Princess Celestia entered the Canterlot Castle breakfast room. Before Nightmare Moon she had treasured the transition between night and day, when it was not quite dark and not quite light, and now her sister had returned to her Celestia treasured it again. Soon she would have to finish raising the Sun and Luna lowering the Moon, but first they could eat and talk and look forward to when their roles were reversed at the evening meal.
“Hello, my sister,” Princess Celestia greeted the Alicorn already sitting at the table.
“Tia,” smiled Princess Luna in greeting, “did thee sleep well?”
“As always, knowing our Ponies are under your eye and protection.”
“And since thou art in that sort of mood, I shall return that and say I sleep well during the day when they are under yours.”
Both Alicorns had little need for food or drink or sleep and that little they had was a deliberate link to their former mortality. During the long centuries of Princess Luna’s exile it had been difficult for both of them to keep a sense of time in the same way as their Ponies felt it. Denying herself sleep as she was responsible for raising the Moon as well as the Sun and forgetting to eat Princess Celestia had found days and nights blurring into each other until one day she had realised that what seemed mere weeks, at most, had been decades when the voice of a Pony she thought of as being almost a colt came from the mouth of a wrinkled and venerable stallion.
This same blurring together had been felt even more strongly by Nightmare Moon as the almost unchanging landscape around her of no seasons and no weather deprived her of external clues as well. She was not even measuring out her time in breaths as there was nothing to breathe on the Moon and so her power was sustaining her even in that way. Princess Celestia had tried to ease the loneliness of her sister, corrupted as she was, but it had been almost as much the absence of clues to the passage of time that had let the exile feel as if it had passed with greater speed than it had.
“You are seeming calm and cheerful this morning Luna,” Princess Celestia commented, settling at the table, her long slender horn glowing to pour herself a bowl of cereal. “I take it the night went well?”
“Oh indeed,” replied Princess Luna, with a secret smile. “The dreams of our Ponies were mostly happy ones and where they were not t’was little trouble to divert them to happier courses or ones where a lesson could be learned or insight gained.”
Princess Celestia nodded, reminded again of how the account of her sister’s fall had become confused when somepony ‘corrected’ day for night. As Goddess of Dreams the fact Ponies slept during her night had drawn no resentment, they were simply entering her other domain, but when the Ponies did stay awake during her night and slept into Princess Celestia’s day there had been the friction. It was hard for Princess Luna to act in one aspect without acting in her other so when she tried to enter and guide those dreams the Moon also responded. Princess Celestia had tried to be patient with the problems this caused but as more Ponies worked or played at night and slept during the day and ‘denied’ Princess Luna their dreams they had both become ever more frustrated. Eventually Princess Luna had become so frustrated at trying to prevent the Moon responding and being denied part of her domain that Nightmare Moon had wanted eternal night so all dreams would belong to her, whenever Ponies slept.
“And no trouble in the waking world?” Princess Celestia asked, suspicious of how quiet her sister was being.
“All was calm, Tia.”
“Good,” Princess Celestia nodded, taking a mouthful of cereal.
“Aside from five Ponies pretending to be Changelings to disrupt Fancy Pants’ party, and four of them being hospitalised.”
“What?” spluttered Princess Celestia, a few flakes of cereal escaping.
“What ‘what’?” Princess Luna asked innocently, having hoped for more of an eruption with her timing.
“My sister, I know that twinkle in your eye, what joke are you hiding?”
“Joke? I would hardly tease thee just because I know how well Fancy Pants and a certain tall bipedal matter of concern got on,” smiled Princess Luna. “And how well it seems he and a certain Pegasus are getting on now, judging from what I saw and some of the images of their dreams.”
“Please, I do not wish details of the latter.”
“They were quite innocent dreams, well at least his were. He dreamed of the flying machines of his world and sharing the sky with her, though he knew she was faster and more agile than the ‘plane’ he imagined.” Princess Luna gave her sister a saucy wink. “She was dreaming of him sharing something else.”
“No details, thank you.”
