Hi All. I am thinking of adding a new model to my jet collection. I am very attracted to the BVM Kingcat, but I cannot understand why it should require so much downthrust on the engine. It looks decidedly odd to me, especially as I have a Bandit , aMaverick and have flown other BVM models including the T33, and they all fly perfectly without such oddities. Even the good old Kangaroo with its high thrustline does not require such a drastic engine angle. I have flown a BVM Bobcat which also flies ok but it still features downthrust on the engine,though not as much.I understand the Kingcat flies extremely well but I would like to understand the engine situation before I lay out the cash!!. I would appreciate enlightenment. Thanks, Paul, BIG BVM FAN.

E-mail BVM and ask 'em!

roo

I have just finished building and test flying my Jetco (see thread) which is a Bobcat / Kingcat style of model although of more modest cost!!

I set the model up with about 10 degrees of down thrust on the turbine as there were no instructions on this. On the first flight I had to punch in all my down trim to get it to fly level under power. On landing I added some more downthrust and the second flight was fine.

It must be a quirk of this model type that will take a smarter brain than me to explain! Although I do agree with you that the Kingcat looks extreem.

John

Thanks for replies. Ross if you have ever emailed Uncle Bob you are a better man than me. He (BVM) is/are not contactable by email.

They have email, just dont put it on the site...

Roo,

PM'd you the e-mail address.

Rgds,
Mark

Many thanks for that Mark. Paul.

The JMP Hustler and the Five jet (not sure who manufactured it) are both of similar design to the KingCat design and both have the turbine thrust line parallel to the centerline of the airframe.

I guess someone figured it out

I am going to take two wild guesses..

1) I bet when you fly a Kingcat really slow, then go to full power it does not climb
or pitch up... I bet it goes straight at a constant altitude..
If you change that engine angle I think that would change also... But nothing is as bad as a Roo..

2) Maybe BV did not want the exhaust plume near the elev... This plume changes size
with air speed and power settings, and it creates quite the low pressure. Maybe it had something
to do with that.

Eddie Weeks

Just to elaborate on what Eddie wrote; The Kingcat is probably one of the best flying sport jets you can buy and the all around flight characteristics are perfect in every aspect. If you attend the jet meets you will often find that when others are grounded due to very strong winds, the Kingcats are one of the few still in the air because they are so stable in both flight and during landings. One of the main reasons I bought mine was because I got tired of going to meets and having limited flying time due to wind conditions. My very first flight was in a 20 mph head wind and it was no problem at all.
There is absolutely no pitch up like the Kangaroo. Go arounds are like any other sport plane, simply throttle up and add elevator without having to worry about the plane pitching up and down. Kangaroo's on the other hand can give you quite a surprise if you don't know what to expect and try to make a go around.
Anyway, go for it Paul and you will be glad. I have about 120 flights on mine now and I think it is great plane...Gary

Thanks for the info lads, especially Gary. I should not have even mentioned the Kangaroo in the same query as the KINGCAT. The Roo requires UPthrust on the engine otherwise when you give it the throttle the nose goes down,unlike both of my Hotspots which have no such bad habits and require neither up nor down thrust. I know very well that the Kingcat is an extremely well sorted plane as designed.Iam just an inquisitive cuss and would like to know the real reason for all that downthrust. It would not stop me buying one but I can hear all the queries at the field already and I would like to be in a position to answer! Of course I have not discounted the fact that I may not understand the (aerodynamic) reason when explained !!!!!!!. Thanks. Paul.

I have had an email asking me to comment on this situation on RCU. I don't have a Kingcat but I do have two Bobcats, similar configuration, but with only slightly angled engines. I do think the engine angle on the Kingcat looks excessive, greater than anything I have seen before. That said, the DH Vampire of similar configuration, had a noticeably angled engine, too, but the DH110, Sea Vixen, did not but the tailplane WAS higher up the fins.

My theory is that the jet efflux accelerates the airflow over the stab. particularly the bottom surface and lowers the pressure, thus increasing the download produced by the tailplane causing a pitchup. If that is the case there are probably three fixes:

1. Raise the tailplane towards the top of the fins.
2. Reduce the camber of the tailplane, paricularly on the lower surface. (but this may have undesirable effects at other speeds and power settings.)
3. Do what was done and adjust the engine thrustline to cause a nose down pitch to balance the noseup pitch produced by the tailplane, the simplest solution.

Regards,

David Gladwin.

David

Having read you post I had a look at the BVM site and, looking at the Kingcat photos the position of the tailplane looks relatively low in relation to the centreline of the wing which would support your theory regarding the jet efflux accellerating the airflow on the underside of the tailplane.

Incidentally the photos of the new composite Bobcat show what appears to be a similiar turbine angle.

John

Yes, John, its actually quite simple to prove my theory. Hold a piece of paper betwween the finger and thumb of each hand at each of the paper end so that the paper "dangles". Now blow over the curved top of the paper (rather like the enhanced airflow created by the jet efflux) and you wil see it rise as the pressure above the paper falls, its a good demo of Bernoulli's theorem, basic aerodynamics, and this, I would suggest, is what causes the pitch up of the Kingcat, such that a huge engine angle is required to control it without elevator input !

Of course it's only theory and the only proof is to run a computer simulation or put the thing in a wind tunnel. More learned people, aerodynamicists, than I may care to comment.

My Bobcats don't have such a large angle and do pitch up, somewhat, on increase of power.

Regards,

David Gladwin.

Thanks for all the replies to this, and to David and John for their input. David I know you flew F3A aerobatics for some time (as I did) so you can appreciate my obsession with motor thrust lines. Regarding your theory I wonder has anybody flown the Kingcat full chat straight and level and hit the engine "kill" switch for an abrupt stop on the engine?. Should not this cause an immediate nose up?. If this were to happen would this prove your theory?. Thanks again, Paul.

Perhaps someone could try it, but at high speed the effect of engine efflux will probably be considerably diminished by the speed of the airlow !

Only one way to find out !
Regards,
David G.