Do You Use Speed Brakes for Anything Unusual?

[carusoam]

Hi everyone!

 

Some notes on our speed brakes…

1) When you use them… you are giving up some of that efficiency that some Mooniacs crave.

2) they disrupt the airflow about 3X the width of the brakes themselves… deploy in the rain for a visual confirmation.

3) essentially they are a wing shortner….  soooo expect a small increase of AOA required to maintain level flight once deployed…

 

SB magic…

4) These devices are speed dependent…

5) at Vne, they have a tremendous effect…

6) For emergency descent, gear down, power out, prop pushed in… combined with the brakes… you will be landing in less than three minutes, no matter what Mooney you fly… a 6k’pm descent…

7) at traffic pattern speeds… they can help reduce airspeed a little, due to the short wing effect and increased AOA…. Similar to pushing the prop all the way in… the down shift maneuver.  
 

8) at landing speed… you are going to need a CloudAhoy app connected to your WAAS device to see the data…. It isn’t something you can feel.. unless your calibrated seat pants are that good.

9) all of this magic come from the placement of where the brakes are on the cord of the wing… if they were closer to the leading edge, they would be more disruptive at slower speeds…  further back, they would be lost in the airflow disruption that is already going on…

10) since airspeed² is the driving force for this drag…  the speed brake effectiveness is more obvious near Vne… and still plenty obvious at Vlo

 

 

other notes…

11) all Long bodies have Speed brakes, as a replacement for full flap slips at low-ish speeds… where the flaps have a tendency to blanket airflow over the horizontal stabilizer…

12) Soooo… they still work at slow speeds, just not as much as they do at gear down speed….

13) Landing with a vertical descent of 6k’pm would be considered slightly unstable by most Mooney pilots….  Unless your traffic pattern was 6k’ agl… 

 

Oddly…

14) flaps are not deployed for emergency descents in Long bodies…

15) The descent rate, at gear down speed is better, than slowing down to Vfe….

16) The procedure includes slowing to extend the gear, then speeding up to enjoy the additionally braking at gear extended speed….

17) Emergency descents are a blast… they aren’t very good for the health of the plane, or the people riding inside…. Or the efficiency of the flight…

18) Since we are on the ground at 60+kias… with an AOA, mandated by the gear geometry, high enough to take off… hitting the brakes above 50kias is sure to skid both tires…

19) It is likely… that deploying the speed brakes will improve the transfer of weight from the wings to the wheels… slightly, not magically…

20) adding the extra detail to a landing procedure… won’t make any friends after the ensuing distraction / gear up discussion…

PP thoughts only not a CfI… my speed brakes get their most use BEFORE each flight… check list item… 

Best regards,

-a-

 

[PT20J]

1 hour ago, PaulM said:

If you look at a video of how the speed-brakes interfere with the air on the top of the wing it interferes with the lift on the upper surface of the wing and causes air flow separation.  That is not simply drag, but reduction of lift.   Stall speed is based on angle of attack, and that does not noticeably change, but the sum total of lift is reduced. It can be a combo of lift reduction and added drag that requires a steeper path angle to maintain the constant airspeed. 

Let's discuss this in reverse order  

According to Newton's second law, F=ma, an unbalanced force will produce an acceleration. In non-maneuvering (i.e., unaccelerated) flight, lift = weight and thrust = drag. If the speed brakes significantly reduced lift, the airplane would accelerate downward. But this isn't what we want. We want a nice, constant descent rate at a steeper angle with constant airspeed. What is happening is that the extra drag causes total drag to exceed thrust. We naturally decrease pitch to maintain airspeed and this brings a small portion of the weight vector into alignment with the flight path to make up for the thrust deficit.

Stall is caused by excessive angle of attack, but stall speed is a function of both angle of attack and wing area (the lift equation is 1/2pV²C_LS, where p = air density, V = TAS, C_L = lift coefficient, and S = wing area). If the speed brake significantly affected lift, it would be equivalent to reducing the wing area, and the stall speed -- but not the stall angle of attack -- would necessarily increase. But it doesn't.

Clearly the speed brake disrupts airflow around it's location. But it is important to remember that the pressure distribution along the chord of an airfoil is not uniform. Most of the lift comes from the forward section. In fact, it is common for aerodynamicists to consider the lift concentrated along the quarter chord line as predicted by thin airfoil theory before modern CFD methods were available. Here is a diagram of the pressure coefficients on the top and bottom of a laminar flow airfoil at high angle of attack from https://www.mdpi.com/2076-3417/12/17/8757. Note that there is very little lift production aft of the 0.5 chord point. Since the speed brake dimension is small compared to the span, it doesn't affect much of the wing area and as it is located so far aft on the chord line this effect is in an area of little lift production.