain farted and was thinking Best Glide since that what the discussion started with Best Glide vs Min sink. Somewhere in my rambling I jumped synapses to Max Range.

Link to post
Share on other sites

Colorado, you also bring up a good point that even though the RFM will often list a particular speed, that speed varies directly with both gross weight and altitude and that is often not noted.

Link to post
Share on other sites

It has something to do with the type certification and the requirements at the time the aircraft was developed. That information is in the R-44 POH because it has to be there and not because robinson felt that you needed it. It should also be in any other newish helicopter POH and not in something like a jet ranger POH.

 

I believe the R-22 certification is under some old rule and the R-44 is under Part 27. I don't really remember the specifics but someone else will...

 

Robinson is ok with you doing anything you want in an auto as long as you don't crash.

 

That’s correct, the R22 type certificate was prior to the rule (§27.71). The R44 type certificate came after the rule; therefore that information was required to be in the R44’s RFM. Minimum rate is often quoted for the R22 at 53kts. @ 90% NR.

 

The rule was proposed in 1982 and the final rule was issued in 1984, adding a new Sec. 27.71 to read as follows:

 

For single-engine helicopters and multiengine helicopters that do not meet the Category A engine isolation requirements of Part 29 of this chapter, the minimum rate of descent airspeed and the best angle-of-glide airspeed must be determined in autorotation at-

 

[a] Maximum weight; and

Rotor speed(s) selected by the applicant.

 

Explanation: For helicopters which do not have prolonged flight capability following an engine failure, crews must be aware of the helicopter's autorotative performance.

 

Data on the minimum rate of descent airspeed and best angle-of-glide airspeed in autorotation is essential for assisting the pilot in best managing altitude and airspeed to reach a desired landing site.

 

This requirement would generate the necessary data which would be incorporated in appropriate portions of the rotorcraft flight manual by proposed changes to Secs. 27.1585 and 27.1587.

 

This information also enhances compliance with rules regarding the overwater operation of aircraft within gliding distance of shore, such as Sec. 135.183 (a).

 

This information is routinely obtained during flight test programs for current rotorcraft and adopting this proposal will not impose added cost or burden to the manufacturer. Proposed Sec. 27.71 provides consistency with an existing requirement for Part 29 Category B helicopters in Sec. 29.71.

 

REF: Proposed Rulemaking. Notice No 82-12

Edited by iChris
  • Like 2
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...