#### Re: Ground AoA/ tailwheel angle

David J. Gall

Since we’re only interested in “ground angle of attack” the analysis centers on the canard with “flap extended” (full aft stick). Set the “ground angle of attack” via the tailspring installed angle/length so that in the three-point attitude the canard can get close to its full-aft-stick CLmax to permit liftoff at minimum speed (and to be able to land at minimum touchdown speed).

Since the tailwheel prevents the airplane from being rotated nose-up while rolling in the three-point attitude, setting the “ground angle of attack” to a too-low pitch attitude that prevents achieving nearly CLmax will mean that the airplane will need to continue accelerating on the ground until the lower angle of attack/CL and the higher speed satisfy the lift equation to overcome gravity. Yes, it is possible that a lower ground angle of attack *might* yield less drag during acceleration so that the higher liftoff speed would be attained in a shorter distance, but who’s got the analysis horsepower for that effort? And anyway, if one thinks that to be the case, the stick also serves as a drag control on the ground – don’t pull full aft stick until close to liftoff speed.

Set the “ground angle of attack” the old-school way like the Piper Cub guy did, so that the “front wing” (canard) can make close to its CLmax while still in the three-point attitude and, thereby, liftoff at minimum flying speed.

You’ll note that I couched references to the CLmax term with a “close to” at every instance; this is so that liftoff actually occurs at something a little bit faster than stall speed. It’s no fun lifting off and climbing out of ground effect only to stall right away. About a one degree lower “ground angle of attack” margin will give a 0.1 CL margin above stall. The liftoff still happens well below (L/D)max so the initial acceleration in ground effect is on the “back side of the power curve” -- some airplanes are so draggy that the margin needs to be more than just one degree, but I think the Q2/Q200 is a fairly low-drag design, so a one degree margin below the CLmax “ground angle of attack” should be sufficient to prevent unexpected settling back onto the runway after liftoff.

Worth every penny you paid for it, my advice is only an opinion and I hereby declaim any and all liability for anything bad that comes of it. (Kudos and accolades for good outcomes may be expressed via copious amounts of cash, as always.)

From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Michael
Sent: Friday, January 7, 2022 6:28 PM
To: main@Q-List.groups.io
Subject: Re: [Q-List] Ground AoA/ tailwheel angle

Finally found the Wainfan references I was looking for; wanted to make sure I wasn't crazy before weighing in. The following is quick, back of the envelope engiNerding meant for illustrative purposes more than anything, so play along for funsies...

Aerodynamically, the best things to reduce takeoff roll are to reduce wing loading and get as close to maximum lift coefficient (Clmax) as possible. Our wings are pretty well fixed (pun intended) so that leaves us getting close to Clmax via mounting angle (angle of attack). Looking *just* at the main wing airfoil, at a close but not perfect takeoff condition since I'm lazy (Reynolds 1M vs ~800k), we see Clmax occurs ~15 degress AoA. We are a tandem wing airplane, so we can't get close to that.

So the next best thing we can do is get as much angle as possible before drag (Cd) goes straight through the roof. That appears to happen in the 10ish degree range:

A good bet here - where we can't achieve max lift - would be to maximize "lifting efficiency", or Cl/Cd for us engineer types. That appears to occur within a neat little plateau from around 10±2.5 degrees...SMACK where Jay has observed it on flying airplanes:

So basically...don't stress it too much. There's a fair bit of wiggle room, and Flying Airplanes don't lie 😉
--
-MD
#2827 (still thinking about planning on visualizing how to finish building)

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