A very unusual fighter plane

[Simon Chaddock]

Given the weather has virtually terminated any reasonable test flying of my Depron Seagull for some weekst myt thoughts turned to making something compact (I have run out of room1) and with a very unusual concept. The Lippisch P13a. Not that surprising really that it never got past the drawing board however Revell even made a plastic kit of it.

It was supposed to be stable up to Mach 2.6 as confirmed by German high speed wind tunnel tests.

Lippisch liked the idea of a ram jet as it is well suited to supersonic flight delivering more thrust the faster it goes.

A full size wooden glider version was under construction at the war's end. The Americans persuaded Lippich and his team to complete it. It and Lippisch were then transported to the US where the glider was tested in Langley's full size low speed wing tunnel. Although stable they discover that the thick wings did not produce the lift suggested by the small scale models used in the German research. Langley suggested that a delta a sharp edged thin wing created much better lift vortices at slow speed and had less drag at high speeds.

Just to add to the oddity the intention was for the ram jet to be coal fired using a replaceable wire 'grate' impregnated with bituminous coal dust!

 

Enough of the history the next issue was could it be realistically EDF powered? The inlet is efficient enough but rather small however the real 'tester' would be the exhaust. It was a wide narrow slot.

A 52 mm diameter inlet would match the FSA of my spare 55 mm EDF. Not ideal as aa 'slow speed' EDF prefers an inlet of 1.2 FSA but acceptable. Fortuitously the 52 mm inlet made the plane 1/10 scale so nice easy scaling.

The next problem was the duct goes from the extreme nose to tail a length of 670 mm. This would make it a long relatively narrow tube. Would there be enough thrust left after losses to actually propel it?

An advantage of a bit of CAD.

More a vacuum cleaner hose and attachment than an EDF duct!.😲

The first task is to assemble and test the complete duct to arrive at a go/no go decision as to whether to proceed any further.

We shall see. 

    

  

Edited December 31, 2022 by Simon Chaddock

[GrumpyGnome]

You sure do like a challenge!

 

Another thread to watch with interest....... good luck! 

[Paul De Tourtoulon]

Nice, the Germans did come out with a lot of 'different' designs,,,

[Eric Robson]

Would it be better to have the D /F at the rear?  A coal fired jet! It would need a fire man and tender to get to England. 😄😄 

[Simon Chaddock]

As the inlet diameter is the major restriction I considered a forward EDF would be a better choice given that the narrow inlet would result in a reduced inlet pressure. EDFs are quite capable of 'collapsing' a light weight duct whereas a circular duct can easily withstand a considerable over pressure. With a forward EDF only a small part of the inlet would have to be strong enough to resit the under pressure.

The other consideration was positioning the battery, With a rear EDF the battery would have to go well forward to achieve a typical delta CofG at 30% root chord. With the duct running the full length there is limited space forward for a battery box and hatch. On the other hand as the EDF is part of the duct it can go well forward as it occupies no extra space. This would likely put the battery in the cockpit area where the P13a does have sufficient space above the duct in that huge fin with perhaps a canopy battery hatch?

Aerodynamically my biggest concern is that very thick delta wing considering what the Langley low speed wind tunnel test discovered, poor lift for the drag created. Not a good scenario for a low powered EDF.😟

 

Just an aside apparently Lippisch did not assist Convair with their delta development despite the apparent similarity of the P13a to Convair's XF92 prototype which was first flown in 1948.

Their engineers independently discovered that a thin wing delta, apart from good supersonic performance actually produced more lift at slow speed as the sharp leading edge actually encouraged the formation of the lift creating upper surface vortex at high angles of attack. A fact well used by deltas ever since.

   

[Chris Walby]

Simon,

 

The Germans did a lot of experimenting and I am continually surprised how well my Do335 Versa with tractor+pusher motors flies (foamboard delta and gusting +30mph today). I would look to employ some cheat holes (tricky with the smooth shape), but perhaps there is room in the underside of the fuz or those thick wings? How about very lightly sprung covers that open once the fan is running /forward travel is achieved?

 

What are you thinking regarding construction material?

[Simon Chaddock]

Dhris W

If I do proceed it will almost certainly be fabricated using sheet Depron to be a super light light weight rather necessary given its likely limited thrust and high drag configuration.

