BE2e Centre of Gravity Position.

[David Davis]

A few years back I acquired a Roy Scott 1/6 scale BE2e, a Great War artillery observation aircraft. I have found the model very difficult to fly.

The wings have an identical 27centimetre chord and the upper wing is staggered 10.5 centimetres ahead of the lower wing. The centre of gravity is shown on the plan at 17 cms from the leading edge of the top wing which puts it just above the rear cabane strut. The model balances slightly nose-down with two finger tips supporting the model in this position.

However, an American poster on RCU has suggested that the c of g should be brought forward to a position above the observer's seat. The observer sat in the front seat of the BE2 aircraft.

Any advice gratefully received.

**LINK**

[MattyB]

I agree with the poster on RCU - there is no way the CG can be as far back as it is marked on the plan, no wonder you had issues. Remember, aircraft with a CG too far forward fly poorly, ones with too rearward a CG fly not at all! His sugggested position looks a sensible starting point to me given you have forward stagger on the top wing.

Edited By MattyB on 29/11/2016 08:00:05

[David Davis]

Thank you Matty. Perhaps you could help me interpret the drawing.

Given that the red crosses mark the 1/4 chord position, do I conclude that the c of g should be moved forward to a point about 1/3 of the chord back from the leading edge of the top wing as illustrated by the "quartered circle" in the drawing?

[MattyB]

Yes. Essentially what he is saying is that you should balance it at or slightly in front of the average quarter chord position for the two wings. The safest position to balance on the top wing for a first flight would be where the dotted line comes up from the solid line marked 70%; you can then edge it back from there to get the handling you like. It should never get anywhere near the position you had it originally though, that was miles too far back, especially given it is not like you need to get the very last iota of soaring performance out of a BE2!

Edited By MattyB on 29/11/2016 08:33:57

[David Davis]

Thanks again Matty.

In order to improve my knowledge of aerodynamics could you please explain what the 70% and 50% are referring to.

70% and 50% of what?

[Jon H]

I have to say i would not be as dismissive of the c/g on the plan. Its in about the same place as my Stampe and there was heated debate on the position of the c/g for that and yet i have found it pretty much perfect. If we assume that 1/3 of the mean aerodynamic chord is about where you want to be (12.5cm aft of top wing l/e if my maths is correct) i dont think that the 17cm described is beyond possible given the type of model.

I would balance it like any other biplane, on the recommended c/g but about 10-15 degrees nose down.

You say its 'an handful' but what specifically is wrong? I ask as even with a wonky c/g it should still be quite docile

[Denis Watkins]

Just as an observation, surely consideration must also be given to the huge control surfaces. 4 big outer ailerons and half the horizontal stabiliser being the elevator, though a slow moving design, the size of these surfaces would have some considerable authority in upsetting the model with rearward c of g.

[Peter Miller]

Personally I would tend to agree with the drawing. The fact that the top wing is set at a very considerable positive angle of incidence will move the cengtre of lift well forward and a centre of lift in front of the CG is not good to put it mildly

On the odd biplane that I design I take the mid posotiob betwen the 1/4 chord of each wing. In other words where he has indicated the 50% line and I put the CG in the middle but my biplanes are rigged with 0 degrees incidence.

With that positive incidence I would want it a bit firther foreaard as shown on the drawing

[Biggles' Elder Brother - Moderator]

I agree with Peter and Matty. The original CoG seems extrordinarliy far back to me. The new proposed position looks far more believable. Let's face it - put it there - at worst it will be sluggishly nose heavy - at best it may well improve matters considerably. There is unlikely to be any significant risk in it being in that new postion - as long as you have enough elevator movment to produce a flair on landing!

BEB

[DH 82A]

Posted by Jon Harper - Laser Engines on 29/11/2016 08:46:51:

I have to say i would not be as dismissive of the c/g on the plan. Its in about the same place as my Stampe and there was heated debate on the position of the c/g for that and yet i have found it pretty much perfect. If we assume that 1/3 of the mean aerodynamic chord is about where you want to be (12.5cm aft of top wing l/e if my maths is correct) i dont think that the 17cm described is beyond possible given the type of model.

I would balance it like any other biplane, on the recommended c/g but about 10-15 degrees nose down.