“Even hers were not too explicit,” Princess Luna shrugged, “but t’was interesting to see what image he conjures up rather than the leader of the Wonderbolts when he thinks of a Spitfire. And the emotions attached to that craft, ‘tis near enough one symbol of his nation.”
The silence lengthened as Princess Luna went back to eating and Princess Celestia waited for her to talk. Eventually Princess Celestia went back to eating as well, hoping in part that this would tempt her sister into trying to repeat the same trick of making her spit some out. She had nigh forgotten her sister’s enjoyment of pranks and when those returned to her life she had realised how important it had been to her to be treated as a sister to tease rather than a Goddess to Worship or a Ruler to Obey.
Princess Celestia sighed and gave in. “No details of the dreams,” she said, “but please tell me about how, I assume, Joe and Rainbow Dash managed to hospitalise four of our subjects.”
“Rarity and Spike helped,” replied Princess Luna, “it had been Rarity that Fancy Pants had invited and had told was welcome to bring as many friends as she liked…”
Princess Luna synopsised what she had seen and done and been told, mentioning that once she had taken the Ponies and the attending medics to hospital she had returned to the gallery to speak to Fancy Pants some more in private. Finding that Fancy Pants thought Joe could tell interesting stories and had encouraged him to set them down on paper was not welcome news. Princess Celestia agreed though that her sister’s judgement on the situation had been a fair one and they both agreed that should anypony be imprudent enough to challenge it then the Royal Wrath would not be Princess Luna’s alone. There had been attempts since Princess Luna’s return and resumption of her throne to play one sister off against the other, but if the examples they’d made of those foolish Ponies had not made it clear this was intolerable then more examples could be always be made.
“Hmm,” Princess Celestia mused, after a pleasant few minutes of discussing amusing methods of example making. “At least Joe, I think, can be trusted enough that if he has to write things down and has the time to think then he would write only what he thought suitable.”
“You are still worried he will say too much?” asked Princess Luna.
“Would be fairer to say that I fear that now,” Princess Celestia replied. “He is an… unstable element, something about him made Discord think he would bring chaos and until we know what I would have preferred his former isolation to continue.” She gave her sister a slight smile. “I am genuinely pleased that Twilight and her friends have extended the magic of friendship to him, that he has their friendship and them his, but compassion and calculation do not align here.”
“Then if thee would prefer Joe to be more isolated,” smiled Princess Luna, picking her moment, “thoust would not be pleased to learn that a few hours hence he will be addressing a class of fillies and colts at the Ponyville school?”
“What?” Princess Celestia spluttered, this time making a more satisfying mess as she’d just sipped at her tea.
“In talking to Fancy Pants he mentioned Joe was teased for lack of knowledge of Pony art and, to resolve the question of if he had not taken time to learn or had not the wits, Joe excused himself by saying he’d needed to devote time meant for learning that to instead prepare a talk on the history of human flight.”
“What? That is… what definition of the word ‘cautious’ is he using now?” Princess Celestia asked rhetorically, before sighing and shaking her head as she closed her eyes. “No,” she admitted, “I think I am being unfair.”
“How?”
Princess Celestia opened her eyes to meet her sister’s. “I have said I think Joe worthy of enough trust to only recount what he thinks suitable…”
“When think he can, rather than be drawn into statements rash,” Princess Luna nodded.
“And I also trust Twilight Sparkle’s judgement and that of Fancy Pants,” continued Princess Celestia. “I do not think anyone they like would not be cautious about what they say to children. You say he has prepared for this?”
“Yes. And that he was invited to do this rather than volunteering.”
“Then I shall allow that, although declining would be the greater caution and what I would have preferred,” nodded Princess Celestia in judgement, “he would be cautious in what he has prepared, and doubly cautious as these are fillies and colts. I shall also trust the teacher there to intervene should Joe’s degree of caution not accord with hers.”
“Though would appear thy first reaction was correct, that thy definition of cautious, for him, would be if to say anything,” Princess Luna mused, “while perhaps his, and his thought of what thee meant, was to take care in the details of what he relates and shares.”
