SR-71A Blackbird Aviation Quote

Fear No Evil. This post is a dedicated to SR-71A Blackbird.

To honor it, I want you to compare the above picture and below picture. I want you to understand that we are fighting our own demons.

Cheers.

CK

Aspect Ratio in Aviaiton

Aspect ratio is the ratio of wing span to its aerodynamic breadth or chord. For most wings the length of the chord is not a constant but varies along the wing, so the aspect ratio AR is defined as the square of the wingspan divided by the area of the wing planform. (Ref: Wikipedia)

I can go on with formulas and other technical details. But let's not get caught up in various representation. What exactly is aspect ratio? What does it do?

Aspect ratio tells you how the wings perform aerodynamically. 

By looking at the value of aspect ratio, you can tell if the wings produce more lift or not. 

Simply put, if the value of aspect ratio is high then the wings produce more lift. 

If the value of aspect ratio is low, then those wings produce less lift. 

Why is it like that?

Check out the picture below:

Aspect Ratio of A = 4

Aspect Ratio of B = 25

And both have same wing area. 

Still the shape is different. Why?

Again, look at the picture below:

It's the Solar Impulse 2 and an A330. Both have different aspect ratios. Solar impulse has high aspect ratio while A330 has lower aspect ratio. Why?

It is to get higher lift to drag ratio. 

Simply put, the lift should be higher than drag. Of course all the aircraft have a positive lift to drag ratio otherwise they won't fly. But we can increase the value of lift while keeping drag value at bay, so that at higher lift to drag ratio, the aircraft produces tremendous lift force.

But if we increase lift the drag also increases. After all their parameters are same. 

How do we increase lift but not drag?  

That is where aspect ratio kicks in. 

With keeping the surface area same, they can reduce drag by extending the wing. As the wing is extended, the amount of air spillage(due to pressure difference) is less from bottom of the wing to top of the wing. 

These so called wingtip vortices try to pull back the aircraft because its a drag force (called induced drag) acting opposite to thrust. 

When you have a reduced wing tip area, then the amount of spill is less and thus drag is reduced.

Still why do we need to extend the wing? Can't we just make the wing tips short? 

Well, surface area directly effects lift force. It's one of the parameters. So when you cut short a wing's tip, the surface area decreases. That does not help gaining lift. We need a way to keep the surface area constant and then make the wing tips short. And the solution is the design of "Aspect Ratios". 

If high aspect ratios provide high lift to drag ratio, then why in the world we need low aspect ratio wings?

Because having high lift to drag ratio is sometimes not the priority. 

Speed is. Maneuverability is. Fuel storage space is. 

What do you think is the aspect ratio of this F-102 Starfighter:

It's 2.35. 

It's not efficient. It has a poor lift to drag ratio than a Cessna 172. 

Then why are they designed like this? 

They are designed for the speed. 

You see, velocity is one another parameter of lift. (Read more about lift)

So, to compensate decreased surface area; velocity is increased. 

F-102 was the first aircraft to break Mach 2. 

So is the Bell X-1 which broke the sound barrier in October, 1947  for the first time in history. Did it have a high aspect ratio? 

Nope. Aspect ratio of Bell X-1 was " 4 ". 

SR-71 Blackbird flew at Mach 3 with an aspect ratio of 1.7.

As you know, SR-71 is a legend. It was from the legendary Skunk Works where everything fly fast and high.

There is another aircraft from Skunk Works that fly high (above 70,000ft) with an aspect ratio of 14. 

That's right - 14. 

That is 9 times more than the aspect ratio of SR-71, and it still flies above 70,000ft.

And that aircraft is the private jet of NSA & CIA. :)

The one and only Lockheed U-2. 

U-2 is a subsonic surveillance aircraft. And when operating at high altitudes, the density of air is less and they don't have the luxury of high speed. The requirement to produce tremendous lift is the highest priority. So in order to compensate, Lockheed engineers gave an high aspect ratio wing to U-2; thereby increasing the surface area and reducing drag. 

Most of the surveillance aircraft that operates at high altitude, where air is scarce, uses high aspect ratio wing. 

Commercial airliners need speed as well as lifting capability. So they settle to have an aspect ratio somewhere between 6 and 10.

Boeing 747 has an aspect ratio of 6.96.

The new Boeing 777-9X has an aspect ratio of 9. I have an article about the new Boeing 777 9X. 

Finally, here is a chart plotted between Mach No. and Aspect Ratio. 

You can see from here that aircraft that has a high aspect ratio has speeds below mach 1.

Aspect ratios are great. By looking at just it's value, we can tell lot about the planes that have them. 

Thank You for reading guys.

CK

The SR-71 Blackbird Chronicles

Built and designed in the 1960s, SR-71 Blackbird is still the fastest, most bad-ass air-breathing airplane in the history of aviation. Below given is one of my favorite stories of SR-71, excerpted from internet directly:

As a former SR-71 pilot, and a professional keynote speaker, the question I’m most often asked is “How fast would that SR-71 fly?”  I can be assured of hearing that question several times at any event I attend. It’s an interesting question, given the aircraft’s proclivity for speed, but  the really isn’t one number to give, as the jet would always give you a little more speed if you wanted it too.  It was common to see 35 miles a minute.

