ground effect -- need opinions asap :)

[Florin777]

This is for a school project, where we have to show the ground effect.
We are measuring thrust, rpm and torque (we get power out from here), as well as power in the motor.
I also have this Prandtl tube for measuring airflow speed, at any point in the propeller wake. From what I understood through research, for the same power in, you get more thrust near ground since the vortices are not allowed to form as much (thus large blade area effective at producing thrust). This also means that downstream velocity component is lower (since airflow out is less vertical -- vortices being cut), which in turn results in slightly higher vertical lift component.
My understand is that if we measure the downstream velocity of the flow out of the prop, the value of airflow speed should decrease with the ground effect, am I right here? I can measure to see the actual results, but want to make sure my theory is fine too.
Also, is there an equation relating the airflow speed in the wake as a furnction of cleanrance from the ground or something like that?
Just want to find other way of using this device that let's me mearure airflow speed at any point.
Opinions and advices would be appreciated.

[JetPlaneFlyer]

No.. you are on the wrong track.
Ground effect does not so much effect prop thrust directly. The main impact of ground effect is that the proximity of the ground disrupts the formation of wing tip (not prop tip) vortexes, which reduces downwash. This reduces induced drag and reduced induced drag means that you will need less thrust and less power to maintain level flight.

I guess theoretically if the prop blades were almost kissing the ground then some very localised 'ground effect benifit' could be observed but it would only impact tha small part of the prop arc that was close to the ground.. In practice i doubt any significant benifit would be recorded due to this issue alone. In fact gliders and jets feel ground effect just as much as prop driven aircraft - check out some of the jet propelled Russian Ecronoplan designs.

http://www.aerospaceweb.org/question/aerodynamics/q0130.shtml


EDIT... I just re-read your post.... are you talking about a prop placed 'vertically' like a helicopter rotor???? If so then 'yes' you are very much on the right track. The prop should see the same benifits as a wing in ground effect. For the same input power thrust (a.ka. lift) should increase and downwash should decrease.

Steve

[Florin777]

Thanks Steve.
Sorry I should have mentioned I have the propeller mounted 'vertically' to resemble a helicopter blade
What I am most interested now is to know if there is a relationship between downwash velocity vs. ground clearance of the propeller. I think it would involve a more thorough analysis of the wingtip vortices?
Thanks.

[nmasters]

http://www.rcgroups.com/forums/showthread.php?p=9069001&highlight=ground+effect#post9069001

http://www.cybercom.net/%7Ecopters/aero/ground_effect.html

http://www.dynamicflight.com/aerodynamics/ground_effect/

[HybriGuy]

Thanks Steve.
Sorry I should have mentioned I have the propeller mounted 'vertically' to resemble a helicopter blade
What I am most interested now is to know if there is a relationship between downwash velocity vs. ground clearance of the propeller. I think it would involve a more thorough analysis of the wingtip vortices?
Thanks.
Yes, there is. Graphically, you could plot the following using exponential decay (http://en.wikipedia.org/wiki/Half_life) or viscosity (http://en.wikipedia.org/wiki/Viscosity) as a general case:

Let h (Height) = ground clearance
- Plot this along the Y-axis

Let |v| (the magnitude of Velocity) = airspeed in the propwash - measured just above the ground; that is, a moment before the propwash encounters the ground
- Plot this along the X-axis

The resulting curve resembles the natural decay function, and follows the same general form. This agrees with Bernoulli and Reynolds, who found velocity decay along the path of fluid travel when blown from a fan/propeller. Remember that you are not concerned with time, and you will need to look at velocity's decay as height increases (not time passing, as in the half-life example). The general form of the graph will not change appreciably in a simple case.

Also, see this HybriCraft, which operates only in ground effect. At the bottom of this page (http://www.walter-group.com/hybricraft_2008_holiday_message.shtml), you can see one with a propeller atop a vertically mounted shaft - just as you suggested.

I hope this helps,

Matthew

[JetPlaneFlyer]

Matthew,
I believe the effect that's expected here would be almost the opposite to that predicted by normal decay in velocity with distance. Due to ground effect you would expect the downwash velocity to reduce as the prop/wing gets closer to the ground. The velocity would be measured a fixed distance from the prop blade, not a fixed distance from the ground.

