When does a cheat hole become a duct?

[Simon Chaddock]

There are many views of the efficacy (I think that is the right word!) of cheat holes but it is a way of overcoming a small inlet area on scale planes.

After all quite a number of full size jets have inlet pressure relief doors.

This posed an interesting question. What if there was no inlet at all?

Could an EDF rely entirely on cheat holes and possibly with sprung loaded doors.

The reason to consider such a layout would be to make an scale EDF model of a rocket plane.

There are several full size designs that have a rear profile big enough for an EDF but of course no inlet.

I did toy with the X15 but my preference is for the A4b - a V-2 with wings on.

By late 1944 a 'glide' version of the V-2 was seen as a way for it to reach London from within Germany itself.

Two were actually tested although with limited success.

The first step will be to built just the tail end as a 'test' duct to see what happens to the performance of the EDF.

Edited By Simon Chaddock on 29/11/2015 22:46:39

[Simon Chaddock]

Having decided to actually build something like this.

I suppose this thread ought to continue on a "Own Design Project Blog".

[Andrew Dunn 1]

I'm neither a modeller or a flyer i just have an interest in all things that fly , always have always will . Now this may sound daft/stupid and please tell me if this is the case but .... is there anyway/possibility you could leave the ends of the wings open and have inlets built into/through the wings ? i know i said it was daft but hey you never know , the other thought i had was could you have inlets disguised with a fine mesh so it looked solid from a distance but would allow air in ?

Andy (member of the daft society)

[J D 8]

Cheat holes are there to get more air to the fan of an EDF or a turbo fan engine's cold air bypass during the slow phase of the flight. At normal flight speed most if not all the air comes from the intake because of ram effect as a result of forward motion which is why the spring loaded doors then shut to help streamline the aircraft on the outside and possibly in the duct its self.

To have any chance cheat holes on your model would have to be rather large and likely not practical.

Why not fit a rocket motor for a rocket plane? They are available, I know flight times would not be long but then neither were those of your prototype. Cheers JD8.

[Cuban8]

What sort of size model are we talking about here? 2 feet tall? 4ft? 6ft? proposed size of fan?

[Bert]

Simon

If you made an inlet at the front, then set say, six profile shapes in the front of the inlet then air could pass through whilst to spectator it would appear to be a solid nose cone.

Bert

[Wingman]

Simon do you reckon there's enough wing area for model flying purposes? - I mean we are talking about a ballistic missile - I wonder how they intended to get it on target 'cos once wings are added the missile becomes affected by air movement which alters the ballistic flight path by an uncalculable amount.

[Simon Chaddock]

Andrew

It would indeed be novel to duct the air down through the wing but I fear the area (and its internal shape) would be nowhere near sufficient.

In the same way a fine mesh is almost 'air tight' when compared to the volume that passes through a free opening of the same area.

Whilst I have not decided on the exact size I had in mind to use my 'spare' 70 mm fan, a cheap and cheerful AEO unit.

Whilst not particularly powerful it is light and electrically efficient at producing the thrust that it does.

If I size the fuselage of the A4b to hold the EDF right at the back it comes out at about 48" tall with a 20" span. A size that is just about manageable as a 'one piece' plane.

With such modest dimensions it would have to be very light which but built in 2mm Depron it should be.

I had in mind to put the cheat holes/ducts just in front of the fan to keep the ducts as short as possible and being in between the fins they would not be so obvious. Although so far back the inlets would tend to be blanketed by the fuselage when at speed I reason that with a very streamlined airframe and close to a 1:1 static thrust any any losses in flight would not be too critical.

Wingman

As I understand it the A4b was really a 'last ditch' solution so the tests were done used normal V-2 'ballistic' guidance.

Of the two known attempts one failed at launch and on the other (January 1945) a wing came off at altitude, presumably as it entered denser atmosphere at 3000 mph!

By March 1945 Von Braun has spirited away most of his team to Austria to ensure they fell into American hands.

[J D 8]

Putting the cheater holes right in front of the fan is the right thing and as you plan to make it light then there is a chance it could fly. [what controls do you plan?] What happens as the airspeed builds up and the flow may tend  to pass the cheater holes is any ones guess.Only one way to find out,try it and see. Good luck. JD8.

Edited By john davies 8 on 30/11/2015 17:13:21

[J D 8]

Hi Simon,Just for fun I did a little experiment tonight with my old Phase 3 F16 EDF jet. Using a kitchen scales I measured the thrust/ airflow by holding it two inches from the scales and running at full power.

As normal the result was 690 grams,then with the main intake covered and air only from the cheater holes it measured 540 grams.Not as much loss as I thought there would be.Interesting though the watts meter read the same for both tests. The F16 has its cheaters some 3 inches in front of the fan so what I said in previous post may be wrong. Cheers John.

[J D 8]

[Colin Leighfield]

Another great idea Simon. Would you use NACA shaped ducts with light spring loaded closers? I reckon that would work, although I'd be inclined to put them a bit further forward, perhaps somewhere around wing root trailing edge, although I appreciate that's a bit further away from the fan than your first idea of between the fins. I just wonder if the airflow that bit further forward might be better quality. Whatever, you will make the idea work, because you always do!

[J D 8]

Not sure what the point of having spring loaded closers would be in this case,under power they would have to be open all the time as there would be no main intake

Would improve streamlining on the glide perhaps but with this prototype I doubt there will be much of one.

[Colin Leighfield]

I just assumed that Simon was thinking of them for appearance purposes.

[J D 8]

Hi Colin,Yes it would look better. All this has got me thinking of taping over the cheat holes of my F16 to see what difference it makes at launch [it will be over long grass] and flat out. This will not be soon as the weather on the west coast is awful.

[Simon Chaddock]

Colin

Strictly the classic NACA duct was a way of efficiently slowing down a fast moving air stream. I believe Initially intended for cooling radiators so they did not actually have protrude and create drag.

In my case the air speed will be very low to start with and I don't want to slow it down any more.

It will thus be more of a 'constant area inlet' starting wide and getting gradually narrower and deeper until it is 1/4 the FSA, in fact almost the opposite shape to the NACA duct!

Spring loaded doors would be entirely cosmetic. In order not to reduce performance they would have be so lightly sprung they would likely only close when the rocket was static standing vertical - which would be appropriate. They would certainly not be fitted initially

Just out of interest none of my EDF jets have cheat holes, this will be the first!

The actual build will be described in the 'Own Design Build Blog' section.

[onetenor]

What about making cheat holes with sliding doors/covers. These could be opened for flight and closed for staic display .. Simple and avoids mucking about with springs and hinges. etc. I don't know if you could do them in Depron but thin balsa or even thinner ply or liteply would do it.

looking good on the other thread

Good luck

John