Why the AI is not an AoA indicator
 

I'm at risk of being called a pedant here, but the AI / AoA thing is a common and dangerous fallacy and it's important to nail it on the head very firmly. Here's an example of why:

Take four flight situations.

First, a descent on approach to land, with a slow speed and relatively high angle of attack. In this situation, the AI will show a small degree of nose down attitude.

Second, a near vertical, full throttle, high speed dive at a very low angle of attack. The AI will show a very high degree of nose down atttitude.

Third, a cruise climb. Angle of attack slightly higher than that for level cruise, but not much, so still a relatively low angle of attack. The AI will show a small nose up attitude.

Fourth, a best angle climb, very high angle of attack. The AI will show a high degree of nose up attitude.

Thus, a high degree of nose down on the AI in the situations above correlates to a LOW angle of attack, whereas a high degree of nose up on the AI correlates to a HIGH angle of attack. In other words, there is no consistent correlation between nose attitude and angle of attack. In the first situation above, you are at risk of stalling with only a small amount of nose down attitude on the AI. In the fourth you are at risk of stalling with a high amount of nose up attitude.

Most important of all, is that this shows the AI alone will tell you nothing about your proximity to the stall. To believe that it has the power to do so will be highly misleading and will likely lead to much more than mere embarrassment.

QDM

Stall, for a given flap/slat setting combination (just flaps on a cub) will always be at the same AoA.

AoA is also a much better indication of loading and proximity to the stall than airspeed or a g-meter. But, unless somebody's done some considerable work to produce operating data based upon AoA, calibrated the instrument, etc. it's not a great deal of use. On an aircraft like the Jaguar, Tornado or Concorde, it's essential. I'd venture that on an aircraft like a super-cub, it's usefulness when you have adequate stall warning from things like lack of wind noise, back-stick and buffet, is at best marginal.

If you simply want to fit it, as something to look, then it's probably a minor mod. If it's on a permit, the PFA'll do it for free, if it's on a CofA it'll cost you £62 through your CAA regional office. They'll probably insist on a suitable placard being fitted saying that it's not a primary flight instrument.

If you want to make proper use of it, that requires calibration, flight testing, preparation of a POH supplement and CAA major mod approval. If you want that, the CAA minimum fee is £309 and you'll need the services of an E3 company, or design and flight-test signatory acceptable to the CAA. Offhand, if you were to ask me to do the job, I'd probably charge you around £2-3k. Trick is to find a design signatory who fancies free use of a super cub for a while in payment, that might be me as well, but even at my eminently reasonable consultancy rates, that's a lot of flying!

G

Thanks, Genghis. It's on a CofA. What's the best way to approach the CAA regional office? I was planning to send them the design drawings and FAA Form 337 documentation and tell them what I hoped to do. Is this the best tack to take with them?

It will be there for information only. I certainly don't plan the whole rigmarole.

QDM

F3G
Please don't Rely on your AI to tell you your AoA! Hold the nose of your aircraft on the horizon - fixed attitude, right? Now close the throttle. Continuing Holding the attitude constant, by your argument means the AoA must be constant...?? It clearly isn't, is it? (unless you have an anti-gravity device fitted, of course)
Put this AI = AoA indicator nonsense out of your mind, "Lest the earth rise up and smite thee....." However, as ol' Lang' said AoA is the difference between where the aircraft is pointed and where it going..... so attitude monitoring is still important

It would be intersting to see if the built in stall indicator of a J3 worked on the PA18. It is a bit of a hassel using it in the UK, but works well in Florida.

Oh you want to know what it is.

When the door is left open of the J3 the bottom half start to rise up near the stall, wonderfull device. But bloody parky in the UK.

Show us a picce when you get it fitted

CC

Note the J3 doesn't have flaps

I confess that minor mods on CofA aircraft isn't my strong point, but I'm pretty sure there's a standard application form.

I'd suggest giving the regional office a call (the numbers are all in CAP455 somewhere), ask to speak to a design surveyor, explain what you want to do, and with any luck they'll send you the paperwork you need and explain any significant points.

But, my instinct is that drawings, the FAA documentation, a few sums showing that the CG change is so small as to be insignificant, the CAA's own form, and the all-important cheque, will probably do it.

A couple of steers for you; do make sure that the attachment to the strut is either clamped on the outside or picks up on an existing bolt, don't drill anything - that's sure to cause problems. Don't run any cable near a wing leading edge, and make sure if it's running from the aircraft power supply (if you've got one!) that it's independently fused at about 50% above the maximum rated current. These are the standard sort of things that they'll look for.

Cheers,

G

Thanks a lot, Genghis. It's simply a wind vane on a stick, fixed to the lift strut with a hose clamp, no electrics. Shouldn't be too much of a problem I hope.

QDM

3QDM/NonRadio/Formationfoto

Quite right to pick me up gentlemen. What I should have said in the context of Kirstey's post was that there was an instrument (AI) that can be used to keep the angle of attack safe when used in conjunction with other instruments.

Having just looked at the Bacon Saver website, there was one line that caused me a little alarm....


My old mountain flying instructor in the States would have had an apoplexy if any of his students had got so close to a ridge on even a minor hill, due to the potential for windshear.

Just out of interest, does any one know what the effect of gusts on this instrument would be and could they cause spurious indications?

