HP Halifax Twin Engine Modification

I well remember the original discussion when that pic first appeared – now as then I could never see the point to creating a twin engined Halifax. Especially as the four Hercules engined version had been developed to overcome the various deficiencies of the earlier version with four Merlins. In other words why bother – perhaps the photo was just a happenstance instance of a photo taken of a conventional Halifax with the two outer engines temporarily removed.

Graham
Cruise is not undertaken in the take off configuration i.e flaps down……You clean up, ie raise flaps to reduce drag in the cruise condition.Thats why you have a high lift system;- for take off;- it allows you get airborne with one low drag point at low speed and then cruise at a second low drag point at higher speed. if you read my post carefully it does say at ““””the flap limiting speed””””. Furthermore cruise is seldom undertaken at the minimum drag even in the flap up configuration because the most economic speed is a combination of three efficiencies;- airframe, engine and propellor.

The excess of power is the same for a four engined Halifax at 28Ton as it is for a twin at 14Ton.

Yes drag does increase with AOA but ithe increase is small compared to the drag rise of flying at higher speeds. Aircraft can easily take off at low weights, even with much reduced power.

I entirely agree that there is no reason to suggest that a twin would have more drag than the real thing. However it does have a lot less thrust, and in modern terms much less Specific Excess Power. A close relative of climb rate. I do however doubt that the Halifax’s unstick speed was only 15 kts below its optimum cruise speed. This would put its cruise speed with 30 kts of the stall. According to Merrick’s last epic, the most economical cruising speed of the Halifax is 190kts (depending upon variant), with a stall at 90 kts. For the Mk.III the numbers are 90 kts and 225 kts, although the latter is quoted at 20,000ft. so not a direct comparison.

I would certainly deny strongly that the induced drag due to angle of attack is “relatively small”. Not on any aircraft I’ve done performance work on – not including the Halifax admittedly. Induced drag increases drastically with angle of attack.

I don’t see any point to it as a taxi trainer, either. Maybe as some kind of maintenance exercise? Or just a joke?

Graham
Remember this take off is being done at not far off Max Landing Weight (MLW). For an aircraft with a stalling speed of about 80-90 knots it’s Velocity min unstick (Vmu) is normally about 10-15 knots below Cd min (min drag). The flap limiting speed (max thrust = max drag) is normally twice the Vmu, so there’s plenty of margin to overcome the relatively small drag due angle of attack. Furthermore I really don’t see why a twin would generate a significantly larger amount of drag than a quad at increased angle of attack.

All aircraft I’ve ever flown have been perfectly controllable in pitch at MLW plus minimum feul load and proceedures for noise abatement/feul efficient take off allow take off on fractional power settings. I don’t see this as being any different.

I’m convinced that if the objective was to fly albeit at min weights for only an hour, it was a very straightforward affair, but like yourself I’m intrigued as what the motivation was make this a worthwhile exercise.

I don’t believe this was a ground taxi trainer as this was the age of ATA pilots who undertook a type conversion on to a four engined bomber just by reading the pilots notes. Pilots went to OCU’s to learn how to take the thing into combat not to ground taxi it.

The aircraft trying to lift off is not at the angle of attack for minimum drag speed. It is at a considerably lower speed, therefore greater angle of attack and greater drag. So the pilot might feel OK travelling along tail up, but hits a wall once pulling the stick back. So to match your description you’d have to look at a take-off speed approaching the cruise speed. The runway length would be considerable, and I don’t think that the tyres would stand for it.

Could it fly – maybe, after a fashion. But you are talking about something with half the power it was designed for, not a lot more than half the power of the original HP56 which might have approached the weight you are postulating (ignoring the effect on the cg of removing all that weight from effectively random areas of the airframe), and no conceivable use. Why would anyone bother?