The reasons for all this are well described in the links a few posts up.

Steve

[HybriGuy]

... expect the downwash velocity to reduce as the prop/wing gets closer to the ground. ...
I think I understand what you're saying, and it seems counter-intuitive to me.

Once the prop./fan works on the air, and no new work gets done on the air, the air will slow down - no matter what the altitude of the prop./fan was when it did the work, right? Isn't this the physics rule for viscosity or friction? It just seems to me that the farther a fluid flows, the more resistance it encounters, and the smaller its velocity becomes.

See attached sketch.

If we consider the air's velocity at several points along its path
(v0, v1, v2, v3, and v4)
for each of three standoff heights
(h=10%, h=90%, and h=150%),

I think the one with the greatest velocity near the ground is the one with the smallest distance between the prop./fan and the ground (h=10%). Doesn't this make sense because the forces resisting the air's flow have had the least chance to slow the airflow?

[JetPlaneFlyer]

Matthew,

You appear to be assuming that the velocity of the downwash immediately as it leaves the prop blades is constant (i.e. Vo being the same in all cases).. This is incorrect when 'ground effect' is considered.

What you are neglecting in your calculations is that ground effect reduces tip vortex and so reduces downwash. This means that as the prop (or wing) gets closer to the ground the downwash it generates decreases. Study the links posted above for a more detailed explanation.

The rate of decay of the downwash velocity after it departs the prop is irrelevant as far as Florin's experiment is concerned because his velocity measurement will, I'm sure, be taken immediately below the prop in all cases.

Steve

[HybriGuy]

Okay, I visited the links above, and reviewed the references in them.

I want to point out something that may be misunderstood by non-helicopter folks. There is a considerable difference between a fan rotating parallel to the ground plane and the helicopter's rotors rotating in the same environment. A heli's rotor system - by design or definition, or whatever you want to call it - causes the rotor blade to approximate an airfoil shape along its circular path. This rotating airfoil produces lift, just as an airplane's airfoil wings do. Yes, ground effect results - under these circumstances - from the interaction of tip vortices with the ground. I agree with that assertion entirely! :)

A fan, like a propeller on an airplane (pusher or tractor prop.), serves as a thrust generator, pumping air through it - accelerating an air mass. The thrust produces an equal and opposite force. When rotating parallel to the ground, this results in a force felt as a lift force. Stanley Hiller built some neat models that did this (Google Image Search). They were ducted and/or shrouded, guarding them from tip-related interactions like vortices.

I think it is incorrect to say that a helicopter pushes itself from the ground by applying thrust. This is exactly what the V-22 Osprey does with its propellers, but the Osprey does not use a helicopter's articulated rotor system to create aerodynamic lift over the top of its blades. A helicopter does. Hiller does not.

Now, I'm not an Aerodynamicist, so please correct me if I'm wrong about that.

Here is an illustration of the difference. Think about the size of these two aircraft. Think about the differences in their rotor diameters. Think about the differences between their principles of operation. Look how tiny the propeller's vortices are, compared with the huge ring state surrounding the entire helicopter.

[nmasters]

This rotating airfoil produces lift, just as an airplane's airfoil wings do. Yes, ground effect results - under these circumstances - from the interaction of tip vortices with the ground. I agree with that assertion entirely! :)

A fan, like a propeller on an airplane (pusher or tractor prop.), serves as a thrust generator, pumping air through it - accelerating an air mass. The thrust produces an equal and opposite force.

That's a distinction without a difference. An airfoil is any shape that produces a desired reaction with the air. A propeller is a group of very small wings flying in a tight circle. The rotor of a helicopter is articulated so that the lift can manipulated for steering and stability purposes. Some early experimental helicopters had ailerons on the blades but the mechanical and structural problems were too much. Aerodynamically it wasn't a bad idea. The reason the flow in the pictures you posted look different is because one is condensation in the core of the vortices and the other is smoke in the outer region. They are both the same, realy :)

[Florin777]

Ok, here is a small portion of my presentation that I want to put on, but need opinions on this. Does it make sense? haha
I don't want to look like I dont know what Im talking about. See attachment for tth table, part of the discussion below.