“Maybe so,” agreed Princess Celestia, “so one precaution I think I shall take.”
==
Joe lingered near the door to the classroom, subduing the temptation to begin checking over his notes one more time. He’d got up early enough that he could have made it to Zecora’s instead had that plan not been cancelled and had got to bed early enough thanks to the abrupt end to the event, lack of time at Pony Joe’s, and speed of the flying chariot that he would not have been too tired. This had given him the time to make the trip into Ponyville to the Spa and pick up his clothes from yesterday first thing.
Unfortunately doing that had given him some fresh doubts. Last night he had regretted not having his sheath knife but now he was very glad. Joe had realised and been scared at the time that he and the Pegasus had been fortunate that the tent pole cast had not struck anything vital and made them murderer and corpse respectively. But walking back to his hut with his knife safely on his belt he had realised how certain him becoming a murderer would have been if he’d had this weapon last night. There was little you could do with a knife other than cut and stab.
Thinking it through as he walked he decided it would have been certain he’d have killed the first one, stabbing into the back of the head and neck with that initial blow rather than striking across it with a pole. Less certain were the others. He might have dodged their leader when he reared to trample or he might have met the descending chest with his blade and, if his knife had not been stuck in that chest, he’d probably have tried to twist aside from the charge and stab the other one in the neck.
Though as well as depending on if he’d still have had his knife that last would also depend on how well he had managed to avoid the horn and, if he hadn’t, how well he’d have been able to strike back despite the shock of the pain. Joe didn’t think his blood had been running as high as against the Manticore where the pain of being mauled had been subsumed by the overwhelming desire to ‘stab what trying to eat me, stab it good’ but he hadn’t noticed the scratch until they started talking rather than fighting. On the other hand even when it got a Pegasus head laid on it there had still been little pain and it was just a scratch.
Seeing a newspaper lying to one side Joe flicked through it for a distraction and tried to see the funny side and managed a slight smile and chuckle at a small item inside it.
There are unconfirmed reports that gatecrashers at the latest party by Fancy Pants were unfortunate that two of the guests there were Holders of the Elements of Harmony. Even without their Elements they were able to object to the intrusion strongly enough there are suggestions it drew the attention of Princess Celestia herself.
Well, that seemed accurate other than that they were guests rather than gatecrashers and it was Princess Luna, since it was nighttime.
==
The fillies and colts were a little unsettled as normally they would continue after morning break with what they had been doing before it. Today though Miss Cheerilee had concluded that portion of their lessons and told them to get out fresh paper, as if they were going to move onto something new. And now she was moving across to the door.
“I have arranged a special treat for you all,” Cheerilee said, opening the door.
After a moment to notice and react Joe walked in through it, deciding that scratches aside it was a shame about the waistcoat and shirt being torn. Or that his formal trousers were dirty from kneeling, and on both knees as he’d tripped to one and knelt to Princess Luna on the other. Something more formal than his normal clothes, even if they were far better tailored now, might have suited today better. But his normal boots were comfier than the newer formal boots as he had broken them in and he’d have wanted to wear his normal trousers so he could wear a belt and his knife anyway.
The Cutie Mark Crusaders grinned at the sight of him as they’d not had it confirmed when Joe would give his talk. All Big Macintosh had been willing to say was ‘Eee’yup’ that Joe had been found and had agreed to do it. The reaction of the other fillies and colts was rather more neutral and some looked a little unsure. There had been a lot of giggling and gossip about the strange creature living on Sweet Apple Acres and suddenly it was in their classroom and looking a lot taller and scarier than they’d expected.
“What is… ‘that’… doing here?” Diamond Tiara sneered as Joe and Cheerilee reached the front of the classroom and turned.
“Diamond Tiara,” said Cheerilee calmly, but with definite menace.
“Oh,” Diamond Tiara squeaked. “Sorry Miss…” She had to pause to force the rest out as Cheerilee looked expectantly at her. “Sorry Mister Joe.”