I would assume that it would display the correct AoA. If a gust hits the wing then the AoA will change...

Cheers
EA:)

Well, you can skim ridges as long as you're 100% positive you're approaching from the upwind side, but it's still a mug's game.

The stupidest thing I have ever done in flying was last year agreeing to go in the jump seat of a Cessna turbine 206 dropping parachutists in the Swiss Alps. I know the area well and the pilot ducked into a very high, steep cirque just after take-off and I thought, 'Uh huh, he's going in here for a sightseeing tour and is going to do a speedy 180 and we'll be out', but no he headed up and West for the saddle (prevailing wind is from the West). Sure, this was a turbine 206, but we were at gross and climbing towards the pass. I just prayed and, of course, we got over, but only just. We crossed the saddle with about 200 feet to spare. He'd done it several times already that day and he knew that day it was OK, but one day it won't be.

We dropped at 10,000 feet agl and beat the first free faller to the ground, but that was a breeze after the flight up.

QDM

EA

Agree in principle, but what about gusts from an angle and also fuselage blanking effects?

Probably not a major issue, but interesting to think about.

F3G,

It would have to be pretty extreme fopr these factors to have a major effect. The vane sits 20% of the chord forward of the leading edge and outside the prop blast. In the Super Cub, it is clamped to the upper pole of the left lift strut, so is very far out.

If it's that gusty, you'll be approaching faster anyway so the proximity to the stall won't be so critical and you'll be ready with a bellyful of power. You wouldn't fly the approach staring at the AoA shifting in gusts in the same way as you wouldn't fly it staring at the ASI fluctuating.

QDM

QDM

Sounds reasonable.

What was interesting to me (at a pedantic level) was taking a reading from one wing and not the other.

Knowing little about these things, I wonder how AoA on jets works ... e.g. a sensor on each wing averaged or some other approach such as a fuselage mounted device.

Best of luck with the installation.

It's on the fuselage in the jet I use. No snags with prop wash you see. It's slightly more complicated than QDMs job but still does exactly the same thing mind!

A simple A of A indicator is the stick. If you paint a line on the fus wall at the correct place, then if the stick is aft of that you have exceeded the critical A of A and the wing will stall regardles of speed, so the message is, always be aware of the stick position. Suggest you hire an aerobatic a/c with a good instructor for an hour and play test pilot and explore some of the corners of the envelope. Have fun!

A very simple one...practically useless....Low speed the stick needs to come back a long way, high speed you pull it back a short distance too fast and you stall the wing (if they don't drop off). So where do you paint the line? right at the back, or just aft of centre?

Cheers
EA:)

Yes, and it would also depend on both the trim and C of G.

It's right in spirit, though, in that the real function of the elevators is to control the angle of attack of the main aerofoil. Just how much elevator deflection is required will depend on speed, as englishal says.

The other thing is that AoA info is not just useful for detecting proximity to stall, but also for flying at best rate and best angle of climb and also best glide.

QDM

I suspect that the stick position is pretty constant, but clearly the rate at which the pitch change occurred would be greater at higher airspeed. Any aeroplane could be made 'unstallable' by limiting the aft stick travel and hence elevator movement (e.g. the Ercoupe)Not sure if the trim would make a diff as it just helps relieve the control surface force. Anyway 3QDM, hurry up and strap the thing on, we're all itching to find out whether it works or not. And then we can maybe construct our own homebuilt versions by nicking the design.....

nonradio,

Cor, luvaduck, gimme a chance. The thing only arrived last week and it needs pop riveting together which is way out of my league, so I'm taking it round toi a metalworking friend next weekend.

Also, when it comes to the minor mod approval the CAA say they will only deal with a licenced engineer, not a mere pilot scumbag such as yours truly, so it may take a bit of time.

I posted this way back in the thread, but here it is again, the homemade version, if you dare:

http://www.supercub.org/woe/woe0402.php#

QDM

Poprivet? SSSHHH....Just get on with it, remove the thread where you foolishly revealed your a/c reg and nail it on pronto.
I've seen the website - got it from Cub Clues (that's why I put the piece of string on the strut!)

keep us all posted:)

Englishal, you still have to pull the stick a long way back at a higher speed to stall the wing. Try flick rolling. As a rule of thumb, the stick posn is broadly constant.

falling out of the sky machine
 

Is it safe to go flying now ??
All this technology could get in the way of enjoying stripping !
A tin of Sparkle for the perspex works quite well, and its cheaper.
IMHO

TTH

John (Harrier) Farley did a great article on AoA in Flyer a wee while back. The impression that I got is that it takes information from all the performance instruments in the panel to give you the information to make a decision on AoA which can more easily be provided by an A0A indicator.

However, is it that critical a requirement? It's down to personal taste so QDM, we're all waiting to see what hoops the CAA made you jump through to fit it :D Bet you wish you had a big engined J3/L4 on a PFA Permit now :D :D

Croqueteer

Is you line on the wall still accurate with landing flap?

:D

More or less right because you already have a pitch down moment with landing flap at near threshhold speed. In reply to a previous sarky comment, you are unlikely to pull the wings of if at or below Va as a full and sudden up elevator will stall the wing and therefore unload it. Anyway, I am too old to have my brain tested like this.