 

To that end I have started fabricating the parts of the duct. Most will be single wall 'vase' printed in LW-PLA.

https://www.youtube.com/watch?v=p_KpshyElgo

This sort of thing prints very well in LW-PLA. The finished article will be 190 mm tall and weigh 6.5 g.

The exception will be the front part of the inlet which will have a double wall and an infill to give it enough strength to withstand the likely negative pressure when the EDF is at full power and static.

[john davidson 1]

Referring to the duct, one option is mounting the fan towards  the rear and using cheat holes to help flow

[Chris Walby]

2 hours ago, john davidson 1 said:

Referring to the duct, one option is mounting the fan towards  the rear and using cheat holes to help flow

But where do the cheat holes go with a fuz that shape?  Although I agree it will need something due to the length of the drain pipe 😉

[J D 8]

  As the fan is going to have to be near the front of the duct for design reasons the only place for cheat holes would be near the nose sort of HS Harrier style or perhaps in the leading edge of the landing skid structure. Just chucking out some ideas Simon. I like your determination with these unusual flying machines. John.

[john davidson 1]

It would have to be a rear mounted fan with the cheat hole or holes on the underside immediately in front, down side is debris entering on landing or take off. Hand launch and throttle closed on touchdown. Done this with an EE Lightning 

[MattyB]

I doubt it will need a source of power - the Earth should repel anything that ugly on sight... 😉

[Simon Chaddock]

I have printed out the complete duct with the 55 mm EDF installed. All printed in LW-PLA

The bit of the inlet that sticks out beyond the fuselage is printed with a double wall with an infill to prevent it collapsing under the possible negative pressure from the restricted area inlet.

The exhaust is just two tubes

With a fishtail exhaust.

The area of the  exhaust slot matches the FSA of the fan although the long tube is a bit bigger to hopefully reduce the duct losses.

It does all hold together but the 'fishtail' vibrates at full thrust. You can hear the 'drone' in the video

https://www.youtube.com/watch?v=vjx8ApC0DDw

it should not be too hard to stiffen the fishtail to remove the drone however the duct does absorb quite a bit of thrust maybe as much a 40% compared to the EDF 'bare'.

The testing did show that the major part of the thrust is lost be the exhaust section rather than the inlet. 

My best guess is that the current level of thrust would be enough to fly the Lippisch P13a but there are a few things I would like to try before actually starting on the airframe. 

 

[PeterF]

Have you made sure that the fishtail section does not have a minimum in the FSA if the height reduces before the width increases. To me, it seems as though you have an area reduction before an area increase. If you do have an minimum in the FSA then this will increase the losses. Does the air flowing out of the slot remain in line with the circular duct or does it spread over the whole width. If it remains in the centre then it may be creating extra losses. I once had ducts from 55mm fans to a slot in a B2 stealth and made the transition over a longer length but not with such a long duct and these had little thrust loss.

[Simon Chaddock]

Peter

The way the shape blended from a circle to a rectangle was also one of my concerns. It was generated by the FreeCad 'loft' function. Quick to do but no claim to give a constant area,

Anyway the Mk2 duct has both a larger exhaust tube at 56 mm diam matching the actual internal diameter of the EDF body rather than the 55 mm of the fan and a revised more bulbous profile fishtail

.

The nozzle stiffening has been further improved at the same time.

It has made a significant difference to the thrust achieved.

Actually building the Lippisch P13a airframe around the duct will generate a whole new set of problems.😉

Edited January 5, 2023 by Simon Chaddock

[J D 8]

   Watching on the you tube an interview with Captain Eric Brown about his interrogations of High up German officials and designers and amongst the model aircraft in the background see a model of the Lippisch P13, probably the Revell one.

[leccyflyer]

Yep - saw that the other day and immediately checked that the Lippisch wasn't one of the nearly five hundred types that Eric Brown had flown in his career. It wasn't.

[Simon Chaddock]

I don't think even the wooden glider version that went to the USA when completed post war ever actually 'flew' but only in a full size wind tunnel.

 

To proceed I need a decent line drawing so I have 'tarted up' the 3 view by removing the fuzzy lines leaving them just one pixel wide.

