Airflow at a Hover in Helicopters

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In this lesson I’ll be covering Airflow at a Hover. For the purposes of explanation, these diagrams refer to helicopters operating at a stationary hover and in a no wind condition. This is because motion or wind from any direction or velocity will change the lift characteristics due to dissymmetry of lift, transverse flow, and translational lift. In another video I will explain how all of these factors come together to affect the helicopter. But for now I’ll keep it simple and cover one subject at a time.

Airflow at a hover can be broken down into two types: In Ground Effect (IGE) and Out of Ground Effect (OGE). This leads to the question, “what is ground effect?” Ground effect is the increase in lift that occurs when a hovering helicopter is within one rotor diameter above a surface. When a helicopter hovers, airflow through the rotor system comes down vertically and vortices develop around the rotor tips. While in ground effect, this down flow of air is restricted due to ground friction as the airflow is redirected out words and away from the rotor disk. Because of this, the high velocity of a downwards flow of air (induce flow) is reduced as well as the rotor tip vortices are pushed away from the helicopter. While hovering out of ground effect, there is nothing to restrict or redirect the downward flow of air. Therefore there is a high velocity of induced flow through the rotor system and the wing tip vortices are allowed to develop without any restriction.

Because there is more induced flow while operating out of ground effect, the pilot must increase the Angle of Incidence in the blades in order to maintain the same Angle of Attack. Keep in mind that Angle of Incidence is a mechanical angle that a pilot can directly control by manipulating the collective. However, the Angle of Attack is in aerodynamic angle that is the product of pitch and induced flow. Due to the fact that the pilot has to increase pitch in order to maintain Angle of Attack while out of ground effect, there is more drag on the rotor system and therefore requires an increase in power to maintain lift. This is why it requires more power to hover OGE than IGE.

One last factor to consider while operating at a hover ground effect is the surface condition. Tall grasses, rough terrain, ground obstructions, water, etc. can all affect airflow as it passes through the rotor disc. This can cause uneven air flow and therefore uneven lift.

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If you’re just getting started and want more information, pictures, and more explanations, I’d recommend reading the Rotorcraft Flying Handbook –

If you’ve already got a basic understanding, and want to further your professional helicopter education with advanced helicopter concepts, I’d recommend reading Cyclic and Collective, by Shawn Coyle –

