Have been trying in vain on the FAA web trying to find the definition of what constitutes an OGE hover as opposed to IGE. Some seem to think two thirds of the rotor diameter. FAR 29.49 mentions OGE but is not forth coming on what is OGE.
I recognize that at 1.5 or 2 rotor diameters that ground effect is negligible enough to be ignored, but as for the definition for the charts, I would have thought that "out of ground effect" is when you are not in ground effect whereas "in ground effect" is a published skid/wheel height.
Effectively, out of ground effect becomes the most power you would ever require in a zero wind hover for the weight & DA.
Whether you define it at 1.5 rotor diameters or 15 rotor diameters, as long as the power advantages of being in ground effect aren't realized then you have OGE.
There is really no such thing as IGE as a discrete thing. There is no place where IGE "starts" or stops. The sweep of power gain with lower altitude starts at maybe 2.0 diameters and can be actually measured at 1.5 diameters. If you could hover the body below ground level, in a hole, so the rotor could get lower to the ground, you can get to about 20% power recovery (where IGE hover power s 80% of OGE hover power). From a practical standpoint, the power gain is about 14% for most helos, since that darn body hits the ground first. As an aside, I believe the power advantage would drop, and things get worse if the rotor gets too close to the ground, as the flow from the rotor starts getting blocked by the ground. Never saw any data on that,(called blackage) although lots exists for the tail rotor working against the vertical fin.
To measure it for yourself, try this on a calm morning, with a friend to carefully read the gages while you fly:
Hover very steadily at 1 inch, where the skids/wheels just touch the gound sometimes, dancing if you will. Keep the rpm exact, and note the torque MP.
Rise to about 2 feet and stabiilize and record. Go to 10 feet and do it again. Then 25 feet, then 50 feet.
Now plot the data as a percentage of HOGE power. In other words, if the OGE was 72% torque, just divide all the readings by 72. If the 1 inch power is 62%, then it will come out as 86%, and the OGE will come out as 100%.
There is no IGE on that chart, is there? Just varying levels of IGE-ness.
Oh, Lord, I hope nobody now posts that the pressure under the rotor is what we are measuring, or this whole thread will turn toward the dark side!
I think part of the issue is what is defined by IGE or OGE, and the FAA doesn't make any hard and fast rules on this. The numbers in the FM are based on the following: for IGE, for Part 27 helicopters anyway (or any that aren't Category A), the IGE height given is the minimum height you can hover at, and do the takeoff as described in the manual (or if nothing is stated in terms of 'hover power +5%, for example, then using a fixed collective takeoff). As you accelerate through translational lift, having started from this height, you won't smite the ground. THis is the reason for the difference in hover height in the charts for example between high skid gear and low skid gear. For OGE, it's the hover height the manufacturer states, and this is probably done solely for reference purposes. There is at least one helicopter that has a Hover OGE chart that you can get to if you start from that height by decelerating from forward flight, but can't climb to from the ground if you are at the power limits stated in the FM (i.e. it takes 100% to hover OGE at 80 feet, but if you start from the ground, using 100% torque, you'll only get to 45', because you haven't got enough power to climb vertically...) Hope this helps the confusion (Nick -notice no mention of pressure under the disk...)
From a practical perspective I think that very few pilots appreciate the improvement and importance of power reduction (or increase in reserve) from a controlled low hover.
Self experiments with the 'A' 76 show an improvement of 5-7% torque by reducing the HIGE from 6ft to 1" above the ground.
The reason I hold such a low hover is in preparation for a manouver such as a vertical ascent to 100ft. Typically a 3ft skid height is satisfactory.
The lower the torque in the hover, the greater the surplus power to initiate the vertical climb and induce intertia plus potential energy into the airframe. At the top of the ascent the inertia and energy in the airframe is converted to Kinetic energy and subsequently airspeed upon rotation....and that is what saves my bacon if the donk stops.
A hover over 1ft begins to absorb torque and potential energy and exposes my arse to increased risk. I want to mitigate the risk by reducing the HV exposure as much as possible. Poor technique, a lack of understanding about ground effect and sloppy hovers all contribute to this. I vote 2/3rds Rotor diameter as the practical limit for GE.
I hope I got the physics correct. Feel free to tell me if I didn't
Setting an actual number on minimum altitude for being OGE is impractical. The point of the OGE charts is that it gives you a maximum power requirement for hover based on weight and DA. Wind and ground effect can reduce this power requirement.
