Question about radiator flaps and their relation to lift.

[FeuerFliegen]

So I've always wondered... many radiator flaps are positioned in a way that looks like it would give it some extra lift (with the expense of drag obviously), in the same way flaps would. 

 

Planes such as the Bf110, and others with similar designed radiators that when fully opened,  seem like opening their radiators would give a similar effect to flaps.

 

Can anyone tell me if they were purposely designed this way, to at least get some benefit from the drag (other than the cooling, obviously)?

 

And if this is true, does the sim take this into effect?

[II./SG.1-MarkWilhelmsson]

An excellent question! Following

[Supercharger]

In case of the Bf110 the radiators where just attached to the wings lower surface. That means that opening or closing of the radiator outlet flap did not improve the camber of the wings airfoil section. The Bf109 and Me210&410 on the other hand have an improved flap design. The flaps at the radiator section of the wing where split in two half s upper and lower. Both half s will move downward if the flaps where set in landing config. that's the only way to improve the airfoil camber with radiator flaps.

In my opinion Bf110 radiator flaps are unable to improve the lift. They where just designed to regulate the cooling air mass flow through the radiator core.

[Canvas25]

The idea would be that the radiator outlet would work as a small split flap. If you're going to increase drag, might as well give some lift?

[Supercharger]

Ok now i unterstand...you focused on the outlet flap only. Flap will be opened with an angle of attack. But to create lift, the air stream on the upper side of the flap(radiator outlet) has to be faster than the air stream underneath, right? I'm not sure about the airspeeds around the flap. I guess the air that comes out of the radiator is much slower than circumambient air. That means the outlet flap generates negative lift.

[Elem]

8 hours ago, Supercharger said:

I guess the air that comes out of the radiator is much slower than circumambient air. That means the outlet flap generates negative lift.

No, just the opposite. Air having passed through a radiator has been heated and the result of that is, it expands so is at a higher velocity. This reaction is used to create a small positive back pressure which will slightly make up for the drag of the radiator. Lift is not a consideration.

[bzc3lk]

8 hours ago, Supercharger said:

 I guess the air that comes out of the radiator is much slower than circumambient air. That means the outlet flap generates negative lift.

 

You might want to read this.

 

https://en.wikipedia.org/wiki/Meredith_effect

[ZachariasX]

The flaps are designed such that they permit an airflow as laminar as possible upon leaving the cooler.

 

 

You can see here in the 109F, it is very much designed to make use of the Meredith effect as is the P-51. You have a narrow entry and then expansion in cross section to slow down the air. This way, you require less force to press it through the cooler (reducing drag accordingly) plus you get the added little push from the expansion of the air as it passes the cooler. The addition of a boundary layer flow show that the main concern of the while arrangement is to let the airflow pass through as easily as possible. Anything you stick out in the airflow will not act as lift device, but as airbrake. Hence also in the 109, the outlet flaos do nothing but try to make a conformal arrangement to the rest of the wing. It is also of note that if the engine runs hot, you open the inlet flap, not only increasing air mass but also increasing airspeed. The 109 cooler has a narrower range of where it is efficient than the Mustang. But I would guess it is still more efficient (cooling vs drag) than the Spitfires‘.

 

Note that the P-51 is definitely a superior solution than what you see in the 109 and most certainly the Spitfire (that features a similar design principle), because it has space for a larger cooler, meaning that it can decelerate the passing air more than wing coolers (due to their limited space) and still permit installing a cooler that is large enough. On top of that, the scoop is offset below the airframe. This means that the turbulent boundray layer of the airframe is not entering the scoop, drastically reducing drag. This is rather revolutionrary. You can see how long it took to make jet intakes offset in such a way. The P-80 jet is less refined aerodynamically than the Mustang.

 

You enjoy a similar effect in your own inlet duct, your nose. The folds in there not only increase surface for more olfactory receptors (better sense of smell), but it also creates a laminar flow though your nose considerably easing your breathing.

[Bilbo_Baggins]

20 minutes ago, ZachariasX said:

The flaps are designed such that they permit an airflow as laminar as possible upon leaving the cooler.

 

 

You can see here in the 109F, it is very much designed to make use of the Meredith effect as is the P-51. You have a narrow entry and then expansion in cross section to slow down the air. This way, you require less force to press it through the cooler (reducing drag accordingly) plus you get the added little push from the expansion of the air as it passes the cooler. The addition of a boundary layer flow show that the main concern of the while arrangement is to let the airflow pass through as easily as possible. Anything you stick out in the airflow will not act as lift device, but as airbrake. Hence also in the 109, the outlet flaos do nothing but try to make a conformal arrangement to the rest of the wing. It is also of note that if the engine runs hot, you open the inlet flap, not only increasing air mass but also increasing airspeed. The 109 cooler has a narrower range of where it is efficient than the Mustang. But I would guess it is still more efficient (cooling vs drag) than the Spitfires‘.

 

Note that the P-51 is definitely a superior solution than what you see in the 109 and most certainly the Spitfire (that features a similar design principle), because it has space for a larger cooler, meaning that it can decelerate the passing air more than wing coolers (due to their limited space) and still permit installing a cooler that is large enough. On top of that, the scoop is offset below the airframe. This means that the turbulent boundray layer of the airframe is not entering the scoop, drastically reducing drag. This is rather revolutionrary. You can see how long it took to make jet intakes offset in such a way. The P-80 jet is less refined aerodynamically than the Mustang.

