Mach Tuck Concern Unjustified ? Help me Understand & I'll pass it on !

[FlyHiGuy]

As a listener-only to a recent flightline debate, I would like to ask some of you more knowlegeable aerodynamicists or practical-world aviators about differences in suseptability to mach tuck between design parameters. Here's a summary/asimmilation of the debate in my own very "mach-knowledge limited" King Air pilot recollection & terminology :

Bizjet pilot: "Hey Avanti Pilot; that's an interesting airplane. How fast does it go ?"

Avanti Pilot: "We're can cruise at just over 400 kts."

Bizjet pilot: "That's pretty good for a turboprop. What mach number do you cruise at at say FL 370 ?"

Avanti Pilot: "We can fly at 0.70 there".

Bizjet Pilot: "0.70 at that altitude ? That sounds a bit dangerously close to getting into a mach tuck condition !" Are you sure ?

Avanti Pilot: "No worries mate. This is a pusher aircraft with a forward canard so the aerodynamics are all different. We don't need to worry about pushing that limit with this aircraft as you guys do in conventional swept-wing jets."

Bizjet Pilot: "Are you sure ? You better check that out with an Avanti authority or factory test/development pilot - it doesn't sound right to me".

****End of Dialogue****

Question for my fellow PPrune professionals: Can someone advise what they think of this ? Maybe a true aerodynamicist, test pilot or Avanti Aero expert. Unfortunately, I can't get this question into Burt Rutan as I know he'd explain it perfectly . . .

I look forward to your response and will try to get the true story back to these Avanti pilots to prevent some sort of scare or incident if they are wrong...

Thanks, FLYHIGUY

[The Nr Fairy]

Can't answer the concern directly, but I'd be interested in having a gander through the RFM.

[Clandestino]

Mmo 0.7

Unless you prefer to second-guess certification test pilots findings, without reasons or arguments, concern is unjustified.

Strength, direction and speed at which mach effects occur is generally related to wing shape, both profile and planform. Saying that Ma 0.70 must be close to mach tuck is rather uninformed, as is responding that the plane doesn't mach tuck because it's canard .

[SNS3Guppy]

There are no mach tuck issues with the Avanti up through Mmo. I flew the airplane for a thousand hours and trained others in the airplane. It has no such bad habits.

The airplane is advertised as a 400 knot airplane at altitude, but it's realistically a 370 KTAS airplane.

[411A]

If one really wants to experience mach tuck in a major way, get yourself belted into an old short-body B707, and with the mach trimmer off, approach M.85....and watch the cows get bigger, in a hurry.
A sight to behold, I kid you not.

[flyboyike]

I never thought any concerns about Mack Trucks were justified.

http://www.koralfarms.com/ConstructionEquipmentImages/Mack%20Tractor%20Products/2005%20MACK%20CH613.jpg

[Pugilistic Animus]

whether naturally or artificially obtained, FAR 25 requires a positive stick force gradient up to MD [demonstrated dive speed] which must be at least 0.07MN above MMO.

if not then MMO must be reduced in order to obtain that result, however stick force gradient may fall to zero at MD. only during training for upset recovery will such a condition be encountered as 411A graphically illustrates in his post, regarding the 707.

so if you remain at or below MMO then you shouldn't encounter any problems provided that the stability augmentation is operative


PA

[ChristiaanJ]

I never thought any concerns about Mack Trucks were justified.Just try..... hit one of those head-on with another one at 65 mph.... True, you may never have any concerns ever again.

CJ

[Keith.Williams.]

Yeah but that one's a BIG MAC....It never fries.

[Daniel_11000]

The P180 Avanti has been tested up to MMO of 0.7, this means that it has been pushed to 0.77. At this Mach No (0.77) it has also been tested for flutter, meaning bonkers has been fired (bonkers are pyro devices which ‘explode’, giving a knock on the surfaces. All modes were tested : wing symmetrical, anti-symmetrical, same for canards, same for fin, same for elevators) ; never the mach-tuck phenomenon has been encountered. Buffet onset envelope has been investigated up to 1.4 g turns, with no negative results. Static stability has never relaxed up to some mach beyond MMO
Max TAS of 402 KTAS (on Avanti II) can be reached easily , but at 26000 ft, surely not at FL 370.
I was there in 1989

[SNS3Guppy]

Max TAS can be reached at FL410 and certainly at 370...if one pushes the airplane. However, there's no need. It cruises quite comfortably at 370 KTAS. While Piaggio claims faster for advertising purposes, realistically it's 20-30 knots slower in cruise. And it's quite comfortable at FL410...I've spent plenty of time there myself in the P.180.

However, It's really an issue of whether Mmo can be reached at altitude, rather than max TAS, isn't it?