Hi

Could it be like that three engined b-17

an aircraft that could not be repaired on site and was made ready to fly to a repair unit
but this one never left ?

cheers
jerry

HP111, ah, although balancing the cg is essential for flight, it’s actually the moment you need to restore, and the outboard engine really isn’t that far forward of the cg. As a simple guide two tons removed from about a meter forward of the cg can be balanced by about 120kg on the tail.

I really don’t understand your point on acceleration with respect to take off, because the drag at unstick is inherently very close to the min drag (ie max LD), further aided by ground effect, normally this is only a small fraction of the point at which max thrust equals max drag, and it’s disproportionately less than high speed because as you quite rightly say, it’s a square law. Remember also the unstick speed will be less than the quad because of its much lower wingloading.

Fundamentally 50% of the thrust will accelerate 50% of the mass to the same speed/distance as 100% of the thrust and 100% of the mass…. for a given section Cl/airspeed the drag is linearly portional to the lift which is required to get the mass airborne. So I can’’t see why the drag, at low airspeed will be significantly more with only two engines, infact with less wetted area, lower velocity airflow over the outer wing, lower rolling resistance logically the twin should be lthe lowest. QED

I’m not so sure;- many twins of the period had lethal engine out handling zones within their flight envelope;- Mosquitoe, Meteor, Canberra* to mention a few;- the Miles Aerovan was certified despite being unable to maintain level flight on one engine in any part of its flight envelop …… “the good engine just delivered you to the crash”.

For test flying as discribed I don’t think they would have batted an eyelid.

*a good friend and former Canberra pilot once told me of the day he lost an engine on initial climb..”I thought I’d breathed my last breath”….I punch a full wing tank into a framers wheat field…..it was better for him to have just a tank arrive in his field rather than the whole aeroplane”

And the single-engine failure case? Minimum control speed? Whatever this was intended to do, it wasn’t to fly.

Post above ^^^^^^^

Of COURSE a 2 engine ‘Fax would fly!
Devoid of crew (save pilot and engineer) turrets, bombs, extra fuel, and pounds of equipment, 2 engine would be enough! (provided all worked correctly)
Especially if more powerful engines installed.

Then again, what would be the point for such an aircraft if it could not carry anything more than the bare minimums??

subscribed, this seems like a very interesting thread

Many thanks for the replies.

I don’t think the Flypast photo’s are a hoax or show an aircraft mid dismantling. The wing leading edges look too good with absolutely no sign of controls/pipes etc sticking through them. I could believe that if this one is being confused with the Radlet example it could be assumed there’s a photo of twin in the process of scraping.

Can I post the images?

The account that I read was that it was actually a hoax. It was in fact a Halifax in the process of being dismantled – I could be wrong of course… The good old Halfax was much modified during its career…. For example what about the “gas powered” Halifaxes! I was told that two actually flew ops – Or was that just a myth too.

Found it;- Flypast May 94 Article Title “Something Missing”

Two photo’s which were found in a former Canadian serviceman album (amongst “girls and pubs”)showing a Hercules engined Halifax with inner engines only, the outboard having been removed and the holes made good with wing skins. The article mentions that the chap who took these was based at RAF Topcliff during his time in the U.K. However that’s not to say this was the location where the photo’s were taken.

Vega,

Apologies – was just checking for pics on this subject. The single finned Halifax was PN323 – this had no engines. Apologies for the false steer.

Like Ant.H, I remember the photo – I think it was in the reader’s letters section of Aeroplane Monthly, but cannot be certain. Think it was photographed at one of the Schools of Technical Training?

I can’t give a publication date, but I do remember the pics being published. It was apparently some sort of ground-running, maintenance training rig and never flew as a twin.

I think the single-fin, ground-bound radio trials airframe was PN323 at Radlett, which is the now well-known cockpit section with IWM.

Maybe not what you are looking for, but the only twin-engined Halifax that I am aware of was one used for some sort of radio trials or ground instruction. However, this was fitted with a single large tailfin.

There were a few photos of it out in internet land somewhere, but typically I cannot find them.

Sorry!