 

You enjoy a similar effect in your own inlet duct, your nose. The folds in there not only increase surface for more olfactory receptors (better sense of smell), but it also creates a laminar flow though your nose considerably easing your breathing.

 

Great post. On a side note, why do you think they omitted the boundary layer bypass outlet going from the F to G model machines?

Edited October 10, 2020 by Bilbo_Baggins

[ZachariasX]

Just now, Bilbo_Baggins said:

Great post. On a side note, why do you think they omitted the boundary layer bypass outlets on the G model machines?

I can only speculate. I also cannot say how much this arrangement reduces drag „in the real world“, but it is for sure something that increases manufacturing effort. At a time where production numbers trumped everything, I think it might have made sense omitting fancy stuff in progressive wing redesigns. Also it might give you another inch of cooler height that might come handy, as engine power goes up.

[Supercharger]

You're absolutely correct, P51 radiator design was very advanced. But SCG_FeuerFliegen was asking for the radiator flap of a Bf110 and if it's creating some lift when it's opened fully? In my opinion you can not compare those two designs(P51&Bf110). So the question was about lift not thrust. When the radiator flap is fully opened with a certain angle of attack it creates a diffusor which in turn creates a pressure drop because of the slowed down airstream exhausted from the radiator. This is why i am pretty sure that an wide open radiator flap did not create any kind of useful lift.

[Guest deleted@50488]

Question is, does il2 fdm model this subtleties?

 

[messsucher]

7 minutes ago, jcomm-in-il2 said:

Question is, does il2 fdm model this subtleties?

 

 

Since everything is simplified because of variety of reasons I would guess no.

[ZachariasX]

15 hours ago, Supercharger said:

But SCG_FeuerFliegen was asking for the radiator flap of a Bf110 and if it's creating some lift when it's opened fully?

No, it just adds drag:

23 hours ago, ZachariasX said:

Anything you stick out in the airflow will not act as lift device, but as airbrake.

 

It doesn‘t matter if the cooler flap is on the wing as it will not increase the camber of the profile or anything like that.

[FeuerFliegen]

On 10/10/2020 at 6:44 AM, ZachariasX said:

Anything you stick out in the airflow will not act as lift device, but as airbrake.

 

Could it not be both?  Unless the airbrakes (or whatever is adding drag) are perfectly pushing directly against the angle of attack, I would assume that based on it's shape, it would also push the plane in one direction or another (whether up, down, to the side, diagonally, etc).  While also adding drag, why could it not also push the plane partially in an upward direction?

 

My original question also applies to other planes too, such as the Tempest.

[ZachariasX]

8 hours ago, SCG_FeuerFliegen said:

Could it not be both?

No. Making things stick out of an airfoil, regardless of the angle, usually acts as a brake. You have to look at the whole profile of the foil you are creating. The only way to get lift is if the things you are sticking out in the slipstream create as a whole over the entire crossection a profile that has more lift than the clean profile. Such are known as flaps (or slats, any kind of high lift device). Any other thing you stick outthere acts as a spoiler, disrupting the airflow. Remember, you are (in your example) not only absorbing the impulse to kick an air molecule downwards instead of past you (this would indeed give a lift vector), but then you also receive trailing turbulences that cancel out all real world lift but leave you with nothing but drag.

[FeuerFliegen]

18 hours ago, ZachariasX said:

absorbing the impulse to kick an air molecule downwards instead of past you (this would indeed give a lift vector)

 

So, yes, it does give some lift?  

 

I never thought that the radiator flaps would give a net gain in lift, but that the designers figured that since they have to add drag regardless to allow more airflow into the radiators, they might as well angle it in a direction that, while obviously not giving a net benefit, would at least give some gross gain to the lift factor.

 

Also I was never suggesting that the radiator flaps on a 110 would affect the lift factor or affect camber of profile, or anything else about the wing itself; I would assume that it is separate from the wing's airflow, even though it is attached to it; just like other parts of the plane, while adding drag, can also push the plane in one direction or another, one of which could be upwards.

 

If there was zero gross gain in lift from having the radiator flaps pointed in a downward, rearward, 45° angle, then is there any reason that this aspect was applied to several aircraft designs?

[ZachariasX]

4 hours ago, SCG_FeuerFliegen said:

So, yes, it does give some lift?  

But it takes away lift as well. You are interested in net lift. So, no, it does not give lift.

 

4 hours ago, SCG_FeuerFliegen said:

or anything else about the wing itself;

It is part of the wing and one thing comes with the other.

 

You also have to understand that you really, really DON‘T want your radiator flaps to give net lift, as it would change the trim of the aircraft. With automatic radiator flaps, this would not only be a nuisance, but a nightmare for flying on instruments.

 

Any designer would spend the rest of the day on the naughty stairs if he ever shared such an idea.

 

Opening flaps add drag and slow down your aircraft. There is no good from that except that you can design the installation such that is hasas little drag as possible while allowing for the required heat exchange.

[FeuerFliegen]

Any idea what this particular design was chosen for then?  It seems to be one of the most common for WW2 planes with inline engines.