BY THE WAY, IF ANYONE LOOKS AT THE TREND OF THOSE NUMBERS, DO THOSE MAKE SENSE AT ALL? I DONT THINK I HAD THE PATIENTE TO WAIT LONG ENOUGH FOR THE FLOW TO "SETTLE", I THINK THERE WAS TOO MUCH TURBULENCE TO GET SOMETHING STEADY.

"Validating the theory

Vertical flow speed component measured above

Due to the airflow turbulence and propeller vibrations, the numbers above cannot be correlated accurately, but an interpretation of the flow speed magnitudes validates the theory as follows: downwash flow velocity is reduced in the ground effect, and vortices are larger as ground clearance increases even when air viscosity slows flow speed to 0 at infinity."

[JetPlaneFlyer]

Florin,

Id have expected the reduction in downwash to be greatest at the tips and the results taken at the tips are exactly what I'd have expected. The increase in velocity near the blade centre as the prop gets closer to the ground is a surprise.
Were velocity measurements taken from a fixed distance from the blade in each case?.. If the Prandtl tube was perhaps mounted on the ground and it's height not adjusted to maintain a constant clearance to the prop blade then this could explain the unexpected results?
Steve

[JetPlaneFlyer]

I want to point out something that may be misunderstood by non-helicopter folks. There is a considerable difference between a fan rotating parallel to the ground plane and the helicopter's rotors rotating in the same environment.

Nope.. A helicopters rotor and a prop are fundamentally the same. They are both rotating wings and both work on exactly the same principals as any wing. The main 'difference' is that a prop has a forward speed through the air wheareas a helicopter is generally close to the hover. This is like comparing a wing that is climbing to one thats in level flight.. they are still both wings.

The movement of a prop through the air does however lead to a prop having considerable twist along it's length to keep all parts near optimum angle of attack. A rotor being optimised for the hover needs little of no twist.. but this is a minor detail.

Steve

[Florin777]

I am unable to take measurements as same height, while moving the ground up.
However, the result that tip flow speed increases with increased height is still there showing the vortex formation as you move ground up.
Now I don't know why the centre has higher speed when closer to the ground, might be just because it's a measurement taken closest to the prop. The decrease in last measurements is just because of air slowing down at high distance from ground, but the vortices are still there, at least that's what i measured..
Not sure if these make sense at all

[JetPlaneFlyer]

Florin,
I think you need to figure out a way to be able to move your Prandtl tube so that it remains a constant distance from the prop at all times.
If the distance changes then you are adding another variable into the results. You will never be sure what variable is causing the change and the experiment becomes meaningless.

Steve

[Florin777]

I don't know if you can see any change in downwash speed if you measure at same distance all the time.
I tried to measure the downwash speeds before the flow hits the ground.
I can at least show the presence of vortices at higher ground clearances. I'm not saying that numbers are exact (not even close to real), but comparing their magnitudes tells the vortex reduction when in the ground effect.

[JetPlaneFlyer]

I don't know if you can see any change in downwash speed if you measure at same distance all the time.

According to ground effect theory you should do.
To make the experiment valid all variables should remain constant apart from the one that you are investigating (i.e. the distance between prop and ground).. If the distance between prop and velocity measurement point also varies then you are sure to get unpredictable results.

Steve

[HybriGuy]

That's a distinction without a difference.
Yes, the way you removed the difference from my post when quoting me makes it seem so. Later, I provided a couple differences. I think it is incorrect to say that a helicopter pushes itself from the ground by applying thrust. This is exactly what the V-22 Osprey does with its propellers, but the Osprey does not use a helicopter's articulated rotor system to create aerodynamic lift over the top of its blades. A helicopter does. Hiller does not.There; now it's a distinction and a difference again.

I could have said, "A propeller and a heli's rotor system are entirely different, each suited to a different aircraft, each doing a different job, managing airflow through them differently - each for its particular purposes." There is a new set of distinctions and differences. I think a propeller is specialized to do something, and a heli's rotor is specialized for something else. I think that makes them different.

