Is there a "high altitude" aerofoil ?

[IO540]

The context is GA.

Compare a typical IFR tourer, say a TB20, with its relatively thick wing, with say the Malibu Mirage with its much thinner (and longer) wing.

The former might have a respectable ceiling, e.g. FL250 if turbocharged, while the latter goes higher, say FL270, but not vastly higher. OTOH the former will be right at its ceiling at FL250, with a high AOA, while the latter will cruise at FL250.

What is the theory behind this wing aerofoil difference, and what are the tradeoffs?

[flyboyike]

I don't think the TB20 is a "typical" anything, what with being a fairly rare and esoteric aircraft. I don't know if the Malibu is all that indicative either, especially since I would think both of these are engine- and not wing-limited as far as ceiling goes.

However, for a little reading on aspect ratios and related issues, go here:


quest.arc.nasa.gov/aero/wright/teachers/pdf/math/Aspect_Ratio_of_Wings.pdf

[411A]

One particular type that I recall that had a completely new wing, specifically designed for high altitude cruising was the Lockheed 1649A Constellation.
Large aspect ratio and quite a thin airfoil section.
Of course, the other reason it needed a new wing was to accomodate the nearly 10,000 US gallons of avgas...not to mention the the rather large central oil tank to keep all the Curtis Wright turbocompound engines turning successfully...

Usually...:}

[Brian Abraham]

Nothing replaces aspect ratio when it comes to altitude performance, The Mirage was designed at the out set to be a high altitude machine (in GA piston terms). The TB-20 by contrast is a development of the 160 horsepower fixed gear TB-9 and as such not designed and optimised for high altitude. The wing airfoil as such has little effect, the 2300 series being used on everything from the RV series of homebuilds and Bonanza to the Citation jets.
The link provides a discussion about the elements influencing the wing design of an aircraft Peter Garrison both designed and built. He is a gentleman who knows about what he talks.
Wing Geometry (http://www.melmoth2.com/texts/Wing%20geometry.htm)

[barit1]

I've found a remarkable table of airfoils used on various aircraft (http://www.public.iastate.edu/~akmitra/aero361/design_web/airfoil_usage.htm).

By the way...re the 1649A...
the rather large central oil tank to keep all the Curtis Wright turbocompound engines turning successfully...

The objective of this was, of course, to insure that all four ran out of oil at the same time? :D

[John Farley]

barit1

Very interesting table. As it happens the only aircraft quoted in that list that I was reasonably familiar with so far as the aerofoil is concerned is the PA-44 Seminole.

The table quotes the Seminole as having the same aerofoil at the root and tip.

The one I used to fly had a section at the root that was clearly very convex on the underside, much more flat bottomed over the middle of the wing and was concave at the tip, including a slight built in LE droop for the last few feet. I remember all this 'cos we tufted the wing and used it to teach some aspects of stalling to aero eng students. Indeed I used to talk up the wing as an example of how you could combine good natural buffet warning (from the root section which exhibited reverse flow on the upper surface when approaching the stall) good general CL max from the main wing and excellent roll control during and after the stall thanks to the LE droop in front of the ailerons.

[411A]

The objective of this was, of course, to insure that all four ran out of oil at the same time?

Oh oh, one would certainly hope not.:}
The central oil tank (78 USGallons, as I recall) was used to refill (in flight) the individual engine's oil tanks (using two electric oil pumps).
On my longest flight in this type (17 hours) the oil required to refill all the oil tanks was an astounding...105 gallons:ooh::ooh:

[IO540]

The URL

quest.arc.nasa.gov/aero/wright/teachers/pdf/math/Aspect_Ratio_of_Wings.pdf

seems to be dead, with various combinations of HTTP and WWW in front of it.

[flynerd]

Try This (http://quest.arc.nasa.gov/aero/wright/teachers/pdf/math/Aspect_Ratio_of_Wings.pdf)

Works here.

[IO540]

Thank you for that URL but it seems to be just an exercise sheet, with no answers to my Q.

[bookworm]

The higher aspect ratio is a result of wanting to cruise closer to the minimum of the drag curve.

In a typical light aircraft low-level cruise, induced drag makes up only a small proportion of the total drag. However at high altitudes, you operate close to the minimum drag speed -- and in particular, at the ceiling of the aircraft you operate at the minimum drag speed, where the induced drag is 50% of the total drag. Increasing the aspect ratio will generally lower the induced drag at the expense of parasite drag.