You say its 'an handful' but what specifically is wrong? I ask as even with a wonky c/g it should still be quite docile

The Stampe has swept back wings,similar to a Tiger Moth, that is why the C.G. is further back.

[David Davis]

Jon, to be specific, the model is extremely unstable. If you click on the link in my original post you can see a video of me flying the model in England in December 2014 if memory serves. The entire flight consists of erratic left-hand circuits. You may also hear me inquiring whether I could fly the model in a right-hand circuit. What you won't see is me struggling to keep the model in the air at all!

When I flew it for the first time in France I had great difficulty in controlling it. My friend Roger Aubard took some pictures of the model in flight but they also do not convey just how difficult it was for me to control the aircraft. I was lucky to get it down in one piece. I must be a better pilot than I give myself credit for! .

I am going to try extra weight in the nose to bring the c of g forward to the recommended position. I may even replace the engine, an OS 70 FL with a Laser 80, Laser 90 or Thunder Tiger 90. The TT 90 will be a better option if I want to use the overhead exhaust!

Edited By David Davis on 29/11/2016 11:54:57

[Jon H]

Posted by David Davis on 29/11/2016 11:51:36:

Jon, to be specific, the model is extremely unstable. If you click on the link in my original post you can see a video of me flying the model in England in December 2014 if memory serves. The entire flight consists of erratic left-hand circuits. You may also hear me inquiring whether I could fly the model in a right-hand circuit. What you won't see is me struggling to keep the model in the air at all!

When I flew it for the first time in France I had great difficulty in controlling it. My friend Roger Aubard took some pictures of the model in flight but they also do not convey just how difficult it was for me to control the aircraft. I was lucky to get it down in one piece. I must be a better pilot than I give myself credit for! .

I am going to try extra weight in the nose to bring the c of g forward to the recommended position. I may even replace the engine, an OS 70 FL with a Laser 80, Laser 90 or Thunder Tiger 90. The TT 90 will be a better option if I want to use the overhead exhaust!

Edited By David Davis on 29/11/2016 11:54:57

Hi Peter, i did watch your video after i posted and my only comments in my head was 'flying too fast to begin with, sorted out nicely with reduced throttle, all looks lovely, i wonder if i can steal it!' I couldnt actually see much wrong with it in the video, perhaps a shade tail heavy and perhaps slightly more elevator movement than needed but all in all i thought it looked pretty much spot on. It wasnt hanging its tail significantly in turns, and it all looked good.

DH82, i am aware that that stampe has swept wings, but using 1/3 MAC as a guide still holds true. I have used the same on other straight wing biplanes i have had in the past and there were all within a smidge of being where i wanted them

[David Davis]

What you didn't see was me on the transmitter struggling to control the model! The flight in France was even more of an ordeal. I'm going to try moving the c of g forwards and see what happens.

If the model should balance at 1/3 MAC (12.5cms from the leading edge of the top wing,) that means that the balance point should be 4.5cms or one and three quarter inches further forward than its current position. That's 16% of the chord.

That should make a difference!

[Jon H]

It will be interesting to see where you end up. When you get it sorted balance it on the recommended c/g and see how far nose down it sits. If its around 10 to 15 degrees i would say its about right, if its more then clearly the plan is either wrong, or something else is at play.

As i have no idea of your experience level please excuse me if i am telling you how to suck eggs, but remember that this type of model wont appreciate being manhandled. Its best to gently encourage it round corners and generally be positive but not wild with the controls. You may find that smaller surface deflections help and i have no doubt rudder with the turns will be a must.

My Flair nieuport is not always the easiest model to fly but it is very rewarding, although i do wish its ailerons were more powerful

[David Davis]

I am a pretty average club flyer. I have been flying r/c models since 1988. I am a club-level instructor and I hold both the British and French A Certificates and the French QPDD which is the equivalent of the BMFA's B Certificate.

I used to fly a Puppeteer and I found that the c/g position as shown on that plan was also too far to the rear. I put in some more lead under the engine and brought the c/ g 2cms further forward. That made the model much nicer to fly but you had to use the rudder a lot on that model too.

