Minimum Power

[flyguy]

Was fooling around yesterday on a very calm day and while doing some flight a minimum controllable speed I got in a mode that I had never seen before and it rather conflicts with my previous concepts of power, lift, and drag. My BC12D-85 stalls at 44 indicated, more or less, power off. Before I had tried determining the airspeed for mininmum required power to maintain altitude and I had thought I had found 1500 RPM and 70 indicated (That is about 65 calibrated on my airplane.) Yesterday I found it to be 1600 at 45 indicated! Lowering the nose required increasing the power to maintain altitude. I had always assumed that operation at 1 mph above stall was a high drag angle of attack for the wing and would requite more power than the best glide speed, for example. Really twisted my head around. I'm going out and check that again. Strange.
Anyone seen the same thing or have any comments? Taylorcraft test pilots are you out there LOL?
Darryl

[Forrest Barber]

Re: Minimum Power

I presume you were shown "operating behind the power curve" as a young pup! Full power, then slow up till just over stall speed, then try to recover without loss of altitude. THEN use 1600 RPM s and do same thing, then do not increase power and try to level out without losing altitude

[flyguy]

Re: Minimum Power

Yep, this "old dog" has been there and done that in every configuration and variation posssible already.
Actually it is a lift/drag curve that predicates the power curve, not the other way around, but the "power curve" is such established popular terminology that I wouldn't even consider trying to refute it.
Went out yesterday and took some data on the ole kneeboard to be sure of what I had seen before.
Started at 70 and worked down maintaining altitude. Results for anyone who might be interested:

70 1800
65 1700
60 1650
55 1600
50 1550
45 1550

To express my amazement at this a little clearer: I have always understood that the LOWEST Drag to Lift was at a speed considerably faster than stall. As in the minimum sink glide speed. I'm not talking about the "back side of the power curve," where you have to hang the airplane on the prop and if you cut the power it will immediately stall. What I am perplexed by is that the power required is lower in what I always assumed was a high drag part of the wing's lift curve.
Tomorrow I'm going to carefully check sink rates at various airspeeds. Pretty interesting to find that some things I thought I understood are not so. What I'm thinking is that I must have seen a wing lift curve vs something besides drag and thought it was L/D.
Just havin' fun,
Darryl

[Forrest Barber]

Re: Minimum Power

keep having fun do it at, Gross weight, constant pressure altitude, record air temperatures and correct if a large spread., is airspeed calibrated?

[Forrest Barber]

Re: Minimum Power

keep having fun do it at, Gross weight, constant pressure altitude, record air temperatures and correct if a large spread., is airspeed calibrated? 23012 is just a good airfoil...

[VictorBravo]

Re: Minimum Power

Tomorrow I'm going to carefully check sink rates at various airspeeds. Pretty interesting to find that some things I thought I understood are not so. What I'm thinking is that I must have seen a wing lift curve vs something besides drag and thought it was L/D.
Just havin' fun,
Darryl

Not that I have ever modified or fooled around with an airplane, mind you, but here's another thing to play with while you are enjoying the built-in efficiency of a Taylorcraft...

The drag curve you are talking about is not only for the wing but it is the entire L/D of the airplane. So it stands to reason that anything improving the L/D of the airplane at low speeds will improve the numbers of your minimum power required. Also realize that there have been many cases of loops, zig-zags and other acrobatics in the lift curve plots of airfoils at various loadings and AOA's!

It might be an interesting academic exercise to note the minimum power required before and after you try some temporary L/D improvements , such as gap seals, fairings, VG's etc. Taylorcrafts have certain concessions that were made to make production or maintenance easier... but those concessions were made when gasoline was 25 cents a gallon or maybe less. A little effort can minimize the effects of these concessions, without any (serious) changes to the airplane.

There are a few big square holes in the wings that allow high pressure air to leak up into the low pressure air (the aileron hinge locations). These act like spoilers, putting "holes" in the spanwise lift distribution curve. Taping off those gaps would allow the airplane to achieve a better L/D.

Fairing the intersections of the struts to the wings and fuselage will improve L/D. Taping the gap between the stabilizer and fuselage will help a tiny little bit. Gap seals at the ailerons will help, but it has been noted on this forum that they can easily reduce the roll rate if not done properly. Having the engine exhaust pointing back instead of down will reduce drag. Wheel pants will reduce drag significantly if they are designed right. Cooling drag is a larger percentage of drag than people realize.

Here's one I want to try one day... temporarily install a 6 inch extension between the flange and the propeller. This should put the propeller out in cleaner air and reduce the propulsion losses due to the blunt cowling. This CANNOT be a permanent mod, because with a heavy metal prop it does apply additional forces to the crankshaft and will wear out the center main bearings faster. A wood prop would not do this, but for obvious reasons it would be a one-time test flight in calm air and low stresses on the engine. Before anyone screams that it will bend the crankshaft, we ran 8 and 10 inch extensions at 4200 RPM in F-1 racing without ill effects, although as mentioned it applies extra forces on the center main bearing in a 4G turn. I am proposing this as a one-shot test to get a data point under controlled conditions.