Because we flew a programmed Mach number on most missions, and never wanted to harm the plane in any way, we never let it run out to any limits of temperature or speed.  Thus, each SR-71 pilot had his own individual high speed that he saw at some point on some mission.  I saw mine over Libya when Khadafy fired two missiles my way, and max power was in order.  Let’s just say that the plane truly loved speed, and effortlessly took us to Mach numbers we hadn’t previously seen.

So it was with great surprise, when, at the end of one of my presentations, someone asked: What was the slowest you ever flew the Blackbird.?  This was a first.  After giving it some thought, I was reminded of a story I had never shared before, and relayed the following:

I was flying the SR-71 out of RAF Mildenhall, England, with my back-seater, Walt Watson; we were returning from a mission over Europe and the Iron Curtain, when we received a radio transmission from home base.  As we scooted across Denmark in three minutes, we learned that a small RAF base in the English countryside had requested an SR-71 fly-past.  The air cadet commander there was a former blackbird pilot, and thought it would be a motivating moment for the young lads to see the mighty SR-71 perform a low approach.  No problem; we were happy to do it.  After a quick aerial refueling over the North Sea, we proceeded to find the small airfield.

Walter had a myriad of sophisticated navigation equipment in the back seat, and began to vector me toward the field.  Descending to subsonic speeds, we found ourselves over a densely wooded area in a slight haze.

Like most former WWII British airfields, the one we were looking for had a small tower and little surrounding infrastructure.  Walter told me we were close, and that I should be able to see the field, but I saw nothing.  Nothing but trees as far as I could see in the haze.  We got a little lower, and I pulled the throttles back from the 325 knots we were at.  With the gear up, anything under 275 was just uncomfortable.  Walt said we were practically over the field, yet there was nothing in my windscreen.  I banked the jet and started a gentle circling maneuver in hopes of picking up anything that looked like a field.  Meanwhile, below, the cadet commander had taken the cadets up on the catwalk of the tower, in order to get a  prime view of the fly-past.  It was a quiet, still day, with no wind and partial gray overcast.  Walter continued to give me indications that the field should be below us, but, in the overcast and haze, I couldn’t see it.

The longer we continued to peer out the window and circle, the slower we got.  With our power back, the awaiting cadets heard nothing.  I must have had good instructors in my flying career, as something told me I better cross-check the gauges.

As I noticed the airspeed indicator slide below 160 knots, my heart stopped, and my adrenalin-filled left hand pushed two throttles full forward.  At this point, we weren’t really flying, but were falling in a slight bank.  Just at the moment, both afterburners lit with a thunderous roar of flame (and what a joyous feeling that was), and the aircraft fell into full view of the shocked observers on the tower.  Shattering the still quiet of that morning, they now had 107 feet of fire-breathing titanium in their face, as the plane leveled and accelerated, in full burner, on the tower side of the infield, closer than expected, maintaining what could only be described as some sort of ultimate knife-edge pass.  Quickly reaching the field boundary, we proceeded back to Mildenhall without incident.  We didn’t say a word for those next 14 minutes.  After landing, our commander greeted us, and we were both certain he was reaching for our wings.  Instead, he heartily shook our hands and said the commander had told him it was the greatest SR-71 fly-past he had ever seen, especially how we had surprised them with such a precise maneuver that could only be described as breathtaking.  He said that some of the cadets’ hats were blown off, and the sight of the plan-form of the plane in full afterburner, dropping right in front of them, was unbelievable.  Walt and I both understood the concept of breathtaking very well, that morning, and sheepishly replied that they were just excited to see our low approach.

As we retired to the equipment room to change from space suits to flight suits, we just sat there: We hadn’t spoken a word since the pass.  Finally, Walter looked at me and said, “One hundred fifty-six knots.  What did you see?”  Trying to find my voice, I stammered, “One hundred fifty-two.”  We sat in silence for a moment.  Then Walt said, “Don’t ever do that to me again.!”  And I never did.

A year later, Walter and I were having lunch in the Mildenhall Officers’ club, and overheard an officer talking to some cadets about an SR-71 fly-past that he had seen one day.  Of course, by now the story included kids falling off the tower, and screaming as the heat of the jet singed their eyebrows.  Noticing our Habu patches, as we stood there with lunch trays in our hands, he asked us to verify to the cadets that such a thing had occurred.  Walt just shook his head and said, “It was probably just a routine low approach; they’re pretty impressive in that plane”.  Impressive indeed.

Little did I realize, after relaying this experience to my audience that day, that it would become one of the most popular and most requested stories.  It’s ironic that people are interested in how slow the world’s fastest jet can fly.  Regardless of your speed, however, it’s always a  good idea to keep that cross-check up — and keep your Mach up, too. 

Respects! 

CK