“To answer the ‘question’,” said Joe, getting his notes out and putting them down on a rather low desk. One scaled to the fact that even the teacher was only two-thirds his height while she remained on all fours. “I have been asked by Miss Cheerilee to speak on the history and principles of human flight, though I am sure most of the principles will be familiar to you already. Either from your knowledge of Physics or from what Apple Bloom, Scootaloo, and Sweetie Belle have likely shared.”
“Is Rainbow Dash coming?” Scootaloo asked hopefully.
“If she is then would be as much a surprise to me as it seems me being here is to you,” smiled Joe.
“She’s probably sick of his presence after yesterday,” Diamond Tiara leaned across and whispered to Silver Spoon.
“Diamond Tiara!” snapped Cheerilee.
“Sorry Miss,” Diamond Tiara repeated.
After a pause Joe fixed her with a look. “Human ears aren’t that prominent, but I am not deaf.”
“Oh!” Diamond Tiara said, noticing Cheerilee also did not appreciate the incomplete apology. “Sorry Mister Joe.”
“And I am sorry about this as well,” Cheerilee murmured to Joe as he bent to arrange his notes.
“Don’t worry,” Joe murmured back, “when I was conducting castle tours I had to deal with worse, though she does remind me why I was never tempted to train for a teaching career.”
Joe straightened again. “Although they overlap chronologically I’ll first talk about lighter-than-air flight and then heavier-than-air flight, that is craft that float in the air by displacing a greater weight of air than they weigh and those that fly though thrust and aerodynamic lift. More succinctly Balloons or Birds.”
“Or Pegasi,” Scootaloo added happily.
“Or most Pegasi,” sneered Silver Spoon, “except you.”
“Girls!” Cheerilee warned as Scootaloo began turning to the insult.
“With lighter-than-air craft there are two methods for filling the envelope,” Joe continued, “use a gas such as Hydrogen or Helium or use a burner to heat the air inside so that it expands and becomes less dense…”
Sketching on the board as he spoke Joe went through some of the advantages of each method, mentioning that as useful as it was with a Hot Air Balloon to get lift from the contents becoming hotter that this was a problem for Hydrogen or Helium Balloons. As the sun warmed them their gas would expand and they would get extra unwanted lift, or conversely if they went high enough for it to become colder then they would lose lift from their gas contracting as well as the air around it becoming less dense.
Joe talked about ballasting with water or sandbags, the latter as he’d seen on the local balloon, and about being able to travel in different directions by using the winds at different altitudes. Then he moved onto being able to travel in the direction of your choice if rather than a balloon it was an airship, though perhaps not as literal a one as the yachts he’d seen in Canterlot. He gave them a smile and said that to his eye those seemed to have too much ship and not enough gas envelope above it.
A few more sketches dealt with the concept of using a rigid frame and individual gas cells within the outer skin and of having gondolas with people or engines slung beneath it or compartments built along the keel of this frame. And that rather than the wings he’d seen on the Canterlot yachts the engines on these airships would be powering propellers such as the one on the model the Cutie Mark Crusaders had displayed. Since there were the examples of the local Balloon and the Canterlot Yachts Joe had decided to not go into great detail, this part wouldn’t be novel or new, so he was quite soon finished.
“And though airships are still in use, they have the advantage of being able to stay aloft almost indefinitely,” Joe concluded, “and there is balloon racing and recreational flight, these have been almost entirely supplanted by heavier-than-air flight. This again I shall divide into two methods, those that fly through a similar means to Rainbow Dash’s tortoise Tank with his rotating rotor or those that fly like the model with fixed wings…”
Joe quickly sketched the cross-section of an aeroplane wing, pointing out the similarity between that and the curve of natural wings and flight feathers, and then continued by speaking of gliders and how it was simple to at least not go straight down. He dropped a piece of paper to show how it curved down and then folded a quick paper plane. After touching on the nobleman who sent his servant off a hill in a glider and that this idea was more popular with those who did it voluntarily, since the servant had given his resignation as soon as he landed, Joe started on the problem of how to sustain the flight. As he started to talk about how fortunately engines were getting light enough he heard a rather undignified and ill-mannerly snort.