Regardless, the airplane has no tuck or flutter tendencies. Flutter isn't an issue as altitude increases, and the airplane operates at a low mach number.

Piaggio insists the forward wing is just that: a forward wing, and insists it's not a canard. I originally underwent factory training by the factory chief pilot...and that point was made repeatedly.

[Daniel_11000]

I confirm that the P180 forward wing is just that, a forward wing , and as such it also been patented. Its main purpoposes are twofold : increase enormously the CG envelope, and ‘protect’ the stall , as it is stalling at a lower alpha than the wing, thus kindly giving a nice pitch down. I agree with you that normally flutter (structural resonance) is more connected with high EAS, but nevertheless, for certification purposes , it should verified free also at limit Mach number.
However I disagree on the Max TAS issue : this is normally referred to level , stabilized flight, and I am pretty sure that 400+ KTAS cannot be reached in cruise condition at
FL 410 – unless you push the aircraft with more powerful engines – the FL at which MAX TAS occurs in sustained level flight is between FL260 and FL270. At this speed/altitude, you also cross, in the flight envelope, the MMo number, which is 0.67 for the Avanti I (I do not remember the value for Avanti II, I suppose a little bit higher).
Kind regards
Daniel

[SNS3Guppy]

.67 M1 could be achieved at FL 410 or 370 for that matter in the P.180, albeit not at MGTOW. This required a higher power setting than what I normally used, and I found that for the best combination of speed and range, a power setting yielding about 370 ktas worked best.

The CG range in the airplane isn't that great. Single pilot, empty with full fuel can put it aft of the envelope, and nearly uncomfortably so, and we found that in normal operations we were nearly at the edge of the envelope much of the time. Often creative use of the loading schedule was necessary to balance the aircraft.

Due to the laminar-type airfoils used, the airplane also displayed a very unusual characteristic which is more reminiscent of early experimental canard-style aircraft; any disturbance to the airfoil causes significant loss of lift. This was most apparent when hand flying and passing in and out of fair-weather cumulus clouds. Upon entering the cloud the control column required movement aft of two inches or more to sustain level flight, else a pitching-down moment would occur. When exiting the cloud, the opposite movement was required. This see-saw action was very apparent when hand-flying in and out of clouds, but transparent when on the autopilot.

[LeadSled]

Folks,
I can vouch for the Mach tuck characteristics of a B707, it could be hard enough that the nose down pitch exceeded the authority of the elevator, and with that elevator load the stab trim motors would stall out. The only successful recovery was to split the spoilers and pull the speedbrakes. The USAF lost at least on KC-135 like this, several civil 707's were lost.

It didn't need to be a -120/138 or -720, the -300 and -320 were no better.

Having said all that, the aerodynamics that cause tuck are well known, unless there is something weird about the Avanti wing (and I don't think there is) M0.70 (or M0.77) would not be fast enough to have an area of pressure shift in transonic flow to produce the centre of lift change that is tuck.

Tootle pip!!

[Dan Winterland]

Early jets did suffer from Mack tuck. Later jets do seem to have the problem designed out. I was always suprised the 707 didn't have a Mach Trimmer considering it's susceptability to the problem. The Handley Page Victor had a Mach trimmer and it predated the 707 by about four years. The early 737s and (IIRC) the 727s had them so a retrofit shouldn't have been too difficult.

[411A]

I was always suprised the 707 didn't have a Mach Trimmer

You must have flown a different type of B707 than I did, as the ones that I flew most certainly did have a mach trimmer installed.
Now, there were many models, and perhaps some didn't, so I would not know about those.

[LeadSled]

Folks,

I've flown every variant of the 707, except the RR Conway powered -300, and the QF 707-138 (not a -120, as many seem to think) and every one featured Mach trim.

The biggest variable I found between them all was the yaw stability, ie; the propensity to dutch roll, followed by the pitch/speed stability on approach, the -320s could be very touchy.

Some strange modifications insisted on by the UK ARB didn't do anything to help the G- registered, although the late D.P Davies would disagree --- but he didn't have to fly them in the real world.

Tootle pip!!

[411A]

I've flown every variant of the 707, except the RR Conway powered -300, and the QF 707-138 (not a -120, as many seem to think) and every one featured Mach trim.


And I've flown those...so I think we've covered the waterfront with discusion re: mach trimmer on the 707...they all have one.
The -138B by the way was a tail-wagging son of a gun on final in gusty winds, with the yaw damper OFF, as it had to be.
The -320 straight-pipe aircraft was the same.
The young button-pushing First Officers of today would have eyes as big as dinner plates if they had one of these old airplanes strapped on...would be interesting to watch, no doubt about it.:}