The velocity would be measured a fixed distance from the prop blade, not a fixed distance from the ground.Oh, I had not read that from Florin's posts. Thank you for pointing that out. I would have approached a ground effect problem like the one he explained from the ground - rather than some fixed distance from the propeller. I think he will have a great deal of difficulty managing his equipment - doing it the way you want him to.

Remember that he wants to measure velocity - a vector. As he gets closer and closer to the ground plane, he has to keep his tube straight into the incident air, or he will receive false airspeed readings. Meanwhile, the direction of the incident air keeps changing, because the ground gets closer and closer to the prop plane.

I guess he could do it either way. It just seems simpler measuring airspeed at or near the ground.

[JetPlaneFlyer]

I guess he could do it either way. It just seems simpler measuring airspeed at or near the ground.

It may be easier but that way you are not isolating the influence of ground effect from othere variables. The whole point of the experiment is to measure the effect of the proximity of the ground on the downwash velocity produced by the prop. To have any validity the velocity must be measured from the same position relative to the prop each time. If you change the point of measurement then velocity is bound to change regardess of ground effect influence.

To be honest i cant see ant fundamental problem in setting the Prandtl tube to a fixed distance from the prop, it should be easy enough.

[Florin777]

I got lost in these disucussions hehe, haven't been able to read them all in detail because I'm really busy with other stuff at school.
After all, is there a good way to measure airflow speed with a Prandtl (pitot) tube, and then make a conclusion on the ground effect based on the results?
What I did when I measure I put the tip of the pitot tube directly in the wake, and was mounted just below the ground (ground is up since it's a pusher setup).. that way it was taking readings 2 inches before the airflow hit the ground, but they were not all too stable. I am guessing a lot of turbulence, plus the flow separation around the tip of the Prandtl tube. Perhaps better to make a small hole in the ground, and measure the speed at the instant it hits the ground?

[Florin777]

would need to know this by today if possible!!
got the presentation tomorrow, and I'm still debating whether to show the prandtl measurements or not... it would be something apart from everyone else hehe

[bwalt822]

would need to know this by today if possible!!
got the presentation tomorrow, and I'm still debating whether to show the prandtl measurements or not... it would be something apart from everyone else hehe

Florin,

Its kind of a Jack@$$ thing to ask for advice and then say you are too busy to take 5 minutes to read it and think about it.

You have had many smart people here tell you what you need to do in order to get accurate results, you can either not take their advice and do the expirement wrong or take the advice and possibly get good data. The measurements that you have taken do not accurately show ground effect. Your measurements show the effect at the ground of a prop coming nearer.

[nmasters]

I think it is incorrect to say that a helicopter pushes itself from the ground by applying thrust. This is exactly what the V-22 Osprey does with its propellers, but the Osprey does not use a helicopter's articulated rotor system to create aerodynamic lift over the top of its blades. A helicopter does. Hiller does not.

Now, I'm not an Aerodynamicist, so please correct me if I'm wrong about that.


You're wrong! The Osprey uses a different and much more complex control system than other VTOs but the rotor produces force in exactly the same way. "thrust" and "lift" are synonyms for "force".

[Florin777]

I did not mean to be rude when saying that I dont really have time to look through all posts in detail.
What I meant is that I am reading them, but some say different things, and I can't decide on what to experiment with at the moment. Time is key now.
I think I will go with the ground totally removed (rather than at higher clearance) and see the results there, and compare with results I already have. I will also try to reproduce 2 of the previous results, see what I get.
Thanks for everything guys

[bwalt822]

Also it seems that if you can mount the motor and move it closer to the ground, then you should just be able to mount the pitot tube to whatever the motor is mounted to instead of the ground.

[nmasters]

What I meant is that I am reading them, but some say different things, and I can't decide on what to experiment with at the moment.
Such is the nature of an open forum. Everybody gets to talk, even those who are wrong but are confident that they are right. That's why a classroom is often a better place for a young person to learn (fewer distractions).