   

Slow and very tedious.

It is not a process I would ever recommend to anyone!

A symmetrical, thick & blunt nosed wing section - ouch!

  

Edited January 6, 2023 by Simon Chaddock

[MattyB]

I would be tempted to make a low tech, flat plate (or Kline-Fogleman) sectioned chuck glider first to understand the aerodynamics and get a decent approximation for the CG.
 

 

Re: the actual section for the final model, I’d certainly be inclined to ditch the scale one and just go with something that is known to work on model deltas of your target size - that could even be a flat plate, but whatever it is it will need fairly low camber and a couple of degrees of washout. This RCGroups thread may help…

 

https://www.rcgroups.com/forums/showthread.php?693652-Airfoil-for-a-delta-wing

Edited January 7, 2023 by MattyB

[Simon Chaddock]

 

MattyB

I am sure almost any wing section would be better than the scale one but then making a scale P13a fly was the task if only to prove how bad a plane it was.

I have a scale EDF Fairey Delta 2 which has a very thin symmetrical section in a true 'triangle' delta. It flies pretty well although the drag rises really rapidly as the angle of attack  increases.

My expectation is that the thick section of the P13a will require so much more thrust to even fly that the angle of attack rise will at least initially be less noticeable.

The P13a is likely to have a short flight duration, even for an EDF!😲. 

Construction in 3 mm Depron will start soon. 

[Shaun Walsh]

The nose of the P13a is just made for an outrunner and a folding prop. 🤣

[Erfolg]

One of my books claimed that the 13a ply glider was a project that Lippisch had pursued to keep his students out of the forces. 

 

I also understood that Lippisch was a consultant on the Delta Dagger etc.?

[Simon Chaddock]

My research suggest that it was the American occupying forces that 'persuaded' Lippisch and his students to complete the DM1 the wooden version . Given that both the completed glider and Lippisch were shipped to the US it is fair to assume that was all part of the deal.

Lippisch was after all a well respected aerodynamicist in tailless aircraft design playing a major role in the development of the Me 163 Komet. 

Convair claimed to have discovered the slow speed characteristics of the thin wing delta independently from the work by Lippisch. The head of Convair's design team did meet Lippisch who tried to convince Convair that a thick Delta would be stable to over Mach 2 as had been confirmed by the German wind tunnel testing. Convair did not agree so the result was Lippisch only advised on delta characteristics.

In 1950 Lippisch left Convair and became interested in ground effect vehicles using a reverse delta wing form.  

[Erfolg]

 

Is it a case of recollections differ?

 

I think the claim with respect the origin of the wooden build is in a book "German X planes", I will check tomorrow.

 

With respect to Lippisches involvement with Convair, i seem to recollect that this in a number of different books, Again I endeavour to look.

 

It could be like the Jet engine, this year I saw a Norwegian gas turbine, that was clearly a jet engine from the 1900s. Or I have read that Brown Boveri built a Gas Turbine for the steel industry, just uneconomic. The Cycle and first Patent  was made by a Scottish clergyman Barber, way back. Who actually invented what. Loads of Patents along the way.

 

[Erfolg]

I have found the reference to the P 13a V1. Not where I expected to find it though.

 

The Refence appears in "LUFTWAFFE, Secret Wings of the Third Reich", Dan Sharp, P 126 "A Porotype Rammer, by any other name.

 

The quote,  Only work of the highest priority rating could protect these students from being drafted. Aeronautical students of the Institute of Darmstadt an Munich approached me in the hope of obtaining work in connection with the P13. Since the end of the war was obviously imminent anyway. I created the project of a wooden flying- glider project of the P13, which the students were to build under the direction of my assistant. Heinmann, in a hanger of the small airfield in Prien Chiemsee.

 

There are bags of photographs of the P13, as found when the USA troops found it. There are 4 pages on the P13.

 

There has been a surprise for me, a book on the D 335, I thought lost, turned up in my search. I have bought another 3 books previously, to help me finish a RMC? Kit I have been building for some years now.

 

I will look tomorrow for the references to Lippisches contribution to the Delta Dagger and Dart projects, although I guess some overstate Lippisches contribution, whilst National Pride will down play whatever he did contribute.