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If I wanted to remove the piston engine from a cheap plane like a Cessna 152 and add small turbine engines with an equivalent thrust but weighing less, where would be the best place structurally to mount them? I'm thinking one small turbine on each side of the tail and two on the wings.

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What problems may I expect?

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    $\begingroup$ I'm not sure if this post is just trolling, or the OP really considers this to be an option. FYI, you can't just "mount 2 turbine engines" to a 172, there is a lot of structure that needs to support it. Not only is this not possible, it isn't even within the realm of reality (FYI, the engine in the front adds a lot of "ballast"), even if you did mount 2 very small engines on it the wings aren't designed for that. VTC as off-topic. $\endgroup$ – Ron Beyer Jul 8 '20 at 22:08
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    $\begingroup$ "Can a Cessna be coverted to a jet?" You mean, like, secretly loaded into a Lockheed C-5? $\endgroup$ – A. I. Breveleri Jul 9 '20 at 0:03
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    $\begingroup$ @Justintimeforfun You have a lot more issues regulatory speaking than just strapping them on and taking it up. You'll need to convert it over to experimental and do a whole lot of flight testing before the FAA will sign off on it. At that point, you're better off buying a kit aircraft with a jet engine that you can fly without as much red tape, money, and risk. $\endgroup$ – Ron Beyer Jul 9 '20 at 2:15
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    $\begingroup$ If there was ever a reason to have an "experimental" category, this is it. $\endgroup$ – Aaron Holmes Jul 9 '20 at 6:23
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    $\begingroup$ A few linguistical issues set aside, there is no reason to downvote this question! $\endgroup$ – Jpe61 Jul 9 '20 at 8:27
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Have a look at the Cri-Cri propeller and jet versions. Yes, jets can be mounted near the nose, but the plane must still balance at within its specified CG range. This is why removing the engine in front and placing jets in back is out (unless you want to really go crazy and mount a Gatling in the front). Remember, it must balance.

However, at Cessna (152) speeds, props are vastly more efficient, so a $turboprop$ is not out of the question and, indeed, these are found on crop dusters.

"Pocket jets" are also found on motor gliders, and I have often thought of putting 4 of these on a sailplane.

However, another key consideration is how much stress an airframe can take. With jets come higher speeds. These planes are built to flying within a certain speed "envelope" regardless of propulsion source.

The "Screamin' Sasquatch" may interest you. Here a jet is mounted on a sturdy bi-plane, and yes, it works.

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  • $\begingroup$ Very nice!!! Will the "SS" fly without its main engine, minus the weight? $\endgroup$ – Justintimeforfun Jul 11 '20 at 17:06
  • $\begingroup$ put a more aerodynamic nose on it? I removed the accepted answer to draw mor answers. If nothing comes of it, I will put it back. Thanks for helping. $\endgroup$ – Justintimeforfun Jul 11 '20 at 17:06
  • $\begingroup$ @Justintimeforfun if you removed the main engine you would have to put all four jets in front to even have a change to keep the CG in spec. However, just like the 737, this would lead to pitch stability issues, requiring a larger tail. They could be mounted on the wings, but this would work better for a glider that is already balanced. If you keep the prop engine (good idea), jets on the wings would help take-off and climb greatly, but watch that Vne. Expect much higher fuel consumption, but the jets can be shut down in cruise. I like them as extra engines. $\endgroup$ – Robert DiGiovanni Jul 11 '20 at 18:33
  • $\begingroup$ Yes you are right so similar to the "SS" they would have to go up front, unless you can put the fuel there. The idea is to climate the weight of the engine to make the plane more efficiant not to give it more thrust. maybe they could be mounted around the engine housing so balance is not a problem. They must make an turbine engine as effective and efficient as a piston engine?. $\endgroup$ – Justintimeforfun Jul 12 '20 at 3:04
  • $\begingroup$ @Justintimeforfun your ideas have merit, unfortunately, you may find fuel consumption will be the deciding factor. In the airspeed regime of the 152, props rule. You will need a much larger fuel tank for your jets. It would be cool to put 2 or 4 on the wings. This was known in the 1950s as Jet Assisted Take Off or JATO. They do attach a very large rotor to a turbine (helicopter), but the bigger, slower, wing is much more efficient. Wing>rotor>prop>turbine. See? $\endgroup$ – Robert DiGiovanni Jul 12 '20 at 10:40
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You asked for it, you got it:

The x number of small non-certified engines will not add redundancy, they are in fact very noisy, and terribly uneconomical (as a reply to your comment).

The good old piston engine is extremely reliable compared to these mini jets. Certified GA pisyon engines are tried and tested, and you can bet your mamas behind that they will function properly the whole service interval. The mini (or micro) jets, not so much...

The mini jets are stupid loud. It is incredible how much noise they make compared to their size. Having multiple of those sitting in front of you screaming like nothing you could ever imagine, well... I think they actually are the most efficient way to convert fuel into noise. Which brings us to:

They will also be disappointingly unefficient (other than producing noise). As Robert already commented in his answer, they would be operating in a suboptimal speedrange. Sure, they are used in hoverboards and other doohickeys that are even slower than a C-152, but the reason they are used in such applications is their size and relative safety compared to piston engines with large propellers.

All that (and Roberts excellent answer) being said, if I had the time, I would certainly build what you have proposed in your question. Just for shits and giggles 😃

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    $\begingroup$ This is great. Thanks. I thought since they would be outside the fuselage they might be quieter. I still have much to learn. $\endgroup$ – Justintimeforfun Jul 12 '20 at 3:09
  • $\begingroup$ The structure of C-152 (or any small aircraft) offers very little sound insulation, as they are single sheet acrylic or aluminium with plastic inner liner. $\endgroup$ – Jpe61 Jul 12 '20 at 8:28
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This is such an interesting question. I myself was thinking of adding triple microjets on both wings, for redundancy.

To add @Robert-DiGiovanni's good answer, the balance seems to be the tricky one. I'm thinking aloud here. Now, the plane must be able to be lift at the ends of both wings with

  • Full tanks, Empty Tanks and ballast
  • Full throttle, Empty Throttle
  • Full Speed, Stall Speed
  • All control surface positions
  • All flaps settings
  • All and the worst combinations of these.

These and without stalling any of the wings, tail, flaps and control surfaces.

In my scenario

  • removing the engine from the front causes nose up
  • adding the engines under the wings causes nose down when idling
  • engines cause nose up in full throttle.

Adding fuel tank to the nose instead of the engine helps little as sooner or later the nose fuel tank is empty and again the forces need to be countered.

Now, what comes to fuel considerations, adding six JetCat P80SE (http://modelaircraftcompany.com/newshop/en/home/76-jetcat-p80-se.html) replaces the consumption with about 100 litres/hour. This is in the same ballpark as Jetranger 206B (100l) or Caravan C208 (150l). Don't expect to get much less for "cruise" instead of "peak", for turbine engines it's about the same (80-90%). For four hour endurance needs 400 litres (dm3) instead of the normal C152 70 litre tank. So the front space is needed for fuel.

As the @jpe61 put it, the noise would be incomprehensible so some kind of noise insulation around the engines is needed, adding to the drag and weight.

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