Thickness is a bit more complex. There's usually an optimum thickness for minimising drag, but that depends on the lift coefficient. From looking through Abbott and von Doenhoff, it does look as if thinner aerofoils are less draggy at higher lift coefficient (i.e. close to the minimum drag speed), while thicker ones tend to work better at lower lift coefficient.

Have you worked out the lift coefficient of your TB20 in high altitude cruise? You only need the mass, wing area and IAS.

[barit1]

John Farley -

I've noticed another anomaly or two (and I'm sure there are more) in that table. The NAA T-28, as I recall, had a concave lower surface at the tip, such as you mentioned on the PA-44. (Many aftermarket STOL kits do likewise, and Pilot DAR tells me the Basler kit for the DC-3 does the same thing.)

Yet the table shows the T-28 w/both root and tip with the NACA 64A215 airfoil, and the coordinates I found for 64A215 show no such concave lower surface.

So I suspect some refinement of the table is in order. For another item, the Howard cabin aircraft DGA-8 thru DGA-15 airfoil is a reflexed airfoil properly identified as 2R212 with the middle "2" a subscript. The plain text table has no means of showing the subscript.

[Brian Abraham]

My USN aerodynamic course notes says about the T-28 airfoil "The basic section is the NACA 642-215. The North American number is (NAA) 64A-215 (the A being the North American design coefficient)." It would thus seem the section is a North American proprietary design. That is, its not a NACA section, but as stated, a NAA (North American) one.

[IO540]

Have you worked out the lift coefficient of your TB20 in high altitude cruise? You only need the mass, wing area and IAS.

1400kg
11.9 m2
95kt

from here (http://www.pilotfriend.com/aircraft%20performance/aerospatiale/7.htm).

The Mirage data from here (http://www.airliners.net/aircraft-data/stats.main?id=315) is

1950kg
16.3m2
and I guess Vbg is going to be around 95kt too.

[bookworm]

Cl = 1400 kg * 9.8 m/s2 / (0.5 * 1.225 kg/m^3 * (95 / 1.94 m/s) * 11.9 m2) = 0.79

That's quite a high lift coefficient, though on the only aerofoil for which I have a polar that, in effect, includes thickness as a parameter (NACA 64 series), it's still in the regime where thicker is better. So I think we'd need more detailed data specific to the aerofoil.

[John Farley]

A point that I don't think has been mentioned yet is that too high a thickness chord ratio can be a problem at high altitude if the mach number of the aircraft is likely to be more than say 0.6. I believe the Westland Welkin tripped over this in the days when the effects of mach were less appreciated.

[bookworm]

if the mach number of the aircraft is likely to be more than say 0.6

Good point! IO-540 and I can only dream of such speeds... :)

[Brian Abraham]

John, I see the Welkin had a NACA 23021 at the root and 23015 at the tip. History does say it had a very small usable speed range at altitude. 21% is certainly thick, although the 749 and 1049 Constellations used a 23018, though they were not expected to operate any where near the Welkins altitude.

[John Farley]

Brian

I agree with your comments although I would add the word 'very' between 'certainly' and 'thick' !

[IO540]

A bit late back to this debate.

Part of the reason for me starting this thread was an attempt at understanding some common myths which go around the GA scene.

For example, some piston pilots say they find the best MPG is around 7000ft. Yet, I cannot reproduce this well. I do indeed find that the lowest altitude at which I can fly at what should be an efficient engine operating point (wide open throttle, 2200rpm, 25LOP) is about 10,000ft, I lose a suprisingly small amount of range if I have to do a slow climb to say 18,000ft (to stay above weather) followed eventually by a descent.

I would have thought that the engine efficiency (NON turbo) would fall at the lower power outputs which are inevitable at high altitudes, resulting in a loss of range as a result of the time spent high up, but this doesn't seem to have much of an impact in practice. I have a GPS-linked fuel flowmeter which gives me a forecast reserve at destination; accurate to better than 1%.

If I take the Lyco HP v. fuel flow graph and extrapolate the (say) 2200rpm line all the way back to the 0HP intercept, there is a significant fuel flow there, a few USG/hr, which I guess represents the friction and pumping losses at 2200rpm. These, I would have thought, were constant at that rpm, but maybe the pumping losses decrease with altitude because the air is thinner and flows in more easily? Also, at altitude there is less exhaust back pressure so the engine will be more efficient.

There are so many effects all working at the same time.

One thing appears certain: roughly speaking, at a constant thrust, you get a constant TAS. So, as one climbs, the IAS obviously has to fall (which it does) even if the LOP fuel flow is kept constant.