“You have a comment, Diamond Tiara?” Joe asked.
“We’ve all seen the train engine,” sniffed Diamond Tiara, “and perhaps we can, barely, accept that a big enough balloon could lift one. But without magic there is no way one could be lifted by wings.”
“You would be surprised how much weight a large aeroplane can carry,” Joe replied, “but that was actually an insightful comment.”
“It was?” asked Diamond Tiara, before shaking her head and muttering that of course it was, Silver Spoon naturally agreeing.
“Let us digress onto the different sorts of motors and engines you can build,” Joe continued, wiping the board. “The steam locomotive that Diamond Tiara referred to can be called an external-combustion engine as fuel is burnt outside of it…” He sketched the firebox and how multiple flues from that passed through the boiler to produce steam that was fed to the cylinders. “Though,” he admitted as he explained the diagrams, “I’ve not looked inside the local train so I am only assuming it is the same. In any case small engines of this sort were used on early airships, but as Diamond Tiara rightly pointed out their power to weight ratio was not good for heavier than air flight…”
Diamond Tiara looked briefly triumphant, until Joe continued.
“There was some success with very small engines on winged models, but the breakthrough for human carrying flight was the internal-combustion engine…”
Diamond Tiara’s face fell and she remained gloomy as Joe talked about how a mixture of fuel and air was fed into a cylinder, or fuel injected into the air inside it, and how this was then ignited either by a sparkplug or by the heat of compression. Glad that he’d paid attention during ‘motor vehicle studies’ Joe drew the four stroke cycle and how multiple cylinders connected to the same crankshaft and flywheel kept each other going and the power smoother. Admitting that this was the old fashioned method Joe also sketched the camshaft and how it pushed cylinder valves in or let their springs close them.
To everyone’s surprise, even her own, Diamond Tiara raised her hoof rather than just interrupt.
“Yes, Diamond Tiara?” Joe asked.
“What sort of fuel is this?”
“The more volatile parts of Oil.”
“Oh?” Diamond Tiara scoffed, trying to make up for her bout of politeness. “And I suppose you’ll tell us how you’d get only the volatile part?”
“Diamond Tiara,” said Cheerilee warningly.
“Sorry Miss.”
“I note that you are not apologising to me,” Joe commented, “but even if rudely put, and put in the hopes of tripping me up…” He gave the class a smile and wink. “Which is more dangerous when you’re tottering about on only two legs.” There was a slight, very slight, ripple of amusement. “It is a fair question and the answer is fractional distillation.”
Joe made a quick sketch of how that worked, that the parts that were more volatile would boil off sooner and get higher or further along before they condensed again. Then he got back to what he had been saying about flight and that though gliders could be steered by shifting weight that things became more complex as you added the weight of engine and fuel. Scootaloo looked quite smug as she remembered that talk on the way towards Sweet Apple Acres.
“Humans have things called Hang Gliders!” Scootaloo said.
“Yes, we do Scootaloo,” nodded Joe, looking at her.
“Oh… sorry to interrupt, Joe,” Scootaloo blushed.
“As Scootaloo has mentioned it,” smiled Joe, “this is how those are controlled.” Yet another wipe of the board and a sketch of pulling and pushing the control bar to shift the angle while using the weight of the pilot as ballast, going into a little more detail than he’d done in the soil on the walk. “And with what is recognised as the first successful powered flight,” he continued, “they used something called ‘wing-warping’ which, before Scootaloo comments…”
Scootaloo blushed slightly again.
“Is what more modern designs try to avoid,” continued Joe, “as they don’t want the wings, or anything else, bending. Instead they use the control surfaces you saw on the model…”
He mentioned how crude aeroplane wings were compared with bird or, with a nod to Scootaloo, Pegasus wings and tried to not be distracted as he continued by memories of how he had explained some of this to Rainbow Dash. But Joe managed to talk about leading edge slats and flaps without blushing and about how the aeroplane wings had some ability to reshape themselves. Then he moved onto aeroplane construction and how this had shifted from wood covered in fabric to having metal frames to having a metal skin be part of the structural strength rather than just a covering for the frame.
