crashed_05 Posted March 17, 2011 Report Share Posted March 17, 2011 Why, on the Robinson OGE hover charts, do the lines steepen suddenly as you get close to max gross weight? My guess is that as you increase gross weight (requiring higher AoA), induced drag increases, not linearly, but at a curve, significantly reducing your hover ceiling.Any thoughts? Quote Link to comment Share on other sites More sharing options...
r22butters Posted March 18, 2011 Report Share Posted March 18, 2011 I have no idea, but, did you notice that, that sharp drop doesn't occur in the Beta, HP, or Standard? What's up with that? Quote Link to comment Share on other sites More sharing options...
Goldy Posted March 18, 2011 Report Share Posted March 18, 2011 I have no idea, but, did you notice that, that sharp drop doesn't occur in the Beta, HP, or Standard? What's up with that? I don't have time right now to look at the chart you mentioned. Just remember that charts are made during the certification process and there are two items that are often misunderstood. First is that the ship is put into each of those altitude or speed situations, when in fact it is often 2 or 3 actual points on a graph and the rest is a straight line drawn between points. The second is that these represent real "limits" of an aircraft. When they decide today is the day to demonstrate flight in a crosswind to the FAA...and the wind is 11 knots, then the crosswind max. becomes 11 knots on the chart...even though the ship might have no problem manuevering in a 30 knot crosswind. My guess is you are thinking there is a lot more data going in to the making of that chart than what really happens during certification. Lil different conditions make for different data and you get a different chart. Not always reality. Quote Link to comment Share on other sites More sharing options...
apiaguy Posted March 18, 2011 Report Share Posted March 18, 2011 "The second is that these represent real "limits" of an aircraft. When they decide today is the day to demonstrate flight in a crosswind to the FAA...and the wind is 11 knots, then the crosswind max. becomes 11 knots on the chart...even though the ship might have no problem manuevering in a 30 knot crosswind." Wrong. That is not how they set the crosswind limits. Quote Link to comment Share on other sites More sharing options...
Goldy Posted March 18, 2011 Report Share Posted March 18, 2011 "The second is that these represent real "limits" of an aircraft. When they decide today is the day to demonstrate flight in a crosswind to the FAA...and the wind is 11 knots, then the crosswind max. becomes 11 knots on the chart...even though the ship might have no problem manuevering in a 30 knot crosswind." Wrong. That is not how they set the crosswind limits. OK, when they want a OGE hover ceiling chart, and the cloud cover ceiling is 11,000 feet, or the pilot decides 11,000 feet is as high as he wants to fly that day, and he puts it in a hover....what number shows up on the top of the hover ceiling chart? Quote Link to comment Share on other sites More sharing options...
apiaguy Posted March 18, 2011 Report Share Posted March 18, 2011 wrong again... they wait for a better day... Please tell me what helicopter manufacturer makes their OGE hover charts and directional control statistics in the manner you suggest. Quote Link to comment Share on other sites More sharing options...
iChris Posted March 19, 2011 Report Share Posted March 19, 2011 (edited) Why, on the Robinson OGE hover charts, do the lines steepen suddenly as you get close to max gross weight? My guess is that as you increase gross weight (requiring higher AoA), induced drag increases, not linearly, but at a curve, significantly reducing your hover ceiling.Any thoughts? You're on track. As you can see from the figure below, the relationships between lift, drag, and angle of attack are not always linear; therefore, as collective pitch increases to meet the demand of increased gross weight you also have a non-linear increase in power required. Note the sharp increase in drag after 8 degrees angle of attack. The methods and procedures used to make these IGE/OGE charts is a combination of flight-testing, along with the normalizing and extrapolation of the test data. AC 27-1B outlines these methods and procedures (See pages B-32 – B34). FAR 27.49, FAR 27.143c)d), and 27.1587 cover some of the other things the above post were talking about. "The objective of hover performance tests is to determine the power required to hover at different gross weights, ambient temperatures, and pressure altitudes. Using nondimensional power coefficients (Cp) and thrust coefficients (Ct) for normalizing and presenting test results minimizes the amount of data required to cover the rotorcraft's operating envelope." "To obtain consistent data, the wind velocity should be less than 3 knots as there are no accurate methods of correcting hover data for wind effects." "Two methods of acquiring hover performance data are the tethered and the free flight techniques." "Hover performance can usually be extrapolated up to a maximum of 4,000 feet." "The regulatory requirement for hover performance, § 27.49, refers to hover in ground effect (IGE). For some applications, such as external load operations, hover performance out-of-ground effect (OGE) is necessary; however, it is not required by this section." Edited March 19, 2011 by iChris 2 Quote Link to comment Share on other sites More sharing options...
crashed_05 Posted March 19, 2011 Author Report Share Posted March 19, 2011 Thanks, Chris! I had a feeling that's what was going on there. What publication did you pull that chart from? Quote Link to comment Share on other sites More sharing options...
iChris Posted March 20, 2011 Report Share Posted March 20, 2011 (edited) Thanks, Chris! I had a feeling that's what was going on there. What publication did you pull that chart from? Helicopter Aerodynamics Volume I, Chapter 4, by Ray Prouty, ISBN 978-0-557-08991-8 Edited June 27, 2012 by iChris 1 Quote Link to comment Share on other sites More sharing options...
Recommended Posts
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.