It might be an interesting experiment to document how these things affect the minimum power required to fly, or how much faster the plane will cruise at these low power settings. Of course, these should be done without making any permanent changes to a classic airplane, and done in a manner "to avoid any imperial entanglements" as Sir Alec Guinness tactfully said in the original Star Wars movie

[Len Petterson]

Re: Minimum Power

Any Homebuilt Taylorcrafts out there what can try out the propextentions?
It would minimaze the " Imperial Entangelments" (I like what expresion")
Andy do you have a wood prop?Len

[DanBrown]

Re: Minimum Power

Started at 70 and worked down maintaining altitude. Results for anyone who might be interested:

70 1800
65 1700
60 1650
55 1600
50 1550
45 1550

Tomorrow I'm going to carefully check sink rates at various airspeeds. Pretty interesting to find that some things I thought I understood are not so.
Darryl

The minimum sink rate of aircraft is less than the best L/D speed (which will give the longest distance in still air). In the glider I had, the best L/D was about 60 and the minimum sink rate at about 45 mph. Aginst a headwind, the speed for the longest distance increases. So it is still true that lowering the nose on final approach increases the touchdown point distance. (When not landing downwind).

Also note that RPM is not the whole story of engine power -- we don't measure manifold pressure with a fixed pitch prop. For example, with the throttle at idle, the RPM wil vary with airspeed. It would be interesting to compare the numbers for maintanining altitude,with the idle and full power RPM at each airspeed along with the idle descent rate and the full power climb rate.

With a GPS, getting the calibrated air speed is easy. Just average the ground speeds on course into the wind and at 180 degrees from that. The course doesn't need to be precise as long as the drift angle is small.

[DanBrown]

Re: Minimum Power

Whoops -- forgot to note that need to correct the TAS for altitude and tempature (standard flight computer stuff) to get correct CAS.

[flyguy]

Re: Minimum Power

I think it would be nice to have a "technical", or aerodynamics keeper/permanent thread on here. Think they call it a "sticker" on our Camaro forum. It would be interesting with all the people here who have a real functional, practical, everyday interest in how our airplanes work, and proven knowledge of it.
I'm just playing with how the machine functions under given conditions that may or may not be applicable to other conditions or aircraft. The idea is to get a understanding of what it really will do that is above the everyday gut knowledge that we all use in daily flying. It is a lot of fun and interesting as I have said, and finally sometimes I find out that what I previously thought was so, just isn't.
I would like to know if someone has made a lightweight short wing BC12D that is optimized for cruise performance rather than aerobatics. Should perform about like the Monocoupe with the same engine. Say an O200.
Would like to try doing something to get rid of the wing-tip losses, although what I know makes me think it might not help much in cruise.
Anyway I'm going to fit in some more fun today.
Darryl
Forrest--Thanks for confirming the 23012 airfoil section, I had some data on it, but lost it, so I was going to look it up again and wasn't absolutely sure of the number. Thought there was something unusual about the number though, like 2 zeros in the middle? ie 230012?
Bill B.-- I would like to take an airframe and build up specialized for cruise speed. Short wing, everything made slick, light weight, 85 HP, wing tip treatment, do a redesign of the cooling flow with thermocouples and such. Customized glass or carbon fiber cowling. Experimental of course. Make one mag electronic. ZOOM.
DC

[1938BF50]

Re: Minimum Power

Pertaining to the original post.....go back and re-read chapter 1 of Stick & Rudder. Lot's of interesting comments that relate to your experimentation.

[VictorBravo]

Re: Minimum Power

In the glider I had, the best L/D was about 60.

Egads! A Nimbus 3 driver!

I competed in a couple of AS-W20's and a Ventus once upon a time

[flyguy]

Re: Minimum Power

LOL on the stick and rudder. Haven't cracked it in a long time. First chapter you say. Strike me with lightning, but there were a few things in the book that I would disagree with the way he put them. (thought it was misleading, if I remember correctly) Was a long time ago.
Oh yeah, almost forgot. Never had noticed before but my Tcart has to be banked much steeper than I would think is average before it goes spiral (roll) unstable. Amazing airplane. As I remember Cessna 150's would take off on you pretty quick maybe 25 degrees.
DC

[Forrest Barber]

Re: Minimum Power

Another thing we did with Leisa-Marie ( the 180 HP clip wing) was to do inverted stall speeds, Vx & Vy too! Yes she had a much higher stall speed like 57 mph as opposed to 48 upright. there were no book figures on the
2 30 12 airfoil .in the inverted condition. the 2 30 15 is also on some ships . The 12 & 15 refer to the % aft where the airfoil goes symetric...

[flyguy]

Re: Minimum Power

Been trying to find a plot of coefficient of lift for the 23012 but could only find 0012 in the old '39 data from NACA. Anyone have a Link for wind tunnel data for older airfoils that includes the 23012?
DC