[Applause] [Applause] [Applause] I hello and welcome back to helicopter lessons in ten minutes or less it's Jacob again and today I'll be going over air flow at a hover now for simplicity of explanation these examples will be referring to helicopters at a stationary hover and a no wind condition if either of these are not true then the lift characteristics are going to change slightly due to dissymmetry of lift transverse flow and translate translational lift some of the stuff that talked about in my previous videos now at a later point will combine all of these into one example that shows how they all influence a helicopter in flight but for now we'll keep it simple and just be covering airflow at hover so what are the two types of airflow at hover we have the angry on effect and we have out of ground effect which begs question what is ground effect well ground effect is going to be the increase in lift that occurs and hovering flight when a helicopter is within one rotor diameter of the surface so what does that look like let's say we have our helicopter right here there's a rotor blade fuselage landing gear right here and we're looking at it from the front now let's say that this rotor diameter is 50 feet now he is considered he or she is considered within ground effect where within 50 feet of the ground so say he's hovering right here I was going to be five foot hover in a 10 foot hover all the way up to 50 feet we're still considered in ground effect now let's look firstly at the out of ground effect so the same rotor disc and helicopter and now say let's say he said 100 feet so he's greater than that 50 feet greater than that rotor diameter above the surface all right now there's going to be a difference in lift characteristics based on the airflow do this these rotor systems so let's start first with in ground effect while we're sitting here at the ground at a hover this air flow is coming in vertically through the disc so what it's doing it's coming into the disc and then as it gets closer to the ground hits that ground effect and it gets pushed out so it's coming vertically and pushed out and away from the disk now also on the tips of the rotor blades it's starting to get a little bit of a rotation swirling a wingtip vortices is a call as it's called and now this is going to affect some ellipse characteristics as well so anger on effect coming in vertically as it gets closer to the crown it's getting pushed outwards so we have this induced flow it's kind of interrupted a little bit with the ground effect we have the wingtip vortices now some of the same stuff is going to happen here in the og so out of ground effect but the lift characteristics will be slightly different so now that we don't have the ground and near proximity to the rotor disk this airflow is coming in and continuing to come straight down through the disc without any kind of ground interference now without that ground interference these rotors and forces are going to be able to develop even bigger than when we were in the ground effect all right so we have differences in airflow we have at any time in a hover we're going to have our wingtip vortices right here and we're going to have that induce flow but when we're in ground effect that induced flow is getting interrupted by the ground friction of the ground resonance and not as much of a high velocity beer is able to travel through the disc vertically and also for these wingtip vortices they're getting pushed out and away from the helicopter because the ground is pushing that air flow away whereas at a GU hover we have this air this mass airflow coming downwards through the disc without any interruption we're having a higher velocity of induced flow of that downward slope air and these wingtip vortices are able to build at a greater rate so this is going to reduce lift so let's take a look at what is actually going on with the lift characteristics we've our blade right here cord line running through the blade there's chord line it's the rotational relative wind I've talked about that my previous videos it's the path that it takes around the helicopter around the mast and then now we'll do the induced flow which is not going to be as much in a in ground effect so here's our induced flow and that combined with the rotational relative wind is going to give us our result that relative wind now this area that's left over is going to be our angle of attack this is the list coming from the blades that are from the yeah from the airfoil all right now vice versa if we were to take this exact same helicopter keep the same pitch angle and put it at oh gee hover this induced flow is it going to be a lot greater so if we were to keep that rotational wind now it has an out mass massive increase and reduce or an induced flow this area right here it's going to reduce our lift right here this angle of attack if we don't change the pitch angle on the blade and so we'll have a loss in lift because right now say we're a stationary hover you know level I'm not gaining or increasing altitude we have this angle of attack now with the reduction in angle of attack its reduction to lift we're can actually going to our sinking unless we increase this angle so what do we do we increase the pitch in the blade using that collective now we have more pitch in the blade to compensate for that increase in adduced flow and all of this is made to keep this angle of attack the same in order to generate the same amount of lift and an out of ground effect is an inground effect what is this called this is your angle of incidence where as this area is your angle of attack but things right over here angle of incidence versus angle of attack so we control with the collective our angle of incidence right here but what we're after is maintaining that angle of attack so we can directly control incidents and aerodynamically the angle of attack is is the the difference there is the combination of the pitch in the blade with the induce flow so more induce flow I have to pull and more pitch in a blade with my collective to increase that angle of attack okay so simply flip when we're in ground effect there's less induced flow velocity less rotor tip vortices than out of ground effect and this is because the ground reduces the amount of induced flowed that can pass through it due to the surface friction while simultaneously restricting these wingtip vortices from forming because it's pushing the airflow out that og hover all of this is able to I guess exacerbate you have a rapid flow of air and nothing that's interfering with these wingtip vortices so one more factor that could potentially affect your hover is any kind of objects on the ground so so you have obstacles on the ground or maybe some high grasses underneath the helicopter or maybe you're over water and there's a bunch of waves this is going to affect how this air flow passes underneath your rotor disk and potentially allow one side of the rotor disk to generate more or less lift than the other side so just another factor to consider but a general nut wraps up air flow in a hover we have in ground effect how to ground effect thanks for watching hit the like below if you enjoy the video and leave a comment for anything in the comment section below if you have any questions or just want to comment on the video thanks again for watching have a good day

26 thoughts on “Airflow at a Hover in Helicopters”

  1. If the helicopter was tethered to a platform below it that is light enough for the helicopter to lift and the diameter of the blade spin fits within the area of the platform, will the helicopter still fly up?

  2. I'm having a real hard time understanding what induced flow exactly is. I've always heard that it was easier to hover in ground effect because there is a "cushion" of air between the rotors and the ground. Is induced flow that 'cushion" of air? I'm thinking its not, because it just doesn't fit with your explanation, i'm trying to figure out how the flow would generate more lift within ground effect, but less out of it.

  3. Thanks for your great explanation. The way I understand from this video, restricted wingtip vortices lead to relatively small downwash over the blades, which generates bigger lift in comparison to OGE. Am I getting it across?

  4. Hi! I am confused on this explanation. You discuss that IGE is the diameter of the rotor blade or less and OGE is greater than the diameter of the rotor blade. What is confusing me is something called IGE and OGE hovering ceilings which for example of a Robinson R22 is somewhere in the thousands of feet for IGE which is way more than the rotor blade of a Robinson R22 which is 25.2 Ft. I am new to learning about helicopters, so is the IGE hover for a Robinson R22 25.2 Ft or is it in the thousands for a hovering ceiling? Very confused? Please help. Tnx!

  5. You mention that ground friction and ground resonance is the reason for a reduction in induced flow but that doesn't really explain why it happens. Perhaps you could mention the pressure increase below the disk as the air cannot escape fast enough. This increase in pressure restricts the downward component and therefore reduces the IF. Just a little further as to why it happens.
    Also, in the U.K. We teach 2/3 of rotor diameter to be truly IGE. Interesting that we use different figures!

  6. Is IGE vs OGE defined by the height of the rotor disk off the ground (Instead of AGL) with the crossover at the measure of the disk diameter? You seem to suggest this, but never state it. Great videos, I agree that some colors could help clarify the images.

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