The point of IGE charts is they predict a still air power requirement for whatever skid/wheel height is published. It's more of an anticipated power requirement rather than a worst case power requirement.
So where does out of ground effect occur? Wherever you aren't in ground effect. Simple, huh?
Does it happen at 2/3, 1, 1.5 or 2 rotors? Doesn't matter. It's not predicting a power requirement at any of those heights, it is predicting a maximum hover power requirement.
Steve, your low hover is a good idea for power requirements, but there is more to consider. If you have hydraulic problems, SAS/AFCS problems, wind gusts, inattentive ground crew, sneezing pilots, rough terrain, etc. then the extra height gives you a safety margin when the helicopter starts moving. I'd rather use an extra 3% torque and avoid dynamic rollovers or decapitations.
That being said, there are times where a lower skid height is much more preferably. The additional excess torque prior to a vertical departure is an excellent example. Not only do you generate more momentum, but the greater the rate of climb that you have adds in efficiency to the rotor. This is probably due to inflowing air from the climb and aiding the blades to avoid vortices from other blades. With the right conditions, you can actually hover climb with less power than it takes to hover. Unfortunately the opposite can happen in the hover descent.
From different books and manuals I read that the concensus (cfr also reference to FAA) is that 1.5 rotor diameters of skids/wheels above ground is considered as OGE. Since the rotor is typically 0.25 diameters above the skids/wheels this means 1.75 diameters measured from rotor to ground.
The book of Shawn Coyle gives a graph on page 23 figure 3.3 which shows that at 1.75 height the ground effect is indeed almost completely vanished, and certainly at 2.
From the engineering books I retain following guidelines :
- Rotor to skid is 0.25
- Normal hover is 0.25 (skids/wheels above ground)
Depending on the hover height 0 to 0.25 diameters the induced power reduction is 60 to 80%, so a saving of 40 to 20%. The part of the engine power (vertical drag, transmission loss, tail rotor) that serves the induced power is roughly 65%. This means that the savings measured at the level of engine power should be 25 to 13%. Guidelines propose however to use a 10% savings.
The book of Coyle also warns against the experiment of setting IGE power plus something and letting the helicopter climb because of HV restrictions.
Ground effect is influenced by a number of factors among which
- the ground it self (grass-concrete...)
- wind. From a wind of 2 times the induced speed of the rotor the ground effect also dissapears, so any test must be at zero wind.
During a practical standards exam for a commercial license, after many questions which I had no clue what the answer was, the examiner asked what the difference was between IGE and OGE. Finally something I thought I could answer.
After a fine dissertation with references to Prouty, George Saunders, physics and the rest, he looked above his glasses and said "try again"
Hmm, I had no other ideas.
"look in the performance section for IGE"
Ah, there it was, on the bottom of the graph, in the performance seciton of a 500, IGE is defined as "Figure 5-4. Hover Ceiling VS Gross Weight, IGE, 3.5 foot skid clearance with a standard landing gear, Allison 250-C20B."
And as far as the FAA was concered "everything else is OGE"
So that may just be the "practical" answer for OGE versus IGE.
Your story reminds me of the demonstration to the "Flat earth" society in the 1800's when a scientist tried to show the curvature of the earth using three floats on a long canal. The scientist aligned the telescope up with the two end floats and showed the center one above, and so concluded that the earth was curved. the Flat Earther then realigned the telescope with the first two floats, and showed the thrid lower, thus cancelling the effect the scientist had showed, he said.
that is the same logic the examiner used! Oh well.....
I may be the one of the examiners who is not an expert. Within the context of a practical test there is the aerodynamic explanation of IGE and OGE and then there is the performance explanation of IGE and OGE.
The main rotor diameter of the R-22 is approximately 25 feet (forgetting the 2 inches). The aerodynamic explanation of IGE is 1.5 to 2.0 of rotor diameter. The manufacturers IGE hover performance is calculated at 2 feet. I have been with applicants who happily explain that IGE hover height for an R-22 is between 37.5 and 50.0 feet less skid height, which is not wrong unless you are making a high density approach to a mountain top and you do not have Pilot Operating Handbook OGE performance.
I understand the difference between theory and practice. When it comes to a FAA Practical Test, and an IGE performance chart is being used to determine IGE performance, I expect an applicant to respond to the question "and for the purpose of this chart what is IGE performance?" the answer to the question should be what the manufacture has published. That may be a "flat earth" response, but it holds true in Lorentz four dimensional space-time with the same validity as the Special Theory of Relativity.