“Yes, Apple Bloom?” Joe said as the Earth Pony filly raised her hoof.
“You mean like what ah did with th’ section of wing on the model?”
“Closer to that than the metal skin on the fuselage as on the central wing it more of a box section and is providing some strength,” Joe nodded, “but, and this is where we are getting towards where my layman’s knowledge ends…” There was a slight stir and Joe wondered if they said ‘laypony’ instead. “I think for proper stressed skin construction the frame and skin are connected so, well, they are already under stress. Like reinforced concrete where the metal bars keep the concrete under compression.”
Joe paused and hoped he hadn’t just said something stupid, either about stressed construction or about if the Ponies had reinforced concrete. The former was the sort of thing where he might have looked it up to be sure and the latter… he thought he had seen construction that would use that but as soon as he used the example he became unsure of it. Dismissing his doubts he moved onto propulsion and how carved wooden propellers had given way to metal ones where the angle of the blades could be adjusted or adjust themselves depending on throttle setting and speed, and how the number of blades had increased and eventually two contra-rotating sets began to be placed on a single shaft.
“But,” Joe said, “the limits began to be reached. You could only spin the propellers so fast and, even with two sets of blades, could only accelerate so much air to that speed with them. These were fast and powerful aeroplanes, but still limited by their piston engines and what they were connected to.” He smiled to the class. “I’m impressed by them at air shows,” he commented, vaguely remembering a dream of one such sort, “but Rainbow Dash would not be.”
“Because she is awesome!” commented Scootaloo, her and the other two Cutie Mark Crusaders watching closely for any reaction from Joe.
“However,” Joe continued, ignoring the interruption, “by that point a different sort of engine had been invented. You’ll be familiar with water turbines since, I assume, those are what are at the hydroelectric plant at your dam. For other sorts of electrical generation, or seagoing ships with their propellers, you can instead have a steam turbine…”
Joe reached a section of notes where he had crossed out a chunk. He wanted to mention the Turbinia but did not want to have to explain what the Spithead Naval Review was, where that boat had shown off her speed and outpaced the ships sent to intercept her. He wanted to mention that although ‘steamship’ gave images of something fairly old-fashioned that some of the most impressive warships ever constructed were still steam powered, but he really did not want to have to describe the nuclear reactors they used to boil the water.
“Same, ah, principle can be applied here as with piston engines, in a way…”
More wiping of the board and more sketching showed how steam, or water, was fed into the turbine to spin it and Joe got the class to agree that if instead the turbine was spun then it would draw the steam, or water, through it instead. That it worked both ways. Then Joe erased part of the turbine and added fuel injectors at a constriction and explained the Jet engine. Air drawn in by the front blades, fuel injected and ignited at the constriction, exhaust out the rear to provide thrust, this air moving a lot faster than a propeller could push or pull it, and passing through the rear blades. Which turned a shaft to turn the front blades to draw in more air. Then Joe modified the diagram twice, first extending the shaft and adding a propeller for a turboprop engine and then erasing that extension and the propeller and the front blades before replacing the front blades with larger ones…
“And as you can see not all the air will go into the engine, but that which doesn’t does provide thrust,” Joe concluded, “and this Turbofan engine is more efficient at lower, sub-sonic, speeds than a pure Turbojet. Are there any more questions on this part of things?”
There was a silence that Joe was not sure whether to regard as encouraging or not. He could hope it was that he’d explained things well but he could fear it was that he’d completely lost them. Diamond Tiara had been rude and had been listening to try to find flaws to expose, but at least she had been listening and understanding well enough that the flaws were problems that had had to be solved.