[Jon H]

I was taught to fly on a Flair Pup, and yup, my dad came to the same conclusion about the c/g and added some extra weight to the cowling under the engine. I have flown one without this weight and it tightened up in loops/turns so some extra nose weight was required

[Alan Hilton]

I maidened a flair pup years ago with the c of g in the plan position I still remember the flight !!! .The BE looks very tail heavy .Rule of thumb on a staggered wing biplanes is 25 % of the chord on the imaginary centeral wing .Which is a good match for the position recommended i.e. The front cockpit position

Alan

[David Davis]

I have decided that my OS 70FL seems to be plenty powerful enough for the model even though Roy Scott's original BE2e had an OS 69 GP two stroke engine. I've also discovered that if I place my retractable tape measure on top of the engine, the c/g moves forward to the desired point! The tape weighs 330 grammes or about 11.5 ozs. I intend to melt the lead in an old paint kettle and to pour the lead into an old sardine tin. That looks about the right size and shape!

The alternative would be simply to replace the OS with a Thunder Tiger F91S which is about 330 grammes heavier than the OS but I fear that it might over-power the model. The BE2s were not fast aircraft, a fact which led to so many casualties.

[Jon H]

You could fit something like a 16x5 prop on the 95 as this would prevent it overspeeding the model. you could also set up a flight mode on your radio so that at the flight of a switch you can make the full range of your throttle stick only move say 50% of the actual throttle range. a friend of mine is going to set this up on his overpowered heinkel 111.

You still need full throttle available for tuning, and its there if you get into trouble so a flight mode might be a good option

[J D 8]

Most early aircraft tend to be one speed machine's, using all of their available engine power to get in the air and remain there.The difference between climbing and level flying speed was often only a few mph. The loss of just a few rpm on the engine would result in a decent.

They were designed to make the best of the very limited power of the heavy [ per HP] motors available at the time. As a result overpowering these types of aeroplane with their somewhat flimsy structure and large [to be effective at slow speeds] control surfaces will result in strange unwanted flight behaviour.

I think you are doing the right thing sticking to your 70FL and should use a fine pitch prop.I also think the CG needs to be at the forward cockpit,at least start there and experiment to find the position that suits you.

It can be both interesting and frustrating trying to sort a wayward aircraft [ see my I16 Rata post ] but with a bit of luck you will get that lovely BE to behave.Cheers John.

[Peter Miller]

Just thought I would mention something slighty related.

My father transferred from the Royal Northumberland Fusiiers after the battle of the Somme to the RFC.

He was an observer on Dh 4s on 18 squadron. The DH 4 could climb to 22,000 feet (Open Cockpit, no oxygen) which was above the height that fighter could reach.

I could go on but won't except to say that obviously by 1918 the engines had a lot more power

[Cuban8]

Wonder how long it took to reach 22,000 ft? Must have been very marginal on power at altitude - don't think the RR Eagle had a supercharger. How on earth did pilots manage to remain conscious at those altitudes without oxygen?

I've been to 10,000 ft just below the summit of mt. tiede (via cable car) and the least exertion had us struggling for breath. Just sitting and enjoying the view was uncomfortable.https://en.wikipedia.org/wiki/Airco_DH.4

 

Edited By Cuban8 on 03/12/2016 12:39:44

[Peter Miller]

Well they didn't know any better. My father did say that at that height it was almost impossible to swing his twin Lewis guns but the enemy fighters just could not reach them. It actually saved his life.

He was going on pilot training but volunteered to go with his pilot on this raid to bomb a bridge. 9 aircraft were to go. THree never took off, three turned back. Their order were to bomb the bridge and then climb for height. The other two simply turned for home...They never got back. MY father's pilot climed.

They were attacked by Fokker DVIIs ( He remembered a yellow DVII with a black chequerboard on the top wing)

They were badly shot up and my father was serious wounded but they got to the22,000 feet and eventually they crashed on our side of the lines.

I recently learned that he was officially credited with 6 kills from the rear cockpit.

He was also awarded the DFC for a very low level photo mission along a German canal

Edited By Peter Miller on 03/12/2016 12:53:26

[Cuban8]

Interesting stuff Peter, many thanks.

[J D 8]

Plus 1 on that.