Supercub info

[MLS-12D]

I have almost completed the longest, most wasteful type checkout in my flying career (thanks, Brampton Flying Club :mad: ). Now I have to write a short written exam.

The POH (for a/c Nos. 18-7409140 and higher) performance figures state that Vx is 45 MPH. The "takeoff speed" is identified as 30-35 MPH. Are those figures correct? They seem awfully low, especially as the stall speed is given as 43 MPH with flaps, and 47 without.

While I've got your attention, can anyone tell me about the battery, which the POH variously describes as "12 volt, 23-ampere" (page 12) and "12 volt, 33-ampere" (page 33).

Sorry to take up your time with these enquiries ... I know that the proper person to ask is my "instructor", but he doesn't have any answers. :rolleyes:

Thanks,

MLS-12D

[FlyingForFun]

MLS,

I can check the POH for my club's Super Cub next weekend if you don't get satisfactory answers before then.

The theory books (if I remember correctly - I dumped most of this stuff out of my brain as soon as I walked out of the exam hall) will tell you that, for propellor-driven aircraft, Vx is the lowest controllable speed.

This is because Vx is defined as the speed at which maximum excess power is available. (Vy, as a point of interest, is the speed at which maximum excess thrust is available.) Propellors can supply a (very) roughly-constant amount of power at all low speeds, dropping off very sharply at high speed, so all we are looking for is a speed where power required is a minimum. We know that power = thrust * TAS, so a low TAS requires a low power. Remember that thrust = drag, and low drag is, generally, obtained at low airspeeds too - the increase in drag as alpha increases turns out to not be significant above the stalling speed when compared to the decrease in power as a result of the low TAS. Hence, low speed = low power required = maximum excess power = Vx.

In practice, many operators specify a higher Vx because they don't want you operating so close to the stall, at speed-unstable angles of attack, when very close to the ground. But I'm not all that surprised that Vx is given as just a couple of knots above stall speed. Not so sure about "takeoff speed", though - personally, I've never looked at the ASI when taking off in any tail-dragger.

As for the batteries, I'd guess that each aircraft has a completely different electrical system, and that there are lots of options. I know that the Super Cubs I flew in America, with transponders and GPS units, certainly wouldn't have been able to be powered by the tiny dynamo on my club's Super Cub. Maybe the best way of finding out is to look at the label on the battery???

FFF
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[MLS-12D]

Actually the POH doesn't say anything about Vx per se; that's my shorthand for what the POH calls "best rate of climb speed".

Like you, I don't watch the ASI during takeoff ... but I would be surprised to learn that one could safely take off with an IAS of less than the aircraft's stall speed, unless there was a "skyhook" involved!:confused:

[Kingy]

Like you, I don't watch the ASI during takeoff ... but I would be surprised to learn that one could safely take off with an IAS of less than the aircraft's stall speed, unless there was a "skyhook" involved

Ground effect mate thats how... Same as landing - if you were to be looking at the ASI at the point of a perfect 3 pointer it would read less than 35mph ... ground effect... STOL pilots friend!

Kingy

[QDMQDMQDM]

Flying a super cub by the numbers in that kind of way seems like a novel idea! They certainly get off the ground bloody slowly with a bit of flap and lots of stuff seems to happen around or below the bottom of my ASI which is at 40 kts. As Kingy says, ground effect is important and one rough field take-off technique is: on the brakes, full power, stick fully back, brakes release, count three, stick fully forward, count three, full flap, pop into the air, bleed off second stage of flap as airspeed increases. Actually, best angle of climb is full flap at about 45mph and 45 degrees, God help us. I've yet to do it!

The approved POH for super cubs is notoriously non-existent and has very little info in it. The battery, according to page 23 of the Univair owner's manual (which isn't the POH), is 12V/33A, but frankly who cares?

The place for super cub info on the net is www.supercub.org.

QDM