“Right,” Joe continued, “I explained this far because although the method with rotors did work with piston engines it only really became practical with turbines, and these are more turbines for providing rotation than jet engines for providing thrust…”
Joe moved onto describing helicopters and made a rough sketch of the rotor assembly, admitting he was not clear on how it worked but as it rotated it also tilted the rotor blades so part of the lift was diverted into forward thrust, or could be diverted to rear or sides to move that way instead. Generally though they flew forwards and could dip their nose for more speed. There would be a problem with the helicopter body spinning in the opposite direction to the rotors but that could be solved by using contra-rotating rotors, like the propellers he’d mentioned, or by putting a small rotor on the tail to counter the torque. Joe admitted he wasn’t sure why Rainbow Dash’s tortoise doesn’t spin, so he assumed magic.
Silence returned to the classroom as Joe finished and waited for any comments. As this lengthened he shrugged to Cheerilee and began gathering his notes together. Seeing this she stepped forward.
“Thank you Joe,” Cheerilee said to him, “I am sure we are all grateful for the talk, and that you did it at such short notice.” She looked to her class. “Now,” she added, doing what she thought Joe should have done, again, rather than just wait, “Are there any questions?”
Scootaloo raised a hoof.
“Yes, Scootaloo?” Cheerilee asked.
“How fast can human machines go?”
“Depends what sort,” Joe replied, putting his notes back down. He spoke briefly of maximum speeds, admitting he was not sure of all the reasons why Helicopters could only reach low sub-sonic speeds. Mentioning the fastest petrol engined propeller planes could approach the speed of sound in a dive but more like five-sevenths that in level flight. And that above Mach 3 or 4 Jets began to run into trouble with heating of the airframe and the stress and heating on the jet turbine blades. “However,” he added, turning back to the board, “if you have enough forward speed you don’t need turbine blades…” He sketched a ramjet. “And,” he smiled, “fly fast enough and the heating of the airframe means you don’t need an engine.”
“Come again?” Applejack said at this counter-intuitive statement.
“Rather than compressing the air inside an engine,” replied Joe, vaguely describing a scramjet, “you can design the airframe so that the airflow over it becomes compressed at carefully selected places, and if fuel is injected into those places then you can get thrust.”
“Whoa!”
“No Scootaloo, that is not something we can build.”
“Aw.”
Deciding on politeness now she wasn’t so surprised Apple Bloom raised a hoof.
“Yes Apple Bloom?” Joe asked.
“What can we build?”
“Models, at the moment,” Joe replied. “But if you keep on with the project then the goal would first be something fairly simple.”
Joe erased the crudely remembered drawing of high end aeronautical experimentation and began sketching a Microlight instead. He ran through the decisions they had made about it being a wooden frame with fabric covered wing and tail and a metal skin for extra protection on the fuselage, and then mentioned the decisions they had to make about if extra bracing or a lower semi-wing would be needed. Joe then talked about why they had put the propeller where it was and the advantages and disadvantages of different places, though he admitted he was not sure about Pony Persistence of Vision.
A filly with a frizzy mane and bottle-end glasses raised her hoof and, once he had got over his shock that somepony other than Apple Bloom, Scootaloo, or Diamond Tiara was asking or commenting, Joe nodded to her. “Yes?”
“Persistence of vision?” she asked.
“I am not sure if I used the right term,” replied Joe. “In the case of an animation it would be how many pictures per second would be needed to give the illusion of smooth movement. In this case it would be how fast the propeller would need to be spinning for it to be seen through as a faint blur rather than the blades being visible.”
Scootaloo raised her hoof.
“Scoots, let somepony else ask questions,” Joe smiled, “especially since you can ask me while we are working on things.”
“Sorry Joe,” said Scootaloo, lowering her hoof.
“No, it’s okay,” Joe reassured her, looking to the class. “Anypony else?” He shrugged. “Right then, what was your next question Scootaloo?”
“How high can human flying machines fly?”
“I should have expected that, the motto of the human version of the Equestria Games is something like ‘Higher, faster, stronger’…” Joe smiled again. “Were you going to also ask how much weight they can carry?”
“Er, maybe?”
“As to height that starts getting into the question of what is flight,” Joe said, going back to sketching. “Near the ground you need wings or rotors or balloons but as you get higher the air gets thinner, so those would provide less support. I’m not sure how accurate this is, but…” He drew some rough altitudes for different things. “And like us the engines I have described need to ‘breathe’, so they have a limit on how high they can work, though one advantage of thinner air is that less drag and less heating and stress on the engines and airframe means you can go faster…”
Joe looked at his sketch for a moment and stepped back.
“And,” Joe admitted, turning to face the class, “I’ve been comparing speeds to the speed of sound, when I should have been saying the speed of sound at ground level and the atmospheric pressure there. Sound travels at different speeds depending on how dense the medium of transmission is.”
“Huh,” said one of the fillies or colts.
“Sorry,” Joe nodded. “It travels faster the denser the thing it is travelling though is, so if something is far enough away and loud enough you might feel a vibration through the ground with your feet… hooves, sorry… before you hear it through the air with your ears. Anyway, that is a digression, again.” He pointed at the sketch. “If you are high enough you are not being supported by the air are you really flying?”
“But humans have gone that high or higher?” asked Scootaloo.
To Cheerilee’s surprise, and concern, as she’d thought he was finished and seemed to now be working without notes Joe wiped the board again. He started explaining rockets and how they could be solid or liquid fuelled and mentioned his amusement when there was a modern sleek stylish thing and then a very simple looking cone for the nozzle sticking out the back. Most of these lifted vertically on the thrust of their engines before tilting slightly and gaining some aerodynamic lift despite being cylinders. The alternative was the sleek stylish thing which was carried to relatively high altitude by a parent aeroplane and then its own engine drove it up even higher to where it was flying more in the sense that something flies if you throw it.
Yet again Joe wiped the board and this time he drew a horizontal line across the middle of it and then a series of ballistic arcs. “This is how it would work if the ground was flat,” he explained, “but as it is curved…” He erased the horizontal line, replaced it with a downward curve, and extended the arcs. “Eventually if something gets high enough then it will miss the ground, or at least might take a few trips around the world before it stops missing as the air slows it. But if it is high enough to not be slowed then it might continue missing and going around like…”
Joe suddenly stopped. He was about to explain orbits and use the obvious analogy of saying ‘like the Moon’ but all at once he remembered who he had met the night before and what she was reputed to do and realised he was touching on mythology and cosmology rather than ballistics. He coughed a few times and hoped he had not gone too pale.
“It will keep going round,” Joe weakly concluded, “and if it is going around high enough than the time it takes to do this will match…”
Another thought occurred to Joe of someone he’d met, most recently, ten days ago and what she was reputed to do. And if she really did rather than it being planetary rotation then the concept of geo-stationary orbit was a nonsense.
“Are you alright Joe?” asked Cheerilee as he looked quite so stunned.
“Ah, sorry,” Joe replied, forcing a smile and looking back at the class. “Anyway. If you are high enough that what you are doing is falling towards the ground and missing does that count as flight?”
“Yes,” said a filly.
“No!” another contradicted.
“Yes!” a colt informed them.
“Nooo!” the second filly said, repeating her contradiction.
The fillies and colts started bickering and seeing Joe and his expression Cheerilee decided to end this. “I think that is all we have time for,” she said, “so everypony say thank you.”
“Thank You!” chorused the class before returning to the debate.
“A pleasure,” Joe said, feeling a little shaky as he gathered his notes and left.
Cheerilee watched him go and wondered what that last part was about. She’d been warned by Princess Celestia to keep an eye on Joe as the Princess feared that Joe would mention things that were inappropriate for the audience or inaccurate for this world, so Cheerilee wondered which that had been if not both. Despite Princess Celestia’s qualms and the ones those qualms had awoken in her Cheerilee thought this had gone quite well so she tapped a hoof to regain the attention of her class and end their quarrelling.
“We shall take lunch now,” Cheerilee told them, “and after lunch you have some very